research on plant based diet

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• Published: 21 October 2021

What makes a plant-based diet? a review of current concepts and proposal for a standardized plant-based dietary intervention checklist

• Maximilian Andreas Storz   ORCID: orcid.org/0000-0003-3277-0301 1  

European Journal of Clinical Nutrition volume  76 ,  pages 789–800 ( 2022 ) Cite this article

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Within the last decades, plant-based diets have received increasing interest for their potential benefits to human and environmental health. The concept of plant-based diet, however, varies widely in its definition. Current definitions range from the exclusion of all animal products to diets that include meat, fish, and dairy in varying quantities. Therefore, the main objectives of this review were twofold: (a) to investigate how researchers use the term plant-based diet in nutrition intervention studies and (b) what types of food a plant-based diet may include. Searching two databases, we found that the term “plant-based diet” evokes varying ideas to researchers and clinicians. Fifty percent of the retrieved studies that included a plant-based dietary intervention completely proscribed animal products and used the term plant-based diet interchangeably with a vegan diet. In contrast, an ~33% of trials included dairy products and 20% of dietary interventions emphasized a semi-vegetarian dietary pattern. Based on specific examples, we point out how the usage of the umbrella term “plant-based diet” may cause significant ambiguity. We often encountered incomplete descriptions of plant-based dietary interventions, which makes comparison and reproducibility of studies difficult. As a consequence, we urge others to use the term “plant-based diet” only in conjunction with a detailed dietary description. To facilitate this process, we provide a template of a standardized plant-based intervention reporting checklist. Finally, the present review also highlights the urgent need for a consensus definition of the term plant-based diet and its content.

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Introduction.

Chronic diseases are increasing in global prevalence and a leading cause of mortality in the world [ 1 , 2 ]. Heart disease, metabolic syndrome, and type-2-diabetes are driven by unhealthy consumption patterns, including refined sugars, processed meats, and high-fat foods [ 3 ]. More recently, however, Western societies noticed an increasing interest in plant-based eating patterns that may favorably affect chronic diseases [ 4 , 5 ]. Meat’s central place in the menus is being increasingly challenged [ 6 ], whereas plant-based diets, (emphasizing grains, vegetables, fruits, legumes, nuts, and seeds), are becoming increasingly popular [ 7 , 8 ].

However, according to Williams and Patel, the concept of “plant-based diet” varies widely in its definition [ 9 ]. Some definitions of “plant-based diet” exclude all animal products [ 10 , 11 ] while others emphasize that plant-based eating patterns “maximiz[e] consumption of nutrient-dense plant foods while minimizing processed foods, oils, and animal foods” [ 12 ]. A popular and widely accepted definition by Ostfeld recently pointed out that a plant-based diet excludes all animal products [ 11 ], whereas other researchers emphasized that a plant-based dietary pattern may include fish, poultry, and yogurt [ 13 , 14 ].

Several well-known authorities in the field of nutrition use the term “plant-based diet” synonymously with the term “vegan diet” - implying automatically that a plant-based diet is characterized by the avoidance of all flesh foods and animal-derived ingredients [ 15 , 16 ]. In contrast, other sources explicitly highlight that a plant-based diet does not mean “being vegetarian” [ 17 ] or vegan [ 18 ].

The term plant-based diet is a rather new term that has been introduced by the scientific community to describe eating patterns that emphasize a large proportion of plant-dominant foods [ 19 ]. Examples include both a vegan and a vegetarian diet (Fig. 1 ).

From right to left: vegan diet (excluding all flesh foods and animal products), lacto-ovo-vegetarian diet (excluding meat, fish, or poultry but including eggs and dairy), pesco-vegetarian diet (excluding meat or poultry but including fish) and omnivorous diet (containing all food groups) [ 56 ]. Modified from Medawar et al. [ 4 ].

The inconsistent usage of the term “plant-based diet”, however, may cause confusion and ambiguity among researchers, clinicians and the general public. Questions arise as to whether a plant-based diet should, by definition, include animal products or not? What does the term plant-based diet actually refer to and how can physicians advocate for something that is not clearly defined?

Since the term “plant-based diet” appears to evoke substantially varying concepts to researchers and clinicians, this review investigated how the medical community uses this term in scientific publications. The main objectives of this review were twofold: (a) to understand how researchers define the term “plant-based diet” in nutrition intervention studies and (b) how frequently this term is used interchangeably with other diets that are more clearly defined (e.g. the vegan diet, a lacto-ovo-vegetarian diet, a pesco-vegetarian diet, etc.).

The electronic database of PubMed was searched using the keywords “Diet”, “Nutrition”, “Intervention” and “Plant-based” combined into the following search string: “Plant-based AND (Diet OR Intervention OR Nutrition)”. We applied the filter “Clinical Trial” and considered only English language articles. Original articles and case reports were included in this review; reviews were not considered. Reference lists of the included articles were manually screened for additional studies to ensure that all relevant trials were identified. In addition to that, we used Google Scholar’s “cited by” function to broaden our search. This allowed us to identify additional studies that could not be retrieved from controlled databases [ 20 ]. We applied no time restriction. The entire review process was conducted by the author (MAS).

Articles were included if they reported a plant-based dietary intervention in human subjects, irrespective of age, gender, race, or ethnicity. To be eligible, articles had to include the term “plant-based diet” or a synonymous term (e.g. “plant-based dietary intervention”, etc.) in the abstract, the introduction or the methods section. We considered only interventions with a minimum duration of 1 week. Studies were included irrespective of their outcome, as we focused solely on the description of each intervention. We also included studies that combined the dietary intervention with other lifestyle modifications (e.g. smoking cessation, exercising, or meditation). Moreover, studies were included regardless of their setting and location (e.g. corporate setting, in- or outpatient setting, etc.).

We did not consider animal studies for this review. Studies that investigated the effects of a single group of plant foods (e.g. plant milks) or of supplements or a particular bioactive plant compound were excluded, as well. In addition to that, surveys about plant-based diets were not considered for this review.

We identified 153 articles that met the aforementioned criteria. Eligible articles were carefully screened for a definition of the term “plant-based diet” and for a description of the dietary intervention. In a second step, we examined whether animal products were allowed as part of the plant-based diet and whether the term “plant-based diet” was used interchangeably with other terms that are more clearly defined (Fig. 1 ).

Finally, we assigned the prescribed diet in each study to one of five pre-defined dietary groups (including the vegan diet, the lacto-ovo-vegetarian diet, the pesco-vegetarian diet, the semi-vegetarian diet and the omnivorous diet). For example, a diet focusing on whole grains, legumes and vegetables but also containing dairy products was assigned to the lacto-ovo-vegetarian group. The intention behind this process was to gain a better understanding of the concept of “plant-based diet” and to examine how the term is used in scientific studies. Figure 1 includes descriptions of the pre-defined dietary groups. We analyzed all data using Microsoft Excel (2016).

The initial search using the electronic database of PubMed yielded 153 articles published between 1992 and October 2020. We identified an additional 26 records by manually screening the reference lists of the retrieved articles and by the usage of Google Scholar. The reference management software “Zotero” (Roy Rosenzweig Center for History and New Media. (2016) Zotero Computer software) was used to identify potential duplicates. We screened 179 records in total. After examination of abstract and title, 75 records remained eligible for full-text review (Fig. 2 ).

The PRISMA flow diagram for the present review detailing the number of identified records, the number of records screened and the full texts retrieved.

Occasionally, multiple studies reported different outcomes of the same intervention (e.g. when authors performed a secondary data analysis). We included only one report when multiple publications were linked to the same intervention or the same group of participants. In case of any doubts, we contacted the authors of the respective publications via email to confirm that the sample was the same.

The search revealed a total of 44 intervention studies that investigated the effects of a plant‐based diet (Table 1 ). We identified 37 clinical studies and 7 case reports. Table 1 provides an overview of the study characteristics (in a reversed chronological order) and shows how the term “plant-based diet” was used in the respective studies.

The majority of the included studies was done in the United States of America. Other countries of origin included (in an alphabetical order): Australia, Canada, Germany, Italy, Japan, New Zealand and Slovenia. More than 2/3 of the included studies were published after 2010 ( n  = 31/44), reflecting the aforementioned growing interest in plant-based diets within the scientific community. We refrained from calculating the mean duration of the studies because this review also includes multiple case reports ( n  = 7) without a precise duration.

All studies included either a definition or a short description of the term “plant-based diet” ( n  = 44/44). However, the descriptions of the dietary interventions varied significantly in detail. In several cases it was impossible for us to determine which foods were in- and excluded in a particular plant-based dietary intervention.

Table 1 shows bulleted summaries of the dietary interventions. Five studies used the term “plant-based diet” interchangeably with the term “vegan diet” [ 21 , 22 , 23 , 24 , 25 ]. Fifty percent ( n  = 22/44) of the included studies completely proscribed animal products. More than 1/3 of studies ( n  = 17/44) included animal products as part of plant-based diet. Occasionally, the dietary description was inconclusive and it was not exactly specified to which extent animal products were allowed [ 26 , 27 , 28 , 29 ].

For example, the authors of a 2008 study used a vegan diet that excluded all animal products but supplemented it with fish oil [ 29 ]. Thus, the (dietary) intervention was technically not vegan. Another example is a 2018 study by Valdez et al. [ 26 ]. The authors investigated the feasibility of engaging college students in a 10-day plant-based dietary intervention. The intervention emphasized the value of a whole-food plant-based diet and minimized processed foods, saturated fats and added sugars. This was also represented in the provided meals at a local restaurant offering whole foods plant-based (vegan) options. While the intervention was presumably vegan, the authors did not clarify whether animal foods were “only” minimized or fully excluded.

The literature research also revealed a case report by Massera et al., who reported a whole-food plant-based diet to reverse angina without medication or interventional procedures [ 27 ]. The dietary intervention consisted primarily of vegetables, fruits, whole grains, potatoes, beans, legumes, and nuts. Again, it was not fully clear whether small amounts of animal products were allowed or proscribed. Based on the studies’ reference list and other studies of this particular group [ 30 ], however, one may assume that the dietary intervention was technically vegan. Nevertheless, all 4 aforementioned articles were not considered in the final dietary group assignment (see below and Fig. 3 ).

Based on missing or inconclusive data, assignment was not possible in few cases.

Finally, the authors of a 2014 study carefully dissected the umbrella term “plant-based diet” and discussed the different dietary patterns used in their study. The study included a vegan diet (excluding all animal products), a vegetarian diet (excluding meat and seafood), a pesco-vegetarian diet, a semi-vegetarian diet and an omnivorous diet [ 31 ]. This study was not considered in the final dietary group assignment.

Approximately 20% ( n  = 9/44) of the retrieved studies allowed participants to consume meat and fish during the plant-based dietary intervention. Moreover, a plant-based diet contained dairy products in 34% ( n  = 15/44) of the included studies. Increased consumption of plant foods was a feature of all plant-based dietary interventions in all studies ( n  = 44/44). Almost 30% ( n  = 13/44) of studies included a “whole-food” aspect and used the prefix “whole-food” to describe the plant-based dietary intervention in greater detail.

In the last step, we assigned the prescribed diet in each study to one of five pre-defined dietary groups. These groups included a vegan diet, a lacto-ovo-vegetarian diet, a pesco-vegetarian diet, a semi-vegetarian diet and an omnivorous diet. In a few cases ( n  = 6), an attribution was impossible due to missing information or inconclusive dietary descriptions. As displayed in Fig. 3 , the majority of studies prescribed a technically vegan diet. It is noteworthy that ~20% of studies allowed participants a semi-vegetarian diet, including fish and meat products.

The term “plant-based diet” evokes different ideas to researchers, scientists and clinicians. The primary aim of this review was to gain a better understanding of how scientists and clinicians define this term. Moreover, we sought to investigate how the term plant-based diet is used in scientific publications and nutrition intervention studies. A broad search strategy revealed 44 studies reporting a plant-based dietary intervention. Fifty percent of the included studies completely proscribed animal products. In ~20% of the retrieved studies, a plant-based diet included meat and fish. One-third of studies allowed the consumption of dairy products. While the majority of trials prescribed a technically vegan diet, 20% of trials included a semi-vegetarian eating pattern.

Our review confirmed the hypothesis that the term “plant-based diet” is used inconsistently within intervention studies. We also demonstrated that researchers have varying ideas about the content of a plant-based diet. Concepts range widely from a traditional vegan diet (excluding all animal-derived products) to a semi-vegetarian diet or even an omnivorous diet.

These findings may have important scientific and clinical implications. Clear definitions of a term or concept are necessary to allow for scientifically sound and reproducible results. According to Kampourakis, any kind of scientific discourse “has to involve concepts, the meaning of which ought to be clear among those participating in the discourse” [ 32 ]. The greater the flexibility in definitions and concepts, the less likely research findings are to be true [ 33 ]. In contrast, adherence to common standards and clear definitions is likely to reduce bias.

In the worst case, the absence of a clearly defined concept may lead to diametrically opposed results in scientific studies. This can be easily translated into clinical practice and is shown hereafter with the aid of two specific examples.

In 1998, Yamashita et al. compared two (isoenergetic) diets designed to lead to weight loss in 36 overweight or obese women in a 16-week parallel-design trial [ 34 ]. One arm of the study emphasized red meat and the other arm emphasized soybeans as the major protein source. Participants with a preference for daily meat consumption were allocated to the first arm. The second arm included subjects with a preference for plant foods who (habitually) ate more chicken and fish than red meat. Nutrients calculated from planned menus revealed a cholesterol content of 54 mg/1000 kcal in the second group. This serves an indirect indicator that their diet contained substantial amounts of animal products, because strict plant-based diets are usually much lower in cholesterol [ 35 ]. The authors found that weight loss was equal with both diets and concluded in their abstract that weight loss “occurred equally with the meat-based and plant-based diet” [ 34 ].

Seven years later, Barnard et al. published the results of a randomized clinical trial which examined the effects of a low-fat plant-based diet on body weight and metabolism [ 25 ]. Sixty-four postmenopausal, overweight women were randomly assigned to either a (low-fat) vegan diet or a control diet (based on the National Cholesterol Education Program guidelines). Adoption of a low-fat, vegan diet was associated with a mean weight loss of 5.8 (±3.2) kg in 14 weeks. Weight loss in the intervention group was significantly greater than in the control group (3.8(±2.8) kg) that regularly consumed meat and other animal products.

The two studies revealed contradicting results but were both published under the same umbrella term “plant-based diet” [ 25 , 34 ]. The basic dilemma could not be clearer. In one of the studies, the term “plant-based diet” was used interchangeably with a vegan diet [ 25 ], whereas, in the other trial, the usage of the term “plant-based diet” implied the regular consumption of fish and chicken [ 34 ]. Although both diets were very different with regard to their food composition, the results were published under the same umbrella term.

The lack of a clear definition of the term ‘plant-based diet’ and its inconsistent usage may cause significant ambiguity among researchers and the public. The term ‘plant-based diet’ may therefore only be useful in the context of a clear definition and a thorough description of the applied dietary pattern. Otherwise, studies including “plant-based diets” are difficult to compare and hard to reproduce.

Reproducibility of research, however, is a fundamental tenet of good science and requires meticulous and complete reporting of interventions parameters [ 36 ]. This is particularly true for nutrition interventions, that vary from study to study in many methodological details [ 37 ]. To facilitate comparison (and reproducibility) of studies, we call for a standardized plant-based intervention reporting checklist. A template including nine items that primarily focuses on the description of the dietary intervention itself is provided below (Fig. 4 ).

Template: the plant-based dietary intervention reporting checklist.

Finally, one must pose the question whether it is justified to call a diet “plant-based” when it contains fish and chicken (at least) twice per week (as it was the case in Yamashita et al.) [ 34 ]. Should a plant-based diet contain animal products after all and if so, to what extent? What makes a plant-based diet and how much “plant-based” is necessary to exert health benefits? In the absence of a clear definition, a seemingly endless number of questions arise during a scientific discourse about plant-based dietary interventions. Although this could indeed stipulate valuable scientific discussions, one may not forget about the public health and environmental aspects behind this controversy, which have become particularly urgent during the last decades.

There is now a general consensus that diets link environmental and human health [ 38 ]. The global transition towards diets high in animal products, ultra-processed foods, and refined sugars exacts a heavy toll on planetary and human health [ 39 , 40 ]. Diets high in saturated fat and meat products were frequently linked to a variety of chronic conditions, including obesity and type-2-diabetes [ 5 , 41 ]. Moreover, they were associated with excessive land use, depletion of natural resources and a loss of biodiversity [ 40 , 42 ]. Promoting animal-free diets that are abundant in land-sparing foods (such as vegetables) is therefore essential to boost environmental protection and human health [ 43 , 44 ].

In this context, Fresán and Sabaté highlighted the alignment of environmental outcomes and human health for plant-sourced foods [ 39 ]. Plant foods are usually less resource-intensive than animal foods [ 45 ]. In addition, they were associated with beneficial effects on cardiovascular and metabolic disorders [ 4 , 46 , 47 ]. Plant-based diets are characterized by a reduced caloric density and a high nutrient density [ 48 ]. They also improve gut microbiota symbiosis [ 48 ], insulin sensitivity [ 49 ], beta-cell function [ 49 , 50 ] and increase postprandial energy expenditure [ 51 ]. The improved postprandial metabolism after plant-based meals [ 52 ] and the reduced energy density of plant-based diets are two of the main reasons why this dietary pattern was frequently linked to weight loss [ 48 , 49 ].

Reducing meat and animal product consumption is an effective way to adopt a healthier diet while simultaneously strengthening environmental protection. To promote plant-based eating patterns, however, large and well-designed public health campaigns are necessary. Physicians play an important role in this process as they are often seen as nutrition authorities and are well-positioned to deliver dietary advice and nutritional prescriptions [ 53 , 54 ]. Another discussion about the value of plant-based nutrition could be a significant barrier to this development. Unfortunately, inconsistent usage of the term “plant-based diet” in the absence of clear definition of the term may exactly lead to such a discussion.

Therefore, it appears of utmost importance that future plant-based dietary interventions declare exactly to which extent animal products were included. A “plant-based dietary intervention” that includes a semi-vegetarian or even an omnivorous diet may lead to “false-negative” results (no health benefits) when compared to a plant-based diet that includes a vegan or lacto-ovo-vegetarian regimen.

Plant-based diets (and vegetarian diets in particular) are nowadays generally perceived in a positive light [ 55 ]. Dissonance about the term and its content should be resolved quickly to avoid potentially arising confusion in the general public. Otherwise, there will be a call for additional research examining the beneficial health and environmental effects of diets low (or free) in animal products. This call would inevitably translate into a substantial loss of time in the race against the growing burden of chronic non-communicable diseases and human-made environmental destruction. Thus, the authors of this paper finally argue that the term ‘plant-based diet’ should only be used in conjunction with a clear definition and a thorough description of its content.

This review has several strengths and limitations that warrant further consideration. The methodology employed in this review included a simple but easily reproducible search strategy with clearly defined in- and exclusion criteria. The outcome-independent search strategy revealed a broad spectrum of different studies. The biggest strength of this review is probably that its findings are of high translational value and highly applicable to future research studies in the field of plant-based nutrition. Our review revealed a problematic trend that requires a fast solution to allow for a better comparison between studies. The provided checklist may serve as an important guide to facilitate this process.

This review also has important limitations. Since the search strategy mainly relied on the electronic databases of PubMed and Google Scholar, it is not inconceivable that potentially relevant studies from other sources were missed. Moreover, it is likely that the English language restriction may also have limited the results.

The concept of “plant-based diet” varies widely in its definition and evokes varying ideas to researchers and clinicians. Many researchers use this term interchangeably with a vegan diet, as 50% of the included studies completely excluded animal products. In contrast, a noticeable amount of trials included dairy products or emphasized a semi-vegetarian dietary pattern. We argue that this inconsistent usage of the term “plant-based diet” may cause significant confusion and makes comparison of studies difficult. Therefore, we call for a rapid consensus definition. In the meantime, we suggest to use the term “plant-based diet” only in conjunction with a detailed dietary description. Our provided checklist may support researchers and clinicians in this process.

Nugent R. Chronic diseases in developing countries. Ann N. Y Acad Sci. 2008;1136:70–9.

Article   PubMed   Google Scholar  

Terzic A, Waldman S. Chronic diseases: the emerging pandemic. Clin Transl Sci. 2011;4:225–6.

Article   PubMed   PubMed Central   Google Scholar  

Yach D, Kellogg M, Voute J. Chronic diseases: an increasing challenge in developing countries. Trans R Soc Trop Med Hyg. 2005;99:321–4.

Medawar E, Huhn S, Villringer A, Veronica WA. The effects of plant-based diets on the body and the brain: a systematic review. Transl Psychiatry. 2019;9:1–17.

Article   Google Scholar  

Storz MA. Will the plant-based movement redefine physicians’ understanding of chronic disease? N. Bioeth. 2020;26:141–57.

Graça J, Calheiros MM, Oliveira A. Attached to meat? (Un)Willingness and intentions to adopt a more plant-based diet. Appetite. 2015;95:113–25.

Jakše B, Jakše B, Pinter S, Pajek J, Godnov U, Mis NF. Nutrient and Food Intake of Participants in a Whole-Food Plant-Based Lifestyle Program. J Am Coll Nutr. 2020;0:1–16.

Google Scholar  

Storz MA Is There a Lack of Support for Whole-Food, Plant-Based Diets in the Medical Community? Perm J. 2018;23. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6307547/ .

Williams KA, Patel H. Healthy plant-based diet: what does it really mean? J Am Coll Cardiol. 2017;70:423–5.

Freeman AM, Morris PB, Barnard N, Esselstyn CB, Ros E, Agatston A, et al. Trending cardiovascular nutrition controversies. J Am Coll Cardiol. 2017;69:1172–87.

Ostfeld RJ. Definition of a plant-based diet and overview of this special issue. J Geriatr Cardiol. 2017;14:315.

PubMed   PubMed Central   Google Scholar  

Tuso PJ, Ismail MH, Ha BP, Bartolotto C. Nutritional update for physicians: plant-based diets. Perm J. 2013;17:61–6.

Shikany JM, Safford MM, Newby PK, Durant RW, Brown TM, Judd SE. Southern Dietary Pattern is Associated With Hazard of Acute Coronary Heart Disease in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) Study. Circulation. 2015;132:804–14.

Akinyemiju T, Moore JX, Pisu M, Lakoski SG, Shikany J, Goodman M, et al. A prospective study of dietary patterns and cancer mortality among Blacks and Whites in the REGARDS cohort. Int J Cancer. 2016;139:2221–31.

Article   CAS   PubMed   PubMed Central   Google Scholar  

Kahleova H, Tura A, Hill M, Holubkov R, Barnard ND. A Plant-Based Dietary Intervention Improves Beta-Cell Function and Insulin Resistance in Overweight Adults: A 16-Week Randomized Clinical Trial. Nutrients. 2018;10.

Kenneally S, Bristor D, Newman G, et al. The evidence supporting a plant-based diet for optimal health and prevention of chronic disease. https://plantbasedhealthprofessionals.com/wp-content/uploads/HE-KEY-BENEFITS-OF-PLANT-BASED-NUTRITION.pdf .

Livingstone K Why you should eat a plant-based diet, but that doesn’t mean being a vegetarian. The Conversation. Accessed 19 Feb 2021. http://theconversation.com/why-you-should-eat-a-plant-based-diet-but-that-doesnt-mean-being-a-vegetarian-78470 .

Hemler EC, Hu FB. Plant-based diets for cardiovascular disease prevention: all plant foods are not created equal. Curr Atheroscler Rep. 2019;21:18.

Joshi S, McMacken M, Kalantar-Zadeh K. Plant-based diets for kidney disease: a guide for clinicians. Am J Kidney Dis. 2021;77:287–96.

Halevi G, Moed H, Bar-Ilan J. Suitability of Google Scholar as a source of scientific information and as a source of data for scientific evaluation—Review of the Literature. J Informetr. 2017;11:823–34.

Lederer A-K, Hannibal L, Hettich M, Behringer S, Spiekerkoetter U, Steinborn C, et al. Vitamin B12 Status Upon Short-Term Intervention with a Vegan Diet-A Randomized Controlled Trial in Healthy Participants. Nutrients. 2019;11.

Kahleova H, Dort S, Holubkov R, Barnard ND. A Plant-Based High-Carbohydrate, Low-Fat Diet in Overweight Individuals in a 16-Week Randomized Clinical Trial: The Role of Carbohydrates. Nutrients. 2018;10.

Bunner AE, Agarwal U, Gonzales JF, Valente F, Barnard ND. Nutrition intervention for migraine: a randomized crossover trial. J Headache Pain. 2014;15:69.

Mishra S, Xu J, Agarwal U, Gonzales J, Levin S, Barnard ND. A multicenter randomized controlled trial of a plant-based nutrition program to reduce body weight and cardiovascular risk in the corporate setting: the GEICO study. Eur J Clin Nutr. 2013;67:718–24.

Barnard ND, Scialli AR, Turner-McGrievy G, Lanou AJ, Glass J. The effects of a low-fat, plant-based dietary intervention on body weight, metabolism, and insulin sensitivity. Am J Med. 2005;118:991–7.

Article   CAS   PubMed   Google Scholar  

Valdez ES, Pottinger H, Urbon-Bonine A, Duncan B. Feasibility of engaging college students in a 10-day plant-based diet. Health Educ J. 2018;77:952–63.

Massera D, Zaman T, Farren GE, Ostfeld RJ. A Whole-Food Plant-Based Diet Reversed Angina without Medications or Procedures. Case Rep Cardiol. 2015;2015:978906.

Snyder DC, Morey MC, Sloane R, Stull V, Cohen HJ, Peterson B, et al. Reach out to ENhancE Wellness in Older Cancer Survivors (RENEW): design, methods and recruitment challenges of a home-based exercise and diet intervention to improve physical function among long-term survivors of breast, prostate, and colorectal cancer. Psychooncology. 2009;18:429–39.

Frattaroli J, Weidner G, Dnistrian AM, Kemp C, Daubenmier JJ, Marlin RO, et al. Clinical events in prostate cancer lifestyle trial: results from two years of follow-up. Urology. 2008;72:1319–23.

Massera D, Graf L, Barba S, Ostfeld R. Angina rapidly improved with a plant-based diet and returned after resuming a Western diet. J Geriatr Cardiol. 2016;13:364–6.

Turner-McGrievy GM, Davidson CR, Wilcox S. Does the type of weight loss diet affect who participates in a behavioral weight loss intervention? A comparison of participants for a plant-based diet versus a standard diet trial. Appetite. 2014;73:156–62.

Kampourakis K. On the Meaning of Concepts in Science Education. Sci Educ. 2018;27:591–2.

Ioannidis JPA. Why most published research findings are false. PLoS Med. 2005;2:e124.

Yamashita T, Sasahara T, Pomeroy SE, Collier G, Nestel PJ. Arterial compliance, blood pressure, plasma leptin, and plasma lipids in women are improved with weight reduction equally with a meat-based diet and a plant-based diet. Metabolism. 1998;47:1308–14.

Resnicow K, Barone J, Engle A, Miller S, Haley NJ, Fleming D, et al. Diet and serum lipids in vegan vegetarians: a model for risk reduction. J Am Diet Assoc. 1991;91:447–53.

McNutt M. Journals unite for reproducibility. Science. 2014;346:679.

Barnard ND, Willett WC, Ding EL. The Misuse of Meta-analysis in Nutrition Research. JAMA. 2017;318:1435–6.

Tilman D, Clark M. Global diets link environmental sustainability and human health. Nature. 2014;515:518–22.

Fresán U, Sabaté J. Vegetarian Diets: Planetary Health and Its Alignment with Human Health. Adv Nutr. 2019;10:S380–8.

Rizvi S, Pagnutti C, Fraser E, Bauch CT, Anand M. Global land use implications of dietary trends. PLoS One. 2018;13:e0200781.

Pan A, Sun Q, Bernstein AM, Schulze MB, Manson JE, Willett WC, et al. Red meat consumption and risk of type 2 diabetes: 3 cohorts of US adults and an updated meta-analysis. Am J Clin Nutr. 2011;94:1088–96.

Dopelt K, Radon P, Davidovitch N. Environmental Effects of the Livestock Industry: The Relationship between Knowledge, Attitudes, and Behavior among Students in Israel. Int J Environ Res Public Health. 2019;16.

Eshel G, Stainier P, Shepon A, Swaminathan A. Environmentally Optimal, Nutritionally Sound, Protein and Energy Conserving Plant Based Alternatives to U.S. Meat. Sci Rep. 2019;9:10345.

Storz MA. A Practical Guide for Physicians and Health Care Workers to Reduce Their Carbon Footprint in Daily Clinical Work. Perm J. 2018;22:17–145.

Shepon A, Eshel G, Noor E, Milo R. The opportunity cost of animal based diets exceeds all food losses. Proc Natl Acad Sci USA. 2018;115:3804–9.

Wright N, Wilson L, Smith M, Duncan B, McHugh P. The BROAD study: A randomised controlled trial using a whole food plant-based diet in the community for obesity, ischaemic heart disease or diabetes. Nutr Diabetes. 2017;7:e256.

Szabó Z, Erdélyi A, Gubicskóné Kisbenedek A, Ungár T, Lászlóné Polyák É, Szekeresné, et al. [Plant-based diets: a review]. Orv Hetil. 2016;157:1859–65.

Najjar RS, Feresin RG. Plant-Based Diets in the Reduction of Body Fat: Physiological Effects and Biochemical Insights. Nutrients. 2019;11:E2712.

Jardine MA, Kahleova H, Levin SM, Ali Z, Trapp CB, Barnard ND. Perspective: Plant-Based Eating Pattern for Type 2 Diabetes Prevention and Treatment: Efficacy, Mechanisms, and Practical Considerations. Adv Nutr. 2021;nmab063.

Storz MA. The Role of Vegan Diets in Lipotoxicity-induced Beta-cell Dysfunction in Type-2-Diabetes: A Narrative Review. J Popul Ther Clin Pharmacol. 2020;27:e22–38.

Marrone G, Guerriero C, Palazzetti D, Lido P, Marolla A, Di Daniele F, et al. Vegan Diet Health Benefits in Metabolic Syndrome. Nutrients. 2021;13:817.

Barnard ND, Levin SM, Yokoyama Y. A systematic review and meta-analysis of changes in body weight in clinical trials of vegetarian diets. J Acad Nutr Diet. 2015;115:954–69.

Truswell AS, Hiddink GJ, Blom J. Nutrition guidance by family doctors in a changing world: problems, opportunities, and future possibilities. Am J Clin Nutr. 2003;77:1089S–1092S.

Karlsen MC, Pollard KJ. Strategies for practitioners to support patients in plant-based eating. J Geriatr Cardiol. 2017;14:338–41.

Corrin T, Papadopoulos A. Understanding the attitudes and perceptions of vegetarian and plant-based diets to shape future health promotion programs. Appetite. 2017;109:40–7.

Turner-McGrievy G, Mandes T, Crimarco A. A plant-based diet for overweight and obesity prevention and treatment. J Geriatr Cardiol. 2017;14:369–74.

Singh PN, Steinbach J, Nelson A, Shih W, D’Avila M, Castilla S, et al. Incorporating an Increase in Plant-Based Food Choices into a Model of Culturally Responsive Care for Hispanic/Latino Children and Adults Who Are Overweight/Obese. Int J Environ Res Public Health. 2020;17. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7370208/ .

Jakše B, Jakše B, Pinter S, Pajek J, Fidler Mis N. Whole-Food Plant-Based Lifestyle Program and Decreased Obesity. Am J Lifestyle Med. 2020. https://doi.org/10.1177/1559827620949205 .

Crimarco A, Dias CH, Turner-McGrievy GM, Wilson M, Adams SA, Macauda M, et al. Outcomes of a short term dietary intervention involving vegan soul food restaurants on African American adults’ perceived barriers, benefits, and dietary acceptability of adopting a plant-based diet. Food Qual Preference. 2020;79:103788.

Morin É, Michaud-Létourneau I, Couturier Y, Roy M. A whole-food, plant-based nutrition program: evaluation of cardiovascular outcomes and exploration of food choices determinants. Nutrition. 2019;66:54–61.

Drost JM, Cook CB, Spangehl MJ, Probst NE, Mi L, Trentman TL. A Plant-Based Dietary Intervention for Preoperative Glucose Optimization in Diabetic Patients Undergoing Total Joint Arthroplasty. American Journal of Lifestyle Medicine. 2019. https://doi.org/10.1177/1559827619879073 .

Chiba M, Nakane K, Tsuji T, Tsuda S, Ishii H, Ohno H, et al. Relapse Prevention by Plant-Based Diet Incorporated into Induction Therapy for Ulcerative Colitis: A Single-Group Trial. Perm J. 2019;23. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6499111/ .

Campbell TM, Liebman SE. Plant-based dietary approach to stage 3 chronic kidney disease with hyperphosphataemia. BMJ Case Rep Cp 2019;12:e232080.

Campbell EK, Fidahusain M, Campbell Ii TM. Evaluation of an Eight-Week Whole-Food Plant-Based Lifestyle Modification Program. Nutrients. 2019;11.

Allen KE, Gumber D, Ostfeld RJ. Heart failure and a plant-based diet. a case-report and literature review. Front Nutr. 2019;6:82.

Towery P, Guffey JS, Doerflein C, Stroup K, Saucedo S, Taylor J. Chronic musculoskeletal pain and function improve with a plant-based diet. Complement Ther Med. 2018;40:64–9.

Ramal E, Champlin A, Bahjri K. Impact of a Plant-Based Diet and Support on Mitigating Type 2 Diabetes Mellitus in Latinos Living in Medically Underserved Areas. Am J Health Promot. 2018;32:753–62.

Najjar RS, Moore CE, Montgomery BD. A defined, plant-based diet utilized in an outpatient cardiovascular clinic effectively treats hypercholesterolemia and hypertension and reduces medications. Clin Cardiol. 2018;41:307–13.

Beauchesne AB, Goldhamer AC, Myers TR. Exclusively plant, whole-food diet for polypharmacy due to persistent atrial fibrillation, ischaemic cardiomyopathy, hyperlipidaemia and hypertension in an octogenarian. BMJ Case Rep. 2018;11.

Null G, Pennesi L. Diet and lifestyle intervention on chronic moderate to severe depression and anxiety and other chronic conditions. Complement Ther Clin Pract. 2017;29:189–93.

Gonciulea AR, Sellmeyer DE. The effect of dietary protein source on serum lipids: Secondary data analysis from a randomized clinical trial. J Clin Lipidol. 2017;11:46–54.

Evans J, Magee A, Dickman K, Sutter R, Sutter C. A Plant-Based Nutrition Program. Am J Nurs. 2017;117:56–61.

Choi EY, Allen K, McDonnough M, Massera D, Ostfeld RJ. A plant-based diet and heart failure: case report and literature review. J Geriatr Cardiol. 2017;14:375–8.

Yadav V, Marracci G, Kim E, Spain R, Cameron M, Overs S, et al. Low-fat, plant-based diet in multiple sclerosis: a randomized controlled trial. Mult Scler Relat Disord. 2016;9:80–90.

Macknin M, Kong T, Weier A, Worley S, Tang AS, Alkhouri N, et al. Plant-based, no-added-fat or American Heart Association diets: impact on cardiovascular risk in obese children with hypercholesterolemia and their parents. J Pediatr. 2015;166:953–9.

Guthrie GE, Bogue RJ. Impact of a Shared Medical Appointment Lifestyle Intervention on Weight and Lipid Parameters in Individuals with Type 2 Diabetes: A Clinical Pilot. J Am Coll Nutr. 2015;34:300–9.

Clinton CM, O’Brien S, Law J, Renier CM, Wendt MR. Whole-foods, plant-based diet alleviates the symptoms of osteoarthritis. Arthritis. 2015;2015:708152.

Bunner AE, Wells CL, Gonzales J, Agarwal U, Bayat E, Barnard ND. A dietary intervention for chronic diabetic neuropathy pain: a randomized controlled pilot study. Nutr Diabetes. 2015;5:e158.

Merrill RM, Aldana SG. Consequences of a plant-based diet with low dairy consumption on intake of bone-relevant nutrients. J Women’s Health. 2009;18:691–8.

Daubenmier JJ, Weidner G, Sumner MD, Mendell N, Merritt-Worden T, Studley J, et al. The contribution of changes in diet, exercise, and stress management to changes in coronary risk in women and men in the multisite cardiac lifestyle intervention program. Ann Behav Med. 2007;33:57–68.

Frattaroli J, Weidner G, Merritt-Worden TA, Frenda S, Ornish D. Angina pectoris and atherosclerotic risk factors in the multisite cardiac lifestyle intervention program. Am J Cardiol. 2008;101:911–8.

Saxe GA, Major JM, Nguyen JY, Freeman KM, Downs TM, Salem CE. Potential attenuation of disease progression in recurrent prostate cancer with plant-based diet and stress reduction. Integr Cancer Ther. 2006;5:206–13.

Gardner CD, Coulston A, Chatterjee L, Rigby A, Spiller G, Farquhar JW. The effect of a plant-based diet on plasma lipids in hypercholesterolemic adults: a randomized trial. Ann Intern Med. 2005;142:725–33.

Berrino F, Bellati C, Secreto G, Camerini E, Pala V, Panico S, et al. Reducing bioavailable sex hormones through a comprehensive change in diet: the diet and androgens (DIANA) randomized trial. Cancer Epidemiol Biomark Prev. 2001;10:25–33.

CAS   Google Scholar  

Colombo C, Muti P, Pala V, Cavalleri A, Venturelli E, Locardi M, et al. Plant-based diet, serum fatty acid profile, and free radicals in postmenopausal women: the diet and androgens (DIANA) randomized trial. Int J Biol Markers. 2005;20:169–76.

Hoffmann I, Groeneveld MJ, Boeing H, Koebnick C, Golf S, Katz N, et al. Giessen Wholesome Nutrition Study: relation between a health-conscious diet and blood lipids. Eur J Clin Nutr. 2001;55:887–95.

Koebnick C, Plank-Habibi S, Wirsam B, Gruendel S, Hahn A, Meyer-Kleine C, et al. Double-blind, randomized feedback control fails to improve the hypocholesterolemic effect of a plant-based low-fat diet in patients with moderately elevated total cholesterol levels. Eur J Clin Nutr. 2004;58:1402–9.

Spiller GA, Miller A, Olivera K, Reynolds J, Miller B, Morse SJ, et al. Effects of plant-based diets high in raw or roasted almonds, or roasted almond butter on serum lipoproteins in humans. J Am Coll Nutr. 2003;22:195–200.

Koertge J, Weidner G, Elliott-Eller M, Scherwitz L, Merritt-Worden TA, Marlin R, et al. Improvement in medical risk factors and quality of life in women and men with coronary artery disease in the Multicenter Lifestyle Demonstration Project. Am J Cardiol. 2003;91:1316–22.

Saxe GA, Hébert JR, Carmody JF, Kabat-Zinn J, Rosenzweig PH, Jarzobski D, et al. Can diet in conjunction with stress reduction affect the rate of increase in prostate specific antigen after biochemical recurrence of prostate cancer? J Urol. 2001;166:2202–7.

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Storz, M.A. What makes a plant-based diet? a review of current concepts and proposal for a standardized plant-based dietary intervention checklist. Eur J Clin Nutr 76 , 789–800 (2022). https://doi.org/10.1038/s41430-021-01023-z

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Plant-based diets and cardiovascular health

Affiliations.

• 1 Department of Nutrition, Harvard T.H. Chan School of Public Health, 655 Huntington Avenue, Building II 3rd Floor, Boston, MA.
• 2 Department of Nutrition, Harvard T.H. Chan School of Public Health, 655 Huntington Avenue, Building II 3rd Floor, Boston, MA; Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA; Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital, Boston, MA. Electronic address: [email protected].
• PMID: 29496410
• PMCID: PMC6089671
• DOI: 10.1016/j.tcm.2018.02.004

Plant-based diets, defined in terms of low frequency of animal food consumption, have been increasingly recommended for their health benefits. Numerous studies have found plant-based diets, especially when rich in high quality plant foods such as whole grains, fruits, vegetables, and nuts, to be associated with lower risk of cardiovascular outcomes and intermediate risk factors. This review summarizes the current evidence base examining the associations of plant-based diets with cardiovascular endpoints, and discusses the potential biological mechanisms underlying their health effects, practical recommendations and applications of this research, and directions for future research. Healthful plant-based diets should be recommended as an environmentally sustainable dietary option for improved cardiovascular health.

Keywords: Vegetarian diet; cardiovascular disease; nutritional epidemiology; plant foods; plant-based diets.

Copyright © 2018 Elsevier Inc. All rights reserved.

Publication types

• Research Support, N.I.H., Extramural
• Research Support, Non-U.S. Gov't
• Cardiovascular Diseases / diagnosis
• Cardiovascular Diseases / epidemiology
• Cardiovascular Diseases / prevention & control*
• Diet, Healthy*
• Diet, Vegetarian*
• Nutritional Status
• Nutritive Value
• Protective Factors
• Risk Factors
• Risk Reduction Behavior*

Grants and funding

• R01 CA095589/CA/NCI NIH HHS/United States
• R01 HL060712/HL/NHLBI NIH HHS/United States
• R01 HL118264/HL/NHLBI NIH HHS/United States

Vegetarian and vegan diets linked to lower risk of heart disease, cancer and death, large review finds

A plant-based diet is associated with a reduced risk of heart disease, cancer and death, according to a large-scale review published Wednesday. 

The research , which appears in the journal PLOS ONE, analyzed the results of nearly 50 studies published from 2000 to 2023.

The studies examined the health effects of either vegetarian diets or vegan regimens, which restrict any food derived from animals, including dairy.

A clear consensus emerged: Both eating patterns were associated with a lower risk of cancer and ischemic heart disease (heart problems caused by narrowed arteries). In particular, the diets seemed to reduce the risk of prostate cancer and gastrointestinal cancers like colon cancer. Vegetarian diets were also linked to a lower risk of dying from cardiovascular disease.

In addition, plant-based diets were associated with a reduction in risk factors for heart disease and cancer, including high body weight, inflammation and LDL or “bad” cholesterol. 

“This research shows, in general, that a plant-based diet can be beneficial, and taking small steps in that direction can make a difference,” said Matthew Landry, one of the review’s authors and an assistant professor of population health and disease prevention at the University of California, Irvine.

“You don’t have to go completely vegan to see some of these benefits,” he added. “Even reducing a day or two per week of animal-based consumption can have benefits over time.”

However, Dr. Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health, pointed out that not everyone who follows a plant-based diet eats the same foods, so levels of healthiness still vary.

“A vegetarian diet could be based primarily on refined starches and sugar, which we see to be the worst dietary pattern,” Willett, who was not involved in the new research, said in an email. 

A healthy plant-based diet, he said, should consist mostly of whole grains, fruits, vegetables, nuts, soy, beans and non-hydrogenated plant oils. 

Why are plant-based diets so healthy?

Researchers are still investigating the mechanisms through which plant-based diets lower the risk of disease. 

Some of it may have to do with preventing obesity, which is linked to heart disease and certain cancers . But the benefits likely extend beyond that, Landry said. 

“Some of it is independent of weight. Even when weight is maintained or doesn’t change, we still see reductions in some of these other clinical health outcomes, especially when it relates to cardiovascular disease,” he said.

One possible reason is that many fruits and vegetables are high in anti-inflammatory nutrients and antioxidants, which can reduce plaque buildup in the arteries.

Plant-based diets also tend to be high in fiber, which helps lower bad cholesterol, said Brie Turner-McGrievy, a professor of health promotion, education and behavior at the University of South Carolina. She published a study in 2014 which found that plant-based diets can reduce risk factors for heart disease, stroke and Type 2 diabetes. The research was included in the new review.

“Soluble fiber that’s found in things like beans and oats is really a powerful tool to help lower LDL cholesterol levels,” she said.

Turner-McGrievy noted, though, that much of that benefit can only be achieved through eating whole foods: “It’s not like you can take a fiber supplement and hope to have these same outcomes.”

Another benefit of a plant-based diet may come simply from the absence of meat, she said. People who are vegan tend to consume less saturated fat than meat eaters. 

“It’s just really hard to lower your saturated fat intake if you’re consuming animal-based foods,” Turner-McGrievy said. “Cheese, for example, is the No. 1 source of saturated fat in the diet.”

Processed meat products such as bacon or salami are also known to raise the risk of cancer , according to the World Health Organization. The agency considers red meat in general to be a “probable human carcinogen.”

Is a vegan or vegetarian diet right for everyone?

According to the Academy of Nutrition and Dietetics, vegetarian and vegan diets are adequate and healthy at all stages of life , including pregnancy, childhood and older adulthood. 

But the new review stopped short of recommending plant-based diets for everyone. 

“During pregnancy, it’s not recommended based on the data that we have to use a strict vegetarian diet,” said Dr. Federica Guaraldi, one of the review’s authors and an endocrinologist at the IRCCS Institute of Neurological Sciences of Bologna in Italy. 

Guaraldi and her co-authors found that the plant-based regimens studied didn’t lower the risk of gestational diabetes or hypertension in pregnant women. One study included in the review suggested that pregnant women who followed a vegetarian diet had lower levels of zinc — which is important for children's growth, development and immune function — than those who ate meat. Another study in the review found that vegetarian mothers had an increased risk of delivering babies with low birthweights. 

The review's authors also cautioned that plant-based diets might lead to vitamin B12 deficiencies in the general population. Landry said that can be addressed by taking a B12 supplement.  

“From my perspective as a dietitian, a healthy plant-based diet — either vegetarian or vegan — can really meet just about all your vitamin and mineral needs,” he said. 

Aria Bendix is the breaking health reporter for NBC News Digital.

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Open Access

Peer-reviewed

Research Article

Cardiovascular health and cancer risk associated with plant based diets: An umbrella review

Roles Conceptualization, Data curation, Formal analysis, Writing – original draft

Affiliations Department of Biomedical and Neuromotor Science, Alma Mater Studiorum–University of Bologna, Bologna, Italy, Interdisciplinary Research Center for Health Science, Sant’Anna School of Advanced Studies, Pisa, Tuscany, Italy

Roles Conceptualization, Formal analysis, Writing – review & editing

Affiliation Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom

Roles Conceptualization, Methodology, Supervision, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation Department of Biomedical and Neuromotor Science, Alma Mater Studiorum–University of Bologna, Bologna, Italy

Roles Conceptualization, Supervision, Writing – review & editing

Affiliation Stanford Prevention Research Center, Stanford University School of Medicine, Stanford, CA, United States of America

Affiliation Department of Translational Medicine, University of Eastern Piedmont, (UNIUPO), Novara, Italy

Roles Conceptualization, Data curation, Writing – review & editing

Roles Conceptualization, Methodology, Supervision, Writing – review & editing

Affiliation IRCCS Istituto delle Scienze Neurologiche di Bologna, Programma Neurochirurgia Ipofisi—Pituitary Unit, Bologna, Italy

• Angelo Capodici, 
• Gabriele Mocciaro, 
• Davide Gori, 
• Matthew J. Landry, 
• Alice Masini, 
• Francesco Sanmarchi, 
• Matteo Fiore, 
• Angela Andrea Coa, 
• Gisele Castagna, 

• Published: May 15, 2024
• https://doi.org/10.1371/journal.pone.0300711
• Reader Comments

Cardiovascular diseases (CVDs) and cancer are the two main leading causes of death and disability worldwide. Suboptimal diet, poor in vegetables, fruits, legumes and whole grain, and rich in processed and red meat, refined grains, and added sugars, is a primary modifiable risk factor. Based on health, economic and ethical concerns, plant-based diets have progressively widespread worldwide.

This umbrella review aims at assessing the impact of animal-free and animal-products-free diets (A/APFDs) on the risk factors associated with the development of cardiometabolic diseases, cancer and their related mortalities.

Data sources

PubMed and Scopus were searched for reviews, systematic reviews, and meta-analyses published from 1st January 2000 to 31st June 2023, written in English and involving human subjects of all ages. Primary studies and reviews/meta-analyses based on interventional trials which used A/APFDs as a therapy for people with metabolic diseases were excluded.

Data extraction

The umbrella review approach was applied for data extraction and analysis. The revised AMSTAR-R 11-item tool was applied to assess the quality of reviews/meta-analyses.

Overall, vegetarian and vegan diets are significantly associated with better lipid profile, glycemic control, body weight/BMI, inflammation, and lower risk of ischemic heart disease and cancer. Vegetarian diet is also associated with lower mortality from CVDs. On the other hand, no difference in the risk of developing gestational diabetes and hypertension were reported in pregnant women following vegetarian diets. Study quality was average. A key limitation is represented by the high heterogeneity of the study population in terms of sample size, demography, geographical origin, dietary patterns, and other lifestyle confounders.

Conclusions

Plant-based diets appear beneficial in reducing cardiometabolic risk factors, as well as CVDs, cancer risk and mortality. However, caution should be paid before broadly suggesting the adoption of A/AFPDs since the strength-of-evidence of study results is significantly limited by the large study heterogeneity alongside the potential risks associated with potentially restrictive regimens.

Citation: Capodici A, Mocciaro G, Gori D, Landry MJ, Masini A, Sanmarchi F, et al. (2024) Cardiovascular health and cancer risk associated with plant based diets: An umbrella review. PLoS ONE 19(5): e0300711. https://doi.org/10.1371/journal.pone.0300711

Editor: Melissa Orlandin Premaor, Federal University of Minas Gerais: Universidade Federal de Minas Gerais, BRAZIL

Received: January 8, 2024; Accepted: March 4, 2024; Published: May 15, 2024

Copyright: © 2024 Capodici et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Cardiovascular diseases (CVDs) and cancer currently represent the leading causes of death and disability worldwide. Studies performed on large cohorts worldwide have identified several modifiable and non-modifiable risk factors. Among them, robust evidence supports diet as a major modifiable risk factor [ 1 ].

A suboptimal diet, marked by insufficient consumption of fruits, vegetables, legumes, and whole grains, coupled with an excessive intake of meat (particularly red and processed), salt, refined grains and sugar, has been shown to notably elevate both mortality rates and disability-adjusted life years. Over time, these dietary choices have led to a concerning increase in health-related issues [ 1 , 2 ].

Additionally, the reduction of products of animal origin in favor of vegetarian ones has been suggested to reduce CVD and cancer risk [ 3 , 4 ]. Several major professional and scientific organizations encourage the adoption of vegetarian and vegan diets for the prevention and treatment of a range of chronic metabolic diseases such as atherosclerosis, type 2 diabetes, hypertension and obesity [ 5 , 6 ]. Ethical, environmental, and socio-economic concerns have contributed to the widespread growth of plant-based diets, particularly vegetarian and vegan options [ 7 – 9 ]. 2014 cross-national governmental survey estimated that approximately 75 million people around the globe deliberately followed a vegetarian diet, while an additional 1,45 million were obliged to because of socio-economic factors [ 10 , 11 ].

At the same time, study heterogeneity in terms of plant-based dietary regimens (from limitation of certain types to the total exclusion of animal products), their association with other lifestyle factors, patient demographic and geographical features, associated diseases, as well as study design and duration, significantly limit the assessment of the real benefits associated with animal-free and animal-products-free diets (A/APFDs). Finally, an increasing number of studies have highlighted the potential threatening consequences of chronic vitamin and mineral deficiencies induced by these diets (e.g., megaloblastic anemia due to vitamin B12 deficiency), especially more restrictive ones and in critical periods of life, like pregnancy and early childhood [ 5 ].

Based on these premises, our umbrella review aims at assessing the impact of animal-free and animal-products-free diets (A/APFDs) on the risk factors associated with the development of cardiometabolic diseases, cancer and their related mortalities in both the adult and the pediatric population, as well as pregnant women.

Search strategy

PubMed ( https://pubmed.ncbi.nlm.nih.gov/ ) and Scopus ( https://www.scopus.com/search/form.uri?display=basic#basic ) databases were searched for reviews, systematic reviews and meta-analyses published from 1st January 2000 to 31st June 2023. We considered only articles written in English, involving human subjects, with an available abstract, and answering to the following PICO question: P (population): people of all ages; I (intervention) and C (comparison): people adopting A/APFDs vs. omnivores; O (outcome): impact of A/APFD on health parameters associated with CVDs, metabolic disorders or cancer.

Articles not specifying the type of A/APFD regimen were excluded. If not detailed, the A/APFDs adopted by study participants was defined as “mixed diet”. Vegetarian diets limiting but not completely excluding certain types of meat/fish (i.e. pesco- or pollo-vegetarian diet) were excluded. Studies focusing on subjects with specific nutritional needs (i.e., athletes or military personnel) -except pregnant women-, or with known underlying chronic diseases (i.e., chronic kidney disease), as well as articles focusing on conditions/health parameters related to disorders different from CVDs or cancer, and, finally, reviews/meta-analyses including interventional studies assessing A/APFDs comparing it with pharmacological interventions were excluded.

Ad hoc literature search strings, made of a broad selection of terms related to A/APFDs, including PubMed MeSH-terms, free-text words and their combinations, combined by proper Boolean operators, were created to search PubMed database: ((vegetari* OR vegan OR Diet , Vegetarian[MH] OR fruitar* OR veganism OR raw-food* OR lacto-veget* OR ovo-vege* OR semi-veget* OR plant-based diet* OR vegetable-based diet* OR fruit-based diet* OR root-based diet OR juice-based diet OR non-meat eate* OR non-meat diet*) AND ((review[Publication Type]) OR (meta-analysis[Publication Type]))) AND (("2000/01/01"[Date—Publication] : "2023/06/31"[Date—Publication])) and Scopus database: ALL(vegetari* OR vegan OR Diet , Vegetarian OR fruitar* OR veganism OR raw-food* OR lacto-veget* OR ovo-vege* OR semi-veget* OR plant-based diet* OR vegetable-based diet* OR fruit-based diet* OR root-based diet OR juice-based diet OR non-meat eate* OR non-meat diet) AND SUBJAREA(MEDI OR NURS OR VETE OR DENT OR HEAL OR MULT) PUBYEAR > 1999 AND (LIMIT-TO (DOCTYPE , "re"))

Research design and study classification

An umbrella review approach [ 12 ] was applied to systematically assess the effect of A/APFDs on risk factors related to CVDs, metabolic disorders and cancer as derived from literature reviews, systematic reviews and meta-analyses ( Table 1 ).

• PPT PowerPoint slide
• PNG larger image
• TIFF original image

https://doi.org/10.1371/journal.pone.0300711.t001

Study selection

The list of articles identified by literature search was split into 5 equivalent parts, each assigned to a couple of readers (AC, DG, CW, ML, AM, FS, MF, AAC, GC and FG), who independently and blindly read the title and then the abstract of each article to define its pertinence. Papers included in the umbrella review had to focus on one/some of the following A/APFDs: vegans, lacto-vegetarians, ovo-vegetarians, lacto-ovo-vegetarians. No restriction was applied for age, gender, ethnicity, geographical origin, nor socio economic status. Primary studies, reviews/meta-analyses not written in English, or focusing on non-previously mentioned dietary regimens (including the Mediterranean diet) were excluded. Abstract meetings, editorials, letters to the editor, and study protocols were also excluded. To reduce study heterogeneity, at least in terms of dietary regimens, we excluded studies based on vegetarian regimens limiting but not avoiding fish or poultry, and prospective trials directly comparing A/AFPDs to pharmacological interventions.

In case of discordance between readers, we resorted to discussion amongst the authors to resolve it, based on the article’s abstract or, if not decisive, the full text. The study selection process is summarized in Fig 1 .

https://doi.org/10.1371/journal.pone.0300711.g001

This review was registered on PROSPERO (Record ID: 372913 https://www.crd.york.ac.uk /prospero/display_record.php?RecordID=372913 ).

Quality literature analysis

Three raters (AC, DG, FS) independently and blindly assessed the quality of the systematic reviews and meta-analyses using the revised AMSTAR-R 11-item tool, developed by the PEROSH group [ 13 ]. In case of disagreement, the score of each item and the final decision were discussed among the three raters.

Data extraction and reporting

Ten investigators (AC, DG, GM, ML, AM, FS, MF, AAC, GC, FG) independently extracted data from eligible articles. Disagreements in data extraction were resolved by consensus. Using a predefined protocol and a Microsoft Excel sheet, the following data were extracted: first author’s affiliation country; type of review; type of diet; target population; number of aggregated participants; total cholesterol; HDL-cholesterol; LDL-cholesterol; triglycerides; apolipoprotein B; C-Reactive Protein (CRP); Body Mass Index (BMI); body weight; fasting glucose; glycosylated hemoglobin (HbA1c); systolic blood pressure; diastolic blood pressure; cardiac events (type; risk); cardiovascular diseases (type; risk); gestational diabetes; gestational hypertension; cancer (type; risk); death due to CVDs/cancer (risk). Data were reported as mean difference (MD), weighted mean difference (WMD), standardized mean difference (SMD), and 95%CI, while the estimated risk could be reported as relative risk (RR), odds ratio (OR), or hazard ratio (HR), according to the data reported by the study authors. Articles assessing the risk of gestational diabetes and hypertension, as well as risk of low birth weight, and their determinants were examined separately.

Results from studies focusing on both vegetarian and vegan diets were analyzed and reported separately if authors had stratified the results according to the type of diet. On the contrary, if data from vegan and vegetarian subjects were mixed, we arbitrarily considered all of them as “vegetarian”.

Group 1: Cardiovascular endpoints and risk factors

I. total cholesterol (tc)..

Eight studies examined the levels of total serum cholesterol (TC) in vegetarians. Two focused on the general population and included 5,561 [ 14 ] and 576 [ 15 ] respectively, and, based on data meta-analysis, found a significant reduction in TC among vegetarians and people who assumed plant-based proteins (MD: -1.56 mmol/L; 95%CI: −1.73, −1.39; and -0.11 mmol/L; 95%CI: −0.22, −0.01, respectively).

Data were confirmed by Wang et al. (N = 832 total; Ovolacto/lacto-vegetarians: 291) [ 16 ], showing a greater dietary effect in subjects with a BMI ranging from 18.5 to 25 kg/m 2 (mean TC reduction: −0.94 mmol/L; 95%CI: −1.33, −0.55), and from 25 to 30 kg/m 2 (−0.58 mmol/L; 95%CI: −0.89, −0.27), than in those with a BMI >30 kg/m 2 (−0.16 mmol/L; 95%CI: −0.30, −0.01), and by Xu et al. (N = 783) [ 17 ], reporting lower TC in overweight and obese people (WMD: −0.37 mmol/L; 95%CI: −0.52, −0.22) adopting a vegetarian diet.

Another systematic review by Elliott et al., including 27 randomized controlled trials on plant based vs. normal western diets [ 18 ], found lower TC levels in vegetarians. These results were in line with other two descriptive reviews, the first including 2,890 overweight/obese adults [ 19 ], the second 8,969 vegetarian children aged 0–18 years [ 20 ]. Furthermore, a meta-analysis by Liang et al. described significantly lower TC (from -0.36 to -0.24 mmol/L) in people adopting plant based diets vs. people adopting western habitual diets [ 21 ].

Moreover, the review and meta-analysis by Dinu et al. [ 14 ], based on 19 studies for a total of 1,272 adults, reported significantly lower levels of TC among vegans than in omnivores (WMD: −1.72 mmol/L; 95%CI: −1.93, −1.51).

II. High-density lipoprotein cholesterol (HDL-C).

Eight reviews focused on the effects of vegetarian diet on serum high-density lipoprotein cholesterol (HDL-C) levels. Six [ 15 , 17 , 18 , 21 – 23 ] found no significant difference between vegetarians and omnivores, when considering normal weight and overweight/obese people. On the contrary, the study by Dinu et al. [ 14 ], based on 51 studies, for a total of 6,194 vegetarian adults, reported a WMD −0.15 mmol/L (95%CI: −0.19, −0.11). Liang et al. [ 21 ] analyzed 4 studies and reported a pooled estimated MD of −0.10 mmol/L (95%CI: −0.14, −0.05; p<0.001) in vegetarian diet adopters vs. western diets adopters. Finally, Zhang et al. [ 22 ] did not find any statistically significant differences in HDL-C levels when assessing vegetarian diets compared to non-vegetarians; on the same note Dinu et al. [ 14 ], analyzing data from 15 studies, for a total of 1,175 adults, found no significant differences in HDL-C levels between vegans and people following other dietary regimens.

III. Low-density lipoprotein cholesterol (LDL-C).

Ten reviews summarized the effect of vegetarian diets on serum levels of low-density lipoprotein cholesterol (LDL-C). Seven [ 14 – 18 , 21 , 23 ] found significantly lower LDL-C levels associated with vegetarian diet, both in the general population and in diabetic patients. In particular, Elliot et al. [ 18 ], analyzing 43 observational and interventional studies, described lower LDL-C in people adopting plant based diets; a significant difference was reported by the study of Liang et al. [ 21 ] based on 68 studies (MD: -0.29 to -0.17), and similar to data by Lamberg et al. [ 15 ], based on 13 RCTs including for a total of 576 participants (MD: -0.14 mmol/L; 95%CI: -0.25, -0.02). The impact of vegetarian diet appeared even greater in overweight or obese people, according to the analysis by Xu et al. [ 17 ], based on 7 RCTs (N = 783; MD: -0.31 mmol/L; 95%CI: -0.46, -0.16). Two reviews [ 19 , 20 ] reported similar results in overweight/obese patients and children aged 0–18 years, but no meta-analyses were conducted. Wang et al. [ 16 ] reported a MD of −0.34 mmol/L (95%CI: −0.57, −0.11; p<0.001) in the general adult population. Ferdowsian et al. [ 23 ] reported an overall reduction of LDL-C associated with vegetarian diet, but no synthesis analyses were performed. Dinu et al. [ 14 ] analyzed 46 studies encompassing 5,583 vegetarians and found a WMD of -1.18 mmol/L (95%CI: -1.34, -1.01). Finally, Viguiliouk et al. [ 24 ] reported a MD of −0.12 mmol/L (95%CI: −0.20, −0.04) in 6 trials involving 602 diabetic patients.

Four reviews identified a significant reduction in LDL-C in vegans as compared to omnivores [ 14 , 19 , 23 , 25 ]. Benatar et al. [ 25 ] analyzed 31 studies, for a total of 3,355 healthy vegan adults and 53,393 non-vegan controls and found MD of -0.49 mmol/L (95%CI: -0.62, -0.36; p<0.0001). Ferdowsian et al. [ 23 ] reported a reduction of LDL-C in healthy vegans, and Ivanova et al. [ 19 ] in overweight patients, but no meta-analysis was performed. Finally, Dinu et al. [ 14 ] analyzed 13 studies, for a total of 728 healthy vegan adults, and found a significant LDL-C reduction (WMD: −1.27 mmol/L; 95%CI: −1.66, −0.88).

IV. Triglycerides (TG).

Seven systematic reviews [ 14 , 16 – 18 , 20 , 23 , 26 ] analyzed serum triglycerides (TG) in vegetarians vs. omnivores. Specifically, Wang et al. [ 16 ] described no differences between the two, with a pooled estimated effect of 0.04 mmol/L (95%CI: −0.05, 0.13; p = 0.4). Zhang et al. [ 26 ] analyzing 12 studies for a total of 1,300 subjects, found a MD of −1.28 mmol/L (95%CI; −2.14, −0.42). Schürmann et al. and Ferdowsian et al. [ 20 , 23 ] reported lower TG in vegetarians in both children and adults but did not perform data meta-analysis. Dinu et al. [ 14 ] analyzed 55 studies including 4,008 vegetarians and found a WMD of −0.63 mmol/L (95%CI: −0.97, −0.30; p = 0.02). Conversely, in the review by Elliott et al. [ 18 ] no differences were reported in triglycerides. Xu et al. [ 17 ] reported a significant increase of TG (WMD: 0.29 mmol/L; 95%CI: 0.11, 0.47) in vegetarians as compared to meat eaters.

The effect of vegan diet on TG remains debated as one review [ 23 ] reported significative changes in TGs (-0.14 mmol/L, CI -0.24 to -0.05), while another [ 14 ] did not find any differences between vegans and omnivores since, after having analyzed 13 studies for 483 vegans, they reported a WMD of -0.52 mmol/L (95%CI: -1.13; 0.09).

V. C-reactive protein (CRP).

Three studies reported lower C-reactive protein (CRP) levels in normal weight, overweight and obese vegetarians as compared to non-vegetarians. Craddock et al. and Menzel et al. reported a WMD of -0.61 mg/L (95%CI: -0.91, -0.32; p = 0.0001) [ 27 ]; -0.25 mg/L (95%CI: -0.49, 0; p = 0.05) [ 28 ], respectively.

Data derived from the analysis by Menzel et al. [ 28 ] in vegan subjects were in line with previously mentioned studies performed in vegetarians (WMD: -0.54 mg/L; 95%CI: -0.79, -0.28; p<0.0001).

Two reviews [ 29 , 30 ] focused on the effects of mixed vegetarian diets on CRP levels. The first [ 29 ] included 2,689 obese patients and found a WMD of -0.55 mg/L (95%CI: -0.78, -0.32; I 2 = 94.4%), while the other [ 30 ], based on 2,398 normal weight subjects found no significant differences between vegetarians and omnivores in the primary analysis; alas, when considering a minimum duration of two years vegetarianism they described lower CRP levels vs. omnivores (Hedges’ g = -0.29; 95%CI: -0.59, 0.01).

VI. Plant-based diets and lipids.

Three studies [ 23 , 26 , 31 ] assessed the lipid profile in people following plant-based diets (without differentiating among diet subtypes) in comparison with omnivores. All of them found significantly lower levels of TC, HDL-C and LDL-C in subjects following plant-based diets. Specifically, Yokoyama et al. [ 31 ] reported a WMD of −1.62 mmol/L (95%CI: −1.92, −1.32; p< 0.001; I 2 = 81.4) for TC, −1.27 mmol/L (95%CI: −1.55, −0.99; p< 0.001; I 2 = 83.3) for LDL-C, −0.2 mmol/L (95%CI: −0.26, −0.14; p< 0.001; I 2 = 49.7) for HDL-C, and −0.36 mmol/L; 95%CI: −0.78, 0.06; p = 0.092; I 2 = 83.0) for TG when considering observational studies, and of −0.69 mmol/L (95%CI: −0.99, −0.4; p<0.001; I 2 = 54.8) for TC, −0.69 mmol/L (95%CI: −0.98, −0.37; p<0.001; I 2 = 79.2) for LDL-C, −0.19 mmol/L (95%CI: −0.24, −0.14; p<0.001; I 2 = 8.5) for HDL-C, and a non-statistically significant increase of TG based on prospective cohort studies. Additionally, Zhang et al. [ 26 ] in their meta-analysis, including 1,300 subjects, found a SMD of -1.28 mmol/L in TG (95% CI -2.14 to -0.42).

Finally, Picasso et al. [ 32 ] did not find any differences in triglycerides for mixed vegetarian diets (MD: 0.04 mmol/L; 95%CI: -0.09, 0.28), but did find statistically significant differences in HDL-C (MD: -0.05 mmol/L; 95%CI: -0.07, -0.03).

VII. Blood pressure.

A . Systolic blood pressure (SBP) . Various studies found significantly lower mean levels of systolic blood pressure (SBP) levels in vegetarians compared to the general population [ 33 – 36 ]. Specifically, Gibbs et al. [ 33 ] reported a SMD of -5.47 mmHg (95%CI: -7.60, -3.34; p<0.00001) in ovo-lacto-vegetarians, as did Lee et al. [ 34 ] reporting a SMD of -1.75 mmHg (95%CI: -5.38, 1.88; p = 0.05); furthermore, they reported a SBP decreased by -2.66 mmHg (95%CI: -3.76, -1.55), in people adopting generic vegetarian diets. Moreover, Garbett et al. [ 35 ] reported a 33% lower prevalence of hypertension in vegetarians vs. nonvegetarians. On the contrary, Schwingshackl et al. [ 36 ], analyzing data from 67 clinical trials overall including 17,230 pre-hypertensive and hypertensive adult patients with a BMI between 23.6 and 45.4 kg/m 2 , followed for 3 to 48 months, did not find any significant reductions in SBP associated with vegetarian diet.

Four reviews investigated the differences in SBP between vegans and non-vegans. Benatar et al. and Lee et al. [ 25 , 34 ] reported significantly lower mean SBP levels in vegans vs. omnivores (MD: -2.56 mmHg; 95%CI: -4.66, -0.45; and WMD: -3.12 mmHg; 95%CI: -4.54, -1.70; p<0.001, respectively). On the other hand, Gibbs et al. [-1.30 mmHg (95%CI: -3.90,1.29)] and Lopez et al. (-1.33 mmHg; 95%CI: −3.50, 0.84; P = 0.230) [ 33 , 37 ] did not find any significant difference in mean SBP levels between vegans and omnivores.

Both reviews [ 32 , 38 ] focusing on SBP in mixed-plant-based dietary patterns found significantly lower levels in vegetarians than in omnivores. The meta-analysis by Picasso et al. [ 32 ], based on 4 RCTs did not find any differences, alas, analyzing 42 cross sectional studies, they described a MD of -4.18 mmHg (95%CI -5.57, -2.80; p<0.00001), in agreement with Yokoyama et al. [ 38 ], who reported a MD of -4.8 mmHg (95%CI: -6.6, -3.1; p<0.001; I 2 = 0) according to the 7 controlled trials, 6 of which being randomized (311 participants), included in the analysis, and of -6.9 mmHg (95%CI: -9.1, -4.7; p<0.001; I 2 = 91.4) based on the other 32 observational studies (21,604 participants).

B . Diastolic blood pressure (DBP) . Garbett et al. [ 35 ] reported reduced mean diastolic blood pressure (DBP) values in vegetarians vs. omnivores, confirmed by the analysis of Gibbs et al. [ 33 ] (WMD: –2.49 mmHg; 95%CI: –4.17, –0.80; p = 0.004; I 2 = 0%) in ovo-lacto-vegetarians, by Lee et al. [ 34 ] [WMD: -1.69 mmHg (95%CI: -2.97, -0.41; p<0.001)] who included 15 randomized controlled trials (N = 856) performed in vegetarians; and by Yokoyama et al. [ 38 ], who highlighted a MD -2.2 mmHg (95%CI: -3.5, -1.0; p<0.001; I 2 = 0%) and -4.7 mmHg (95%CI: -6.3, -3.1; p<0.001; I 2 = 92.6%) according to data from 7 controlled trials (N = 311) and 32 observational studies (N = 21,604), respectively. Conversely, Schwingshackl et al. [ 36 ] did not find significant differences between vegetarians and non-vegetarians.

Three reviews [ 25 , 34 , 37 ] examined the impact of vegan vs. non-vegan diet on DBP and described statistically significant reductions. Benatar et al. described reduction of DBP, corresponding to a MD of -1.33 mmHg (95%CI: -2.67, -0.02) [ 25 ]. Lee et al. described a reduction in DBP of a WMD of -1.92 mmHg (95%CI: -3.18, -0.66; p<0.001) [ 34 ]. Finally, Lopez et al. [ 37 ] described the same reduction amounting to WMD: -4.10 mmHg (95%CI: -8.14, -0.06).

Four studies agreed upon the lower mean DBP levels in subjects following mixed vegetarian diets as compared to omnivores [ 32 – 34 , 38 ], quantified as MD -3.03 mmHg (95%CI: -4.93, 1.13; p = 0.002) by Picasso et al. [ 32 ], and −2.2 mmHg (95%CI: −3.5, −1.0; p<0.001) and −4.7 mmHg (95%CI: −6.3, −3.1; p <0.001) by the analysis performed on clinical trials and observational studies, respectively, by Yokoyama et al. [ 38 ].

VIII. Body weight and body mass index (BMI).

Berkow et al. [ 39 ] identified 40 observational studies comparing weight status of vegetarians vs. non-vegetarians: 29 reported that weight/BMI of vegetarians of both genders, different ethnicities (i.e., African Americans, Nigerians, Caucasians and Asians), and from widely separated geographic areas, was significantly lower than that of non-vegetarians, while the other 11 did not find significant differences between the two groups. In female vegetarians, weight was 2.9 to 10.6 kg (6% to 17%) and BMI 2.7% to 15.0% lower than female non-vegetarians, while the weight of male vegetarians was 4.6 to 12.6 kg (8% to 17%) lower and the BMI 4.6% to 16.3% lower than that of male non-vegetarians. The review by Schürmann et al. [ 20 ], focusing on 8,969 children aged 0–18 years old found similar body weight in both vegetarian and vegan children as compared to omnivore ones. Dinu et al. [ 14 ] analyzed data from 71 studies (including 57,724 vegetarians and 199,230 omnivores) and identified a WMD BMI of -1.49 kg/m 2 (95%CI: -1,72, -1,25; p<0.0001) in vegetarians when compared to omnivores.

Barnard et al. [ 40 ] found a significant reduction in weight in pure ovolactovegetarians (−2.9 kg; 95% CI −4.1 to −1.6; P<0.0001), compared to non-vegetarians from control groups; furthermore, they found in vegans the mean effect was of -3.2 kg (95% CI: -4.0;-2.4, P: <0.0001); overall they included 490 subjects in their analysis, excluding subjects who did not complete the trials.

Benatar et al. [ 25 ]–including 12,619 vegans and 179,630 omnivores from 40 observation studies–and Dinu et al. [ 14 ]–based on 19 cross sectional studies, for a total of 8,376 vegans and 123,292 omnivores–reported the same exact result, with a mean lower BMI in vegans vs omnivores, equal to -1.72 kg/m 2 (95%CI: -2.30, -1.16) and -1.72 kg/m 2 (95%CI: -2.21,-1.22; p<0.0001), respectively. The meta-analysis by Long et al. [ 41 ], performed on 27 studies, reported a MD of -0.70 kg/m 2 (95%CI: -1.38, -0.01) for BMI in vegans vs. omnivores. A systematic review and meta-analysis by Agnoli et al. [ 42 ] found mean BMI to be lower in subjects adhering to mixed vegetarian diets as compared to omnivores. Additionally, Tran et al. [ 43 ] described weight reductions in clinically healthy patients, as well as in people who underwent vegetarian diets as a prescription, but no meta-analysis was performed.

Finally, Huang et al. [ 44 ] found significant differences in both vegans and vegetarians, who were found to have lost weight after having adopted the diet as a consequence of being assigned to the intervention group in their randomized studies. For vegetarians the WMD was -2.02 kg (95%CI: -2.80 to -1.23), when compared to mixed diets, and for vegans the WMD was -2.52 kg (95%CI: -3.02 to -1.98), when compared to vegetarians.

IX. Glucose metabolism.

Viguiliouk et al. [ 24 ] found a significant reduction in HbA1c (MD: −0.29%; 95%CI: −0.45, −0.12) and fasting glucose (MD: −0.56 mmol/L; 95%CI: −0.99, −0.13) in vegetarians vs. non-vegetarians.

The meta-analysis by Dinu et al. [ 14 ], reported for vegetarians (2256) vs omnivores (2192) WMD: -0.28 mmol/L (95%CI: -0.33, -0.23) in fasting blood glucose.

These findings were confirmed by Picasso et al. [ 32 ] who found a MD of -0.26 mmol/L (95% CI: -0.35, -0.17) in fasting glucose in mixed-vegetarian diets as compared to omnivores.

A meta-analysis by Long et al. [ 41 ], based of 27 cross sectional studies, showed a MD for homeostasis model assessment of insulin resistance -measured as HOMA-IR, a unitless measure ideally less than one- of -0.75 (95%CI: -1.08, -0.42), fasting plasma glucose in vegetarians who adhered also to an exercise intervention as compared to omnivores.

Lee & Park [ 45 ] reported a significantly lower diabetes risk (OR 0.73; 95%CI: 0.61, 0.87; p<0.001) in vegetarians vs. non-vegetarians, being the association stronger in studies conducted in the Western Pacific region and Europe/North America than in those from Southeast Asia.

Regarding vegans, the review by Benatar et al. [ 25 ] determined a mean reduction of 0.23 mmol/L (95%CI: -0.35, -0.10) of fasting blood glucose in vegans (N = 12,619) as compared to omnivores (N = 179,630). The finding was in line with Dinu et al. [ 14 ], who reported a WMD of -0.35 mmol/L (95%CI: -0.69, -0.02; p = 0.04) of fasting blood glucose in vegans (n = 83) than omnivores (n = 125).

A systematic review, finally, including 61 studies [ 42 ] found mean values of fasting plasma glucose, and T2D risk to be lower in subjects following mixed vegetarian diets as compared to omnivores.

X. Cardiovascular events.

Huang et al. [ 46 ] found a significantly lower risk of ischemic heart disease (IHD) (RR: 0.71; 95%CI: 0.56, 0.87), but no significant differences for cerebrovascular mortality between vegetarians and non-vegetarians. The review by Remde et al. [ 47 ] was not conclusive, as only a few studies showed a reduction of the risk of CVDs for vegetarians versus omnivores, while the others did not find any significant results.

Dybvik et al. [ 48 ] based on 13 cohort studies for a total of 844,175 participants (115,392 with CVDs, 30,377 with IHD and 14,419 with stroke) showed that the overall RR for vegetarians vs. nonvegetarians was 0.85 (95%CI: 0.79–0.92, I 2 = 68%; 8 studies) for CVD, 0.79 (95%CI: 0.71–0.88, I 2 = 67%; 8 studies) for IHD, 0.90 (95%CI: 0.77–1.05, I 2 = 61%; 12 studies) for total stroke, while the RR of IHD in vegans vs. omnivores was 0.82 (95%CI: 0.68–1.00, I 2 = 0%; 6 studies).

The meta-analysis by Kwok et al. [ 49 ], based on 8 studies including 183,321 subjects comparing vegetarians versus non-vegetarians. They identified a significant reduction of IHD in the Seventh Day Adventist (SDA) cohort, who primarily follow ovo-lacto-vegetarian diets, while other non-SDA vegetarian diets were associated only with a modest reduction of IHD risk, raising the concern that other lifestyle factors typical of SDA and, thus not generalizable to other groups, play a primary role on outcomes. IHD was significantly reduced in both genders (RR: 0.60; 95%CI: 0.43, 0.83), while the risk of death and cerebrovascular disease and cardiovascular mortality risk reduction was significantly reduced only in men. No significant differences were detected for the risk of cerebrovascular events.

The meta-analysis by Lu et al. [ 50 ] -657,433 participants from cohort studies- reported a lower incidence of total stroke among vegetarians vs. nonvegetarians (HR = 0.66; 95%CI = 0.45–0.95; I 2 = 54%), while no differences were identified for incident stroke.

The descriptive systematic review by Babalola et al. [ 3 ] reported that adherence to a plant-based diet was inversely related to heart failure risk and advantageous for the secondary prevention of CHD, particularly if started from adolescence. Another review by Agnoli et al. [ 42 ], confirmed a lower incidence of CVDs associated with mixed vegetarian diets as compared to omnivorous diets. Finally, Chhabra et al. [ 51 ] found that vegetarian diet, particularly if started in adolescence and associated with vitamin B intake, can reduce the risk of stroke.

Gan et al. [ 52 ] described a lower risk of CVDs (RR 0.84; 95% CI 0.79 to 0.89; p < 0.05) in high, vs. low, adherence plant based diets, but the same association was not confirmed for stroke (RR 0.87; 95% CI: 0.73, 1.03).

Group 2: Pregnancy outcomes

The meta-analysis by Foster et al. [ 53 ], performed on 6 observational studies, found significantly lower zinc levels in vegetarians than in meat eaters (-1.53 ± 0.44 mg/day; p = 0.001), but no association with pregnancy outcomes, specifically no increase in low children birth weight. The finding was confirmed by Tan et al. [ 54 ], who similarly reported no specific risks, but reported that Asian (India/Nepal) vegetarian mothers exhibited increased risks to deliver a baby with Low Birth Weight (RR: 1.33 [95%CI:1.01, 1.76, p =  0.04, I 2 = 0%]; nonetheless, the WMD of neonatal birth weight in five studies they analyzed suggested no difference between vegetarians and omnivores.

To our knowledge, no reviews/meta-analyses have assessed the risk of zinc deficiency and its association with functional outcomes in pregnancy in relation to mixed or vegan diets.

Group 3: Cancer

The meta-analysis by Parra-Soto et al. [ 55 ], based on 409,110 participants from the UK Biobank study (mean follow-up 10.6 years), found a lower risk of liver, pancreatic, lung, prostate, bladder, colorectal, melanoma, kidney, non-Hodgkin lymphoma and lymphatic cancer as well as overall cancer (HR ranging from 0.29 to 0.70) determined by non-adjusted models in vegetarians vs. omnivores; when adjusted for sociodemographic and lifestyle factors, multimorbidity and BMI, the associations remained statistically significant only for prostate cancer (HR 0.57; 95%CI: 0.43, 0.76), colorectal cancer (HR 0.73; 95%CI: 0.54, 0.99), and all cancers combined (HR 0.87; 95%CI 0.79, 0.96). When colorectal cancer was stratified according to subtypes, a lower risk was observed for colon (HR 0.69; 95%CI: 0.48, 0.99) and proximal colon (HR 0.43; 95%CI: 0.22, 0.82), but not for rectal or distal cancer.

Similarly, the analysis by Huang et al. [ 46 ], based on 7 studies for a total of 124,706 subjects, reported a significantly lower overall/total cancer incidence in vegetarians than non-vegetarians (RR 0.82; 95%CI: 0.67, 0.97).

Zhao et al. [ 56 ] found a lower risk of digestive system cancer in plant-based dieters (RR = 0.82, 95%CI: 0.78–0.86; p< 0.001) and in vegans (RR: 0.80; 95%CI: 0.74, 0.86; p<0.001) as compared to meat eaters.

Additionally, DeClercq et al. [ 57 ] reported a decreased risk of overall cancer and colorectal cancer, but inconsistent results for prostate cancer and breast cancer; this was substantiated by Godos et al. [ 58 ] found no significant differences in breast, colorectal, and prostate cancer risk between vegetarians and non-vegetarians.

The umbrella review by Gianfredi et al. [ 59 ], did describe a lower risk of pancreatic cancer associated with vegetarian diets.

Dinu et al. [ 14 ] reported a reduction in the risk of total cancer of 8% in vegetarians, and of 15% in vegans, as compared to omnivores. They described lower risk of cancer among vegetarians (RR 0.92; 95%CI 0.87, 0.98) and vegans (RR: 0.85; 95%CI: 0.75,0.95); nonetheless, they also described non-significant reduced risk of mortality from colorectal, breast, lung and prostate cancers. Regarding the latter, a meta-analysis by Gupta et al. [ 60 ] on prostate cancer risk found a decreased hazard ratio for the incidence of prostate cancer (HR: 0.69; 95%CI: 0.54–0.89, P<0.001) in vegetarians as compared to omnivores from the evidence coming from 3 studies. In the vegan population, similar results were observed from the only included study (HR: 0.65; 95%CI: 0.49–0.85; p<0.001).

Group 4: Death by cardiometabolic diseases and cancer

According to Huang et al. [ 46 ], the mortality from IHD (RR: 0.71; 95%CI: 0.56, 0.87), circulatory diseases (RR: 0.84; 95%CI: 0.54, 1.14) and cerebrovascular diseases (RR: 0.88; 95%CI: 0.70, 1.06) was significantly lower in vegetarians than in non-vegetarians.

The analysis by Dinu et al. [ 14 ] performed on 7 prospective studies, overall including 65,058 vegetarians, reported a 25% reduced mortality risk from ischemic heart diseases (RR 0.75; 95%CI: 0.68, 0.82; p<0.001), but no significant differences were found analyzing 5 cohort studies in terms of mortality from CVDs, cerebrovascular diseases, nor colorectal, breast, prostate, and lung cancer. Regarding vegans, they analyzed 6 cohort studies, and found no differences in all-cause mortality, but significant differences in cancer incidence (RR: 0.85; 95%CI: 0.75, 0.95), indicating a protective effect of vegan diets.

The literature search did not identify studies focusing on mortality risk for cardiometabolic and cancer diseases in vegans.

Quality of the included studies

The quality of the 48 reviews and meta-analyses included in this umbrella review was assessed through the AMSTAR-R tool. Results are reported in S1 Table . Overall, the average quality score was 28, corresponding to mean quality. However, 36 studies (75%) scored between 60% and 90% of the maximum obtainable score, and can, therefore, be considered of good/very good quality. The least satisfied item on the R-AMSTAR grid was #8 -scientific quality of included studies used to draw conclusions-, where as many as 19 studies (39.6%) failed to indicate the use of study-related quality analysis to make recommendations. This finding should be read in conjunction with the missing quality analysis in 15 studies (31.3%)–Item #7 scientific quality of included studies assessed and documented-. Item #10, regarding publication bias, was the second least met item, in which 18 studies (37.5%) did not perform any analysis on this type of bias. 16 studies (33.3%) lacked to indicate careful exclusion of duplicates (Item #2), but also the presence of conflict of interest (Item #11). This point is certainly another important piece to consider in the overall quality assessment of these articles. All these considerations give us a picture of a general low quality of the publications found, lowering the strength of evidence as well as the external validity of the results.

This umbrella review provides an update on the benefits associated with the adoption of A/AFPDs in reducing risk factors associated with the development of cardiometabolic diseases and cancer, considering both the adult and the pediatric population, as well as pregnant women.

Compared to omnivorous regimens, vegetarian and vegan diets appear to significantly improve the metabolic profile through the reduction of total and LDL cholesterol [ 14 – 21 , 23 , 25 ], fasting blood glucose and HbA1c [ 14 , 24 , 25 , 37 , 39 – 41 ], and are associated with lower body weight/BMI, as well as reduced levels of inflammation (evaluated by serum CRP levels [ 27 , 30 ]), while the effect on HDL cholesterol and triglycerides, systolic and diastolic blood pressure levels remains debated. A much more limited body of literature suggested vegetarian, but not vegan diets also reduce ApoB levels further improving the lipid profile [ 61 ].

It should be remarked that, in the majority of the cases, people adopting plant-based diets are more prone to engage in healthy lifestyles that include regular physical activity, reduction/avoidance of sugar-sweetened beverages, alcohol and tobacco, that, in association with previously mentioned modification of diet [ 62 ], lead to the reduction of the risk of ischemic heart disease and related mortality, and, to a lesser extent, of other CVDs.

The adoption of vegan diets is known to increase the risk of vitamin B-12 deficiency and consequent disorders–for which appropriate supplementation was recommended by a 2016 position paper of the Academy of Nutrition and Dietetics’ [ 5 ], but, apparently, does not modify the risk of pregnancy-induced hypertension nor gestational diabetes mellitus [ 53 , 54 ].

The three meta-analyses [ 46 , 55 , 57 ] that analyzed the overall risk of cancer incidence in any form concordantly showed a reduction in risk in vegetarians compared to omnivores. These general results were inconsistent in the stratified analyses for cancer types, which as expected involved smaller numbers of events and wider confidence intervals, especially for less prevalent types of cancers.

The stratified analyses in the different reviews did not show any significant difference for bladder, melanoma, kidney, lymphoma, liver, lung, or breast cancer. Conversely the three meta-analyses that addressed colorectal cancer [ 55 , 57 , 58 ] showed a decrease in risk in two out of three with one not showing a significant difference in vegetarians versus omnivores for the generic colorectal tract.

Interestingly, one review [ 55 ] showed how analysis with even more specific granularity could reveal significant differences in particular subsets of cancers, e.g., distal, and proximal colon. Also, another recent review found significant results for pancreatic cancer [ 59 ].

Our umbrella review seems consistent with other primary evidence that links the consumption of red processed meats to an increased risk of cancers of the gastro-intestinal tract [ 63 ]. The association certainly has two faces, because while a potential risk of cancer given by increased red meat consumption can be observed, the potential protective factor given by increased fruit and vegetable consumption, shown by other previous evidence, must also be considered [ 64 ].

It has also been described that vegetarians, in addition to reduced meat intake, ate less refined grains, added fats, sweets, snacks foods, and caloric beverages than did nonvegetarians and had increased consumption of a wide variety of plant foods [ 65 ]. Such a dietary pattern seems responsible for a reduction of hyperinsulinemia, one of the possible factors for colorectal cancer risk related to diet and food intake [ 66 , 67 ]. In the same manner, some research has suggested that insulin-like growth factors and its binding proteins may relate to cancer risk [ 68 , 69 ]. This dietary pattern should not be regarded as a universal principle, as varying tendencies have been observed among vegetarians and vegans in different studies. This pattern of consumption may potentially negate the anticipated beneficial effects of their diets.

Also, some protective patterns can be attributed to the effects of bioactive compounds of plant foods, these being primary sources of fiber, carotenoids, vitamins, minerals, and other compounds that have been associated with anti-cancer properties [ 70 , 71 ]. The protective patterns are likely attributed to the mechanistic actions of the many bioactives found in plant foods such as fiber, carotenoids, vitamins, and minerals with plausible anti-cancer properties. These ranged from epigenetic mechanisms [ 72 ], to immunoregulation, antioxidant and anti-inflammatory activity [ 73 , 74 ].

Finally, increased adiposity could be another pathway by which food intake is associated with these types of cancers. Since our umbrella review has demonstrated that vegetarian diets are associated with lower BMI, this might be another concurrent factor in the decreased risk for pancreatic and colorectal cancers in vegetarians.

Inflammatory biomarkers and adiposity play pivotal roles in the genesis of prostate cancer [ 75 , 76 ], hence the same etiological pathways might be hypothesized even for the increase of this type of cancer in people adopting an omnivorous diet.

The study presents several noteworthy strengths in its methodological approach and thematic focus. It has employed a rigorous and comprehensive search strategy involving two major databases, PubMed, and Scopus, spanning over two decades of research from 1 st January 2000 to 31 st June 2023, thereby ensuring a robust and exhaustive collection of pertinent literature. By utilizing an umbrella review, the research enables the synthesis of existing systematic reviews and meta-analyses, providing a higher level of evidence and summarizing a vast quantity of information. Furthermore, its alignment with current health concerns, specifically targeting cardiovascular diseases and cancer, makes the study highly relevant to ongoing public health challenges and positions it as a valuable resource for informing preventive measures and dietary guidelines. The deployment of blinded and independent assessments by multiple raters and investigators fortifies the research by minimizing bias and reinforcing the reliability of the selection, quality assessment, and data extraction processes. Quality assessment is standardized using the revised AMSTAR-R 11-item tool, and transparency is fostered through registration on PROSPERO, thus enhancing the credibility of the study. Lastly, the study’s detailed analysis and reporting, particularly the extraction of specific health measures such as cholesterol levels, glucose levels, blood pressure, and cancer risks, contribute to the comprehensiveness of the data synthesis, thereby underlining the overall integrity and significance of the research.

Main limitations to data analysis and interpretation are intrinsic to the original studies and consist in the wide heterogeneity in terms of sample size, demographic features, and geographical origin of included subjects, dietary patterns–not only in terms of quality, but, even more important and often neglected, quantity, distribution during the day, processing, cooking methods–and adherence, and other lifestyle confounders. In this regard, it is worth to mention that the impact of diet per se on the development of complex disorders (i.e. CVDs and cancer) and related mortality is extremely difficult to assess [ 71 ], especially in large populations, characterized by a highly heterogeneous lifestyle. It should also be considered the heterogeneity in dietary and lifestyle habits among countries, according to which the adoption of A/AFPDs could modify significantly habits in some countries, but not in others, and consequently have an extremely different impact on the risk of developing cardiometabolic disorders and cancer [ 25 ]. Furthermore, due to the nature of umbrella reviews, the present work may not include novel associations which were excluded from the analyzed reviews, as the main aim was to summarize secondary studies, such as reviews and meta-analyses. Finally, studies assessing the benefit of A/AFPDs on cancer risk are also limited by the heterogeneity in the timing of oncological evaluation and, therefore, disease progression, as well as in the histological subtypes and previous/concomitant treatments [ 72 – 75 ].

In conclusion, this umbrella review offers valuable insights on the estimated reduction of risk factors for cardiometabolic diseases and cancer, and the CVDs-associated mortality, offered by the adoption of plant-based diets through pleiotropic mechanisms. Through the improvement of glycolipid profile, reduction of body weight/BMI, blood pressure, and systemic inflammation, A/AFPDs significantly reduce the risk of ischemic heart disease, gastrointestinal and prostate cancer, as well as related mortality.

However, data should be taken with caution because of the important methodological limitation associated with the original studies. Moreover, potential risks associated with insufficient intake of vitamin and other elements due to unbalanced and/or extremely restricted dietary regimens, together with specific patient needs should be considered, while promoting research on new and more specific markers (i.e. biochemical, genetic, epigenetic markers; microbiota profile) recently associated with cardiometabolic and cancer risk, before suggesting A/AFPDs on large scale.

Supporting information

S1 table. r-amstar..

https://doi.org/10.1371/journal.pone.0300711.s001

S2 Table. PRISMA 2020 checklist.

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The worldwide phase out of animal agriculture, combined with a global switch to a plant-based diet, would effectively halt the increase of atmospheric greenhouse gases for 30 years and give humanity more time to end its reliance on fossil fuels, according to a new study by scientists from Stanford University and the University of California, Berkeley.

A new model suggests that phasing out animal agriculture over the next 15 years would have the same effect as a 68 percent reduction of carbon dioxide emissions through the year 2100. (Image credit: Getty Images)

“We wanted to answer a very simple question: What would be the impact of a global phase-out of animal agriculture on atmospheric greenhouse gases and their global-heating impact?” said Patrick Brown , a professor emeritus in the department of biochemistry at Stanford University. Brown co-authored the paper with Michael Eisen, a professor of genetics and development at UC Berkeley.

Based on the model, published in the open-access journal PLoS Climate , phasing out animal agriculture over the next 15 years would have the same effect as a 68 percent reduction of carbon dioxide emissions through the year 2100. This would provide 52 percent of the net emission reductions necessary to limit global warming to 2 degrees Celsius above preindustrial levels, which scientists say is the minimum threshold required to avert disastrous climate change.

The changes would stem, the authors say, from the spontaneous decay of the potent greenhouse gases methane and nitrous oxide, and the recovery of biomass in natural ecosystems on the more than 80 percent of humanity’s land footprint currently devoted to livestock.

“Reducing or eliminating animal agriculture should be at the top of the list of potential climate solutions,” Brown said. “I’m hoping that others, including entrepreneurs, scientists and global policymakers, will recognize that this is our best and most immediate chance to reverse the trajectory of climate change, and seize the opportunity.”

Brown is also the founder and CEO of Impossible Foods, a company developing alternatives to animals in food production. Eisen is an advisor to the company. Both Brown and Eisen stand to benefit financially from the reduction of animal agriculture.

Unlocking negative emissions

Brown and Eisen are not the first to point out that ongoing emissions from animal agriculture are contributing to global warming. But what has not been recognized before, they say, is the much more impactful “climate opportunity cost” – the potential to unlock negative emissions by eliminating livestock.

“As the methane and nitrous oxide emissions from livestock diminish, atmospheric levels of those potent greenhouse gases will actually drop dramatically within decades,” Brown said. “And the CO 2 that was released into the atmosphere when forests and wild prairies were replaced by feed crops and grazing lands can be converted back into biomass as livestock are phased out and the forests and prairies recover.”

Brown and Eisen used publicly available data on livestock production, livestock-linked emissions and biomass recovery potential on land currently used to support livestock to predict how the phaseout of all or parts of global animal agriculture production would alter net anthropogenic, or human-caused, emissions from 2019 levels. They then used a simple climate model to project how these changes would impact the evolution of atmospheric greenhouse gas levels and warming for the rest of the century.

They examined four dietary scenarios: an immediate replacement of all animal agriculture with a plant-only diet; a more gradual and, the authors say, more realistic, 15-year transition to a global plant-only diet; and versions of each where only beef was replaced with plant-only products.

For each hypothetical scenario, the scientists assumed that non-agricultural emissions would remain constant and that the land formerly used for livestock production would be converted to grasslands, prairies, forests and the like that will absorb atmospheric carbon dioxide.

“The combined effect is both astoundingly large, and – equally important – fast, with much of the benefit realized by 2050,” Brown said. “If animal agriculture were phased out over 15 years and all other greenhouse-gas emissions were to continue unabated, the phase-out would create a 30-year pause in net greenhouse gas emissions and offset almost 70 percent of the heating effect of those emissions through the end of the century.”

While the complete phase out of animal-based agriculture was projected to have the largest impact, 90 percent of the emission reductions could be achieved by only replacing ruminants such as cattle and sheep, according to the model.

While their paper does not explore the particulars of what a global phaseout of animal agriculture would entail, the authors acknowledge that “the economic and social impacts of a global transition to a plant-based diet would be acute in many regions and locales” and that “it is likely that substantial global investment will be required to ensure that people who currently making a living from animal agriculture do not suffer when it is reduced or replaced.”

But, they write, “in both cases, these investments must be compared to the economic and humanitarian disruptions of significant global warming.”

Changing attitudes

Many will scoff at the idea that billions of people can be convinced to switch to a plant-only diet within 15 years. To these skeptics, Eisen points out that other revolutions have happened in less time. “We went from having no cellphones to cellphones being ubiquitous in less time than that. Electricity, cars, solar panels – all became common in a relatively short period of time,” Eisen said.

Moreover, Brown added, societal attitudes toward food are far from fixed. “Five hundred years ago, nobody in Italy had ever seen a tomato. Sixty years ago, nobody in China had ever drunk a Coke. Mutton was once the most popular meat in America,” he said. “People around the world readily adopt new foods, especially if they are delicious, nutritious, convenient and affordable.”

The scientists have made all of the raw data they used, as well as their calculations and the computer code used to carry out the calculations, publicly available so that others can make up their own mind.

“The great thing about science is that, in the end, it all comes down to whether the conclusions are supported by the evidence,” Brown said. “And in this case, they are.”

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Can diet help with advanced breast cancer? All indications are positive

Women with breast cancer who exclusively ate a whole-foods, plant-based diet lost weight, improved cholesterol levels and other key metabolic factors, had less fatigue, and perceived that they felt sharper mentally and generally more well.

The outcomes are from a small study by researchers at the University of Rochester Medical Center and Wilmot Cancer Institute. Study participants were individuals with stage 4 breast cancer, who will be on lifelong treatment.

These patients are typically excluded from dietary studies, but with their survivorship numbers growing, it presented an opportunity to make an impact both short- and long-term, said research leader Thomas M. Campbell, MD, an assistant professor of Family Medicine at URMC and an expert on using plant-based diets to improve health.

What Did the Clinical Trial Require?

The study included 30 patients who were on stable treatment and could tolerate food.

Researchers randomly divided participants into two groups: One received standard care, and the intervention group ate meals provided by the research team for eight weeks. The diet consisted solely of fruits, vegetables, whole grains (including whole grain pasta), legumes (beans), potatoes, and nuts and seeds. Participants agreed to avoid animal-based foods (meat, eggs, and dairy), and all oils and added solid fats. They also took a daily multivitamin.

Weekly assessments occurred, and the study reported 95 percent compliance.

"It's exciting to see that these major dietary changes were feasible, well-tolerated, and acceptable to the clinical trial participants," Campbell said.

No calorie restriction was involved. Individuals were encouraged to eat as often as they wanted of food that was "on plan."

How A Clean, Plant-Based Diet Makes a Difference

The women started with an average BMI of 29.7, which is borderline obese. The patients in the whole-foods plant-based group lost one-two pounds per week for eight weeks, without mandated exercise.

This is significant because individuals with breast cancer often gain weight during treatment, which is risky. Why? Too much body weight increases insulin levels and hormones (estrogen and testosterone) in the blood, which can fuel cancer.

Another encouraging study result: researchers saw a reduction in blood samples of IGF-1, a growth factor that has been associated with many common cancers, as well as less inflammation.

"Although we cannot say anything yet about whether the diet can stop cancer progression from this small study, we saw preliminary results that suggest favorable changes within the body, which is very positive," Campbell said.

To better understand the implications for cancer growth, the team is collaborating with Isaac Harris, PhD, at Wilmot, in a bench-to-clinic investigation recently funded by the American Cancer Society.

Scientists know that cancer cells rely on amino acids to survive, and the patients who followed the plant-based diet had changes in their blood levels of amino acids. Harris is studying the effect of amino acid composition on cancer cell survival, and the effect of the amino acids on various cancer drugs.

The journal Breast Cancer Research and Treatment published the primary study, which is believed to be the first of its kind. The breast cancer trial had enough significant results that two additional papers were also published from the dietary intervention: a second study in the same journal, and a third study in Frontiers in Nutrition, all in March 2024.

How to Start Making Healthy Changes

Patients should first consult with their oncologists or healthcare providers before making major dietary changes, Campbell said. This is especially important for people who take blood thinners or insulin medications.

Examples of food provided in the breast cancer clinical trial included peanut soba noodles, steel cut oatmeal, banana flax muffins, sweet potato enchiladas, and Mediterranean white bean soup.

To get started with plant-based recipes and meal ideas that are simple and affordable, Campbell recommends these websites: plantyou.com, shaneandsimple.com, and monkeyandmekitchenadventures.com.

Several factors influence a person's motivation to eat healthier, Campbell added, including family support, taste preferences, and cooking ability.

Whether a person makes dramatic changes overnight, or simply decides to swap out an occasional meal in favor of a plant-based recipe can be a good choice: "You only need five-10 plant-based recipes that are easy, tasty, and convenient enough that you will make them regularly to have a substantial overhaul in your diet," he said.

Higher food costs are often cited as a reason to shirk a plant-based diet, but in 2023 co-author Erin Campbell, MD, published a separate study showing that the diets leading to the biggest health improvements -- including Dietary Approaches to Stop Hypertension or the DASH diet, which is also plant-based -- were the same or cheaper in terms of food costs compared to standard American diets with ultra-processed foods and restaurant take-out.

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Materials provided by University of Rochester Medical Center . Original written by Leslie Orr. Note: Content may be edited for style and length.

Journal References :

• Thomas M. Campbell, Erin K. Campbell, Eva Culakova, Lisa M. Blanchard, Nellie Wixom, Joseph J. Guido, James Fetten, Alissa Huston, Michelle Shayne, Michelle C. Janelsins, Karen M. Mustian, Richard G. Moore, Luke J. Peppone. A whole-food, plant-based randomized controlled trial in metastatic breast cancer: weight, cardiometabolic, and hormonal outcomes . Breast Cancer Research and Treatment , 2024; 205 (2): 257 DOI: 10.1007/s10549-024-07266-1
• Erin K. Campbell, Thomas M. Campbell, Eva Culakova, Lisa Blanchard, Nellie Wixom, Joseph J. Guido, James Fetten, Alissa Huston, Michelle Shayne, Michelle C. Janelsins, Karen M. Mustian, Richard G. Moore, Luke J. Peppone. A whole food, plant-based randomized controlled trial in metastatic breast cancer: feasibility, nutrient, and patient-reported outcomes . Breast Cancer Research and Treatment , 2024; DOI: 10.1007/s10549-024-07284-z
• Jean Lee, Erin K. Campbell, Eva Culakova, Lisa M. Blanchard, Nellie Wixom, Luke J. Peppone, Thomas M. Campbell. Changes in the consumption of isoflavones, omega-6, and omega-3 fatty acids in women with metastatic breast cancer adopting a whole-food, plant-based diet: post-hoc analysis of nutrient intake data from an 8-week randomized controlled trial . Frontiers in Nutrition , 2024; 11 DOI: 10.3389/fnut.2024.1338392

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The Impact of Plant-Based Dietary Patterns on Cancer-Related Outcomes: A Rapid Review and Meta-Analysis

Esther molina-montes.

1 Department of Nutrition and Food Science, University of Granada, 18014 Granada, Spain; se.rgu@vneleb

2 Institute of Nutrition and Food Technology (INYTA) ‘José Mataix’, Biomedical Research Centre, University of Granada, Avenida del Conocimiento s/n, E-18071 Granada, Spain

3 Instituto de Investigación Biosanitaria ibs.GRANADA, 18012 Granada, Spain; [email protected]

Elena Salamanca-Fernández

4 CIBER of Epidemiology and Public Health (CIBERESP), 28029 Madrid, Spain

5 Andalusian School of Public Health (EASP), 18014 Granada, Spain

Belén Garcia-Villanova

Maria josé sánchez.

6 Department of Preventive Medicine and Public Health, University of Granada, 18016 Granada, Spain

Long-term cancer survivors represent a sizeable portion of the population. Plant-based foods may enhance the prevention of cancer-related outcomes in these patients. We aimed to synthesize the current evidence regarding the impact of plant-based dietary patterns (PBDPs) on cancer-related outcomes in the general population and in cancer survivors. Considered outcomes included overall cancer mortality, cancer-specific mortality, and cancer recurrence. A rapid review was conducted, whereby 2234 original articles related to the topic were identified via Pubmed/Medline. We selected 26 articles, which were classified into studies on PBDPs and cancer outcomes at pre-diagnosis: vegan/vegetarian diet ( N = 5), provegetarian diet ( N = 2), Mediterranean diet ( N = 13), and studies considering the same at post-diagnosis ( N = 6). Pooled estimates of the associations between the aforementioned PBDPs and the different cancer outcomes were obtained by applying random effects meta-analysis. The few studies available on the vegetarian diet failed to support its prevention potential against overall cancer mortality when compared with a non-vegetarian diet (e.g., pooled hazard ratio (HR) = 0.97; 95% confidence interval (CI): 0.88–1.06). The insufficient number of studies evaluating provegetarian index scores in relation to cancer mortality did not permit a comprehensive assessment of this association. The association between adherence to the Mediterranean diet and cancer mortality reached statistical significance (e.g., pooled HR = 0.84; 95% CI: 0.79–0.89). However, no study considered the influence of prognostic factors on the associations. In contrast, post-diagnostic studies accounted for prognostic factors when assessing the chemoprevention potential of PBDPs, but also were inconclusive due to the limited number of studies on well-defined plant-based diets. Thus, whether plant-based diets before or after a cancer diagnosis prevent negative cancer-related outcomes needs to be researched further, in order to define dietary guidelines for cancer survivors.

1. Introduction

The latest global cancer statistics have reported that there were 17.0 million new cases of cancer and 9.5 million cancer-related deaths (excluding non-melanoma skin cancer) in 2018 [ 1 ]. It is estimated that 43.8 million cancer patients who survived the disease for five years or more were alive in 2018, with lung, breast, colorectal, prostate, and gastric cancer contributing more prominently to this number [ 2 , 3 ]. Data from Europe (EU-28) are also impressive, with more than 12.1 million prevalent cancer patients in 2018 [ 3 ]. Parallel to the increase of the number of new cancer patients over the past decades, the proportion of cancer survivors has also grown. Survival rates for cancer have increased steadily for patients diagnosed with cancer from 1999 to 2007 [ 4 ]. This increased survival may be partially attributed to improved diagnosis and treatments, better access to high-quality services and earlier diagnosis [ 5 ]. However, several factors can lead to worse cancer prognosis, among which unhealthy dietary choices warrant special consideration [ 6 ].

Plant-based food (fruits, vegetables, cereals, nuts and seeds, legumes, and vegetable oils) are the main source of fiber and other bioactive compounds in the diet [ 7 ]. Particularly, plant bioactives including fiber, sulfur compounds, carotenoids, and polyphenols, present in foods such as cruciferous and allium vegetables, tomatoes, green tea, and whole grain cereal, have well-known anticarcinogenic properties [ 7 ]. Plant-based foods are therefore likely to exert anti-inflammatory and anti-oxidative effects against the development of cancer [ 7 ]. Thus, whereas an unhealthy diet is an established risk factor for several cancer types, eating plant-based foods to achieve a healthful diet has been associated with a reduced cancer risk according to the latest report on diet and cancer, released by the World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) [ 8 ].

A considerable number of studies examined the association between individual nutrients or foods and cancer mortality [ 9 , 10 ]. Most of these studies have focused on associations between pre-diagnosis dietary intake and cancer-related outcomes, while only a few studies have assessed how post-diagnosis dietary intake affects these outcomes [ 11 ]. Moreover, nearly all have focused on overall cancer mortality, whereas studies on cancer-specific mortality are scarce [ 11 , 12 , 13 , 14 ]. In addition to the above, findings from these studies are far from conclusive. For example, inconsistent associations were reported between fruit and vegetable consumption and cancer mortality in a meta-analysis of sixteen prospective studies [ 15 ]. For other plant-based foods, including nuts [ 16 ] and legumes [ 9 ], strong associations with cancer mortality have been observed. In contrast, olive oil consumption was not found to be associated with cancer mortality in several studies [ 17 , 18 ]. Therefore, until now, there is no sufficient evidence to recommend dietary factors for cancer mortality prevention.

Considering, the overall diet (i.e., the dietary pattern) is more appropriate in view of the complex nature of the diet, and the interrelation between its constituents (foods and nutrients) [ 19 ]. Dietary patterns can be assessed using the conceptual framework of a dietary pattern (a priori), or empirically (a posteriori), and allow to establish more robust associations than when considering nutrients or foods on an individual basis [ 20 ]. It is also of great importance that dietary patterns, especially a priori dietary patterns, are more suitable to derive dietary guidelines for the prevention of cancer [ 8 , 21 ]. However, not much attention has been paid to the influence of diet as a whole on cancer mortality outcomes. So far, the WCRF/AICR cancer prevention guidelines establish the same recommendations for cancer survivors as for the general population. Adherence to these guidelines by means of the WCRF/AICR 2007 score has been associated with a lower cancer mortality risk in a meta-analysis combining only three studies on this association [ 14 ].

Thus, whether specific dietary guidelines reduce cancer mortality risk in the population (before the diagnosis of cancer) or in cancer survivors (from post-diagnosis) remains unclear. Dietary guidelines accounting for the holistic effects of plant-based foods within a healthy diet for preventing cancer mortality are crucial in this regard. Their effects have been thoroughly examined for cancer incidence [ 13 , 22 ], but not with respect to cancer mortality. Hence, the aim of this review was to synthesize the current evidence regarding cancer mortality. For the purpose of this review, plant-based dietary patterns (PBDPs) were deemed to be the vegetarian diet (VD) in all its variations and the Mediterranean diet (MD). For the latter, we focused on a priori-derived MD indexes (e.g., the MD score, the alternate MD index (aMED), and others, in their various versions), whereas empirically derived a posteriori patterns were discarded. Furthermore, we distinguished between pre-diagnostic and post-diagnostic effects of PBDPs on cancer outcomes including overall cancer mortality, cancer-specific mortality, and cancer recurrence.

2. Materials and Methods

A rapid review methodology was chosen to conduct the review in a timely manner, following a pre-established protocol for systematic reviews (with the exception of several steps that were omitted to speed up the process, such as methodological quality assessment), and according to the general methodology of the Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) guidelines [ 23 , 24 , 25 ]. Whenever possible, results extracted from the selected studies were pooled in a meta-analysis. The steps are described in more detail below.

2.1. Search Strategy

Leading electronic databases (Medline via PubMed) were searched from 1999 (no studies were available before) until 20 April 2020. The search strategy used (with a combination of Mesh terms, keywords, and operators) is detailed in Appendix A . The flow diagram illustrating the process of identifying and selecting studies is shown in Appendix B .

2.2. Study Eligibility Criteria

The review included all original studies in humans that provided data on cancer mortality or survival in relation to PBDPs. These studies were intervention studies, cohort studies, and case-control studies. Systematic reviews or meta-analyses were not eligible, but they were included in the search to further retrieve original studies on the topic by manual search. The most updated reviews were selected as references to encompass all the available studies [ 11 , 26 , 27 , 28 ]. Studies that were published thereafter and those studies not included in these previous reviews were included too.

With reference to the outcomes, we considered studies the primary outcome of which was overall and/or cancer-specific mortality, and studies reporting results on other related outcomes such as recurrence and progression of the disease. Cancer survivors did not include survivors of cervical lesions or adenomas in the colon since these are considered benign or non-malignant lesions according to common histological classification of tumors. In addition, eligible studies were those evaluating dietary patterns by questionnaire assessment.

PBDPs were defined as: (i) a diet excluding meat and meat products, flesh from any animal, and seafood, or all foods of animal origin, in the strictest sense (vegetarian and vegan diet, respectively); and (ii) a diet featuring a higher consumption of fruits, vegetables, legumes, and nuts over foods of animal origin. We considered the vegetarian diet and Mediterranean diet (MD) as plant-based dietary models. For the former, vegetarian population studies or studies that ascertained compliance with the vegetarian diet standards were eligible. To establish the latter, only a priori-derived MD indexes were included due to their high translational capacity into dietary prevention guidelines. In the light of the different adaptations that have been applied to the first MD score (MDS) proposed by Trichopoulou et al. [ 29 , 30 ], no restrictions were applied to these indexes.

All possible comparisons between vegetarians, vegans (or both combined) with respect to omnivores or different combinations of vegetarianism (e.g., lacto-ovo vegetarians) were considered. We also considered studies that established comparisons by levels of adherence to PBDPs. Furthermore, studies assessing the association between PBDPs and cancer mortality both before and after cancer diagnosis were included.

Studies conducted in children and adolescents (aged <18 years) were excluded, as were those not reporting any risk estimate (odd ratio (OR), relative risk (RR), or hazard ratio (HR) and the corresponding 95% confidence interval (CI)) on the association between dietary patterns and the aforementioned cancer-related outcomes. Studies written in languages other than English or Spanish were also excluded.

2.3. Data Collection and Analysis

Selection of studies.

Studies were first screened by title and abstract by two reviewers (E.M.M. and E.S.F.) and the final study selection was performed based on a full text review. When there were several articles reporting results based on the same study population, we included the study reporting the most updated data. Any discrepancies were resolved by consulting a third reviewer (M.J.S.).

2.4. Data Extraction and Management

Data extraction was performed by three reviewers (E.M.M., E.S.F., and B.G.V.) using a predefined standardized form to collect information on: (a) study characteristics: authors and year(s), study description and design, country, study population characteristics with regard to sample size, gender, and age; (b) the assessment methods used to collect information on diet (questionnaires and tools) and cancer-related outcomes (cancer registry or medical records data), also distinguishing between pre- and post-diagnosis association studies; (c) the measured outcomes: overall cancer mortality, cancer-specific mortality, and cancer recurrence; (d) the PBDPs under consideration (vegetarian, Mediterranean, and their variations), type (population-based or a priori), and the comparison groups (e.g., high vs. low adherence or vegetarians/vegans vs. omnivores); and (e) the reported results: measures of effect size (OR, HR, and RR, with 95% CI) and confounding variables considered for adjustment.

2.5. Presentation of Results

The results of all studies were presented in tabular format and summarized narratively by type of dietary pattern and cancer-related outcome. To summarize these studies, we described their results and risk estimates adjusted for all potential confounders. When possible, these results were pooled in meta-analyses.

2.6. Meta-Analysis

We calculated the summary estimates (from log HR/RRs in cohort studies, for dichotomous outcomes and comparisons between exposure groups) and corresponding 95% CIs when at least two studies reported results on the same outcome, using random effects models to account for possible heterogeneity between studies [ 31 ].

The Cochran Q test and I2 test statistic (assuming heterogeneity if I2 > 50%) were used to assess heterogeneity between studies. Publication bias was assessed by Egger’s test and visual inspection of the funnel plots [ 32 , 33 ]. A p-value of < 0.05 was deemed as statistically significant. The “metafor” package in R version 3.6.3 (R software) was used for these analyses.

A total of 2234 publications was identified. In addition, we considered the most up-to-date reviews addressing the association between dietary patterns and cancer mortality [ 11 , 12 , 13 , 14 ]. These reviews added 25 studies, not previously retrieved. Of these, 930 were excluded in the first step to retain original articles published from 1999. Then, 1256 studies were excluded on the basis of title and abstract, and 73 publications were reviewed using full text. Forty-seven publications were excluded for not fulfilling the inclusion criteria (no cancer related outcomes = 19; not about dietary patterns but single nutrient/food studies = 14; not an original study = 1; a posteriori indexes = 5; a priori but not MD = 8). Thus, 26 studies were included in this review: 5 VD-like studies [ 34 , 35 , 36 , 37 , 38 ], 12 MD-like studies [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ], 2 provegetarian diet studies [ 52 , 53 ], and 6 post-diagnosis studies [ 54 , 55 , 56 , 57 , 58 , 59 ]. The study by Key et al. was not included because no risk ratios on cancer mortality were reported [ 60 ]. Results are presented in the following sections.

3.1. Vegetarian and Vegan Diet

With reference to cancer mortality, characteristics of the studies evaluating VDs (vs. nonvegetarians) and cancer mortality are shown in Table 1 . These studies were based on large cohorts of known vegetarians and vegans also including nonvegetarians/nonvegans, who were followed-up for different outcomes including cancer mortality. Some of these cohorts were the Adventist Mortality Study, the Adventist Health Studies, the Oxford and Heidelberg Vegetarian Studies, the European Prospective Investigation into Cancer and Nutrition (EPIC) Oxford Vegetarian Study, and the Health Food Shoppers Study, which were combined in some of the selected publications [ 35 , 38 , 55 ]. Some studies based the VD assessment on questions regarding whether the participants followed a VD or not (e.g., Health Food Shoppers study); on the reported consumption of meat or fish using close-ended questions (e.g., Heidelberg study); or on dietary questionnaires aiming to identify non-consumers of meat or fish (e.g., the Adventist Health Study). The studies by Orlich et al. and Key et al. [ 37 , 61 ] were based on two different arms of the Adventist Health Study. Both were included to account for overall and cancer-specific (colorectal) mortality. All studies assessed the association between vegetarian eating habits and cancer mortality before cancer diagnosis (all excluded prevalent cancer cases at recruitment), and all assessed cancer mortality and cause-specific mortality using national mortality registry data (death certificates).

Characteristics of the studies evaluating VD (vs. nonvegetarian diet) and cancer mortality including cause-specific cancer mortality.

RR = relative ratio; CI = confidence interval; F = females; M = males; FFQ = food frequency questionnaire; 24-HR = 24 h recall; CRC = colorectal cancer; NA = not available; n = number of events; N = total population (or vegetarians). Adjustments: 1: sex; 2: sge; 3: smoking; 4: physical activity; 5: alcohol intake; 6: education; 7: body mass index (BMI); 8: race; 9: income; 10: marital status; 11: region; 12: sleep; 13: hormone replacement therapy (HRT) use; 14: history of peptic ulcer and inflammatory bowel disease (IBD); 15: family history of colorectal cancer; 16: energy; 17: treatment for diabetes; 18: aspirin/statins; 19: supplemental Ca/vitamin D; 20: fiber; 21: colonoscopy; 22: supplements; 23: study/method of recruitment; 24: parity; 25: oral contraceptive use; 26: HRT use; 27: diabetes; 28: blood pressure.

The study by Key et al. [ 34 ] compared death rates between vegetarians and nonvegetarians and showed significant heterogeneity between the five included cohorts. This heterogeneity was probably driven by the fact that the studied populations differed with regard to socio-demographic factors and definitions of the vegetarian population groups. Risk estimates were adjusted for age, sex, and smoking, despite the fact that smoking was not collected in some of the included cohorts. Unreliable associations were reported by duration of vegetarianism due to the limited sample size. In addition, participants were asked whether they considered themselves vegetarians, but no further dietary information was collected. Two of the cohorts (the Oxford Vegetarian and Health Food Shoppers) were included in the article by Appleby et al. [ 35 ]. Later on, Appleby et al. [ 38 ] compared vegetarian vs. nonvegetarian dietary habits in relation to cancer mortality (overall and by cancer type) in some of these previous cohorts (the Oxford Vegetarian and EPIC Oxford Study from the UK). This study also combined vegans and vegetarians in the analyses but considered as a reference group, regular meat eaters (intake >5/week). Importantly, low meat eaters (intake <5/week) and fish eaters (i.e., pesco-vegetarians) were considered in separate categories. The latter study also adjusted for every possible confounding variable related to medical history and lifestyle habits. The studies by Orlich [ 37 , 61 ] based the assessment of vegetarian diet on the dietary information collected with a validated food frequency questionnaire (FFQ) among 73,308 participants of the Adventist Health Cohort. Using this dietary data, the authors were able to classify participants into vegans, lacto-ovo vegetarians, pesco-vegetarians, semi-vegetarians, and nonvegetarians, according to the frequency of intake of particular animal foods (eggs/dairy <1/month, fish >1/month, meats <1/week, and meats >1/week, respectively). This study was, indeed, one of the few reporting risks associated with cancer mortality comparing different types of vegetarians to nonvegetarians, as well as all kinds of vegetarians vs. nonvegetarians. No significant associations were reported with respect to cancer mortality in any of these subgroups. Finally, the study by Chang-Claude et al. [ 36 ], included 1225 vegetarians and 679 health-conscious nonvegetarians from Germany, who provided dietary information on usual frequency of consumption of plant-based foods, milk, eggs, fish, meat, and processed meat. Mortality ratios were compared for vegans and vegetarians, while RRs associated with cancer mortality and adjusted for relevant confounders were reported for vegetarians and vegans combined vs. nonvegetarians. None of these studies considered the influence of cancer treatment or other prognostic factors on the associations.

Pooled estimates of effect sizes and 95% CIs for cancer mortality comparing vegetarians vs. non-vegetarians are shown in Figure 1 and described below. Whenever possible, we removed overlapping cohorts between the studies (e.g., those included in Key et al. (1999) and Appleby (2002) [ 34 , 35 ]). There was no evidence for publication bias according to funnel plots and Egger tests.

Meta-analysis under random-effects model (REM) with regard to vegetarian diet (VD) (vs. nonvegetarian diet) by subgroups of overall cancer mortality and cause-specific mortality. Q and I 2 statistics are indicated for each subgroup analysis together with the pooled estimate (rhombus).

3.1.1. Overall Cancer Mortality

Overall cancer mortality was evaluated in four articles [ 35 , 36 , 37 , 38 ], with one article reporting estimates for two different cohorts [ 35 ]. The articles by Appleby [ 35 , 38 ] combined the study populations of the Oxford Vegetarian Study, the Health Food Shoppers Study, and the EPIC Oxford Vegetarian Study. While there seemed to be population overlap in the Oxford Vegetarian Study between the articles [ 35 , 38 ], it was not possible to retrieve separate risk estimates by study populations. The overall analysis was based on 139,174 participants and 1661 cancer mortality events. The mean age of study participants varied greatly between the studies (from 43 to 58 years), as well as the level of confounding adjustment: from minimal by age, sex, and smoking adjustment [ 35 ] to full adjustments [ 36 , 37 , 38 ]. Pooled estimates showed that VD (vs. nonvegetarian) was not significantly associated with overall cancer mortality (RR = 0.97; 95% CI: 0.88–1.06). There was no significant heterogeneity between the studies (I 2 = 30%, p = 0.24).

3.1.2. Breast Cancer Mortality

There were three articles on the association between VD and breast cancer mortality [ 34 , 35 , 38 ], which together analyzed the association within six cohorts. Potential population overlap between the Oxford Vegetarian Study cohorts could not be resolved. After pooling risk estimates, based on 228 breast cancer events among 83,985 participants, a non-significant association was observed between VD (vs. nonvegetarian) and breast cancer mortality (RR = 0.99; 95% CI: 0.67–1.44). Heterogeneity tended to be significant and became less apparent after exclusion of the Heidelberg Study included in Key et al. [ 34 ]. The association remained, however, nonsignificant (data not shown).

3.1.3. Colorectal Cancer Mortality

Three studies including six cohorts reporting results for 279 colorectal cancer events among 83,985 participants, evaluated the association between VD and colorectal cancer mortality [ 34 , 35 , 38 , 61 ]. Vegetarian diet (vs. nonvegetarian) compliance was not significantly associated with colorectal cancer mortality after pooling risk estimates of these studies (RR = 1.03; 95% CI: 0.84–1.26). There was no heterogeneity among the studies (I 2 = 0%, p = 0.54).

3.1.4. Cancer Mortality for Other Cancer Types

Prostate and lung cancer mortality was evaluated in two [ 34 , 35 ] and three [ 34 , 35 , 38 ] articles, respectively. Both showed nonsignificant associations in pooled analyses. Gastric cancer mortality was also evaluated in two of the cohorts included in the study by Key et al. [ 34 ], but no consistent associations were reported. For other cause-specific cancer mortality, only pancreas, ovary, and lymphatic/hematopoietic cancers were evaluated in relation to VD in the study by Appleby et al. [ 38 ]. Interestingly, there was a significant inverse association of VD (vs. nonvegetarian) with pancreas cancer mortality (RR = 0.44; 95% CI: 0.26–0.76) and lymphatic/hematopoietic cancer mortality (RR = 0.47; 95% CI: 0.30–0.73), but a nonsignificant association with ovarian cancer mortality. Low meat eaters vs. regular meat eaters were also found to have lower pancreatic cancer mortality risk (RR = 0.54; 95% CI: 0.35–0.85) in this study.

3.2. Provegetarian Diets

Our literature search retrieved two studies on the association between provegetarian diets and cancer mortality [ 52 , 53 ]. Characteristics of these studies are shown in Table 2 .

Characteristics of the studies evaluating provegetarian diets and cancer mortality including cause-specific cancer mortality.

F = females; M = males; FFQ = food frequency questionnaire; 24-HR = 24 h recall; CRC = colorectal cancer; NA = not available; n = number of events; N = total population. Adjustments: 1: sex; 2: age; 3: smoking; 4: physical activity; 5: alcohol intake; 6: education; 7: BMI; 8: race; 9: energy intake; 10: use of aspirin and multivitamins; 11: family history of medical conditions; 12: menopausal status; 13: HRT use; 14: weight change; 15: medical history; 16: use of medication for hypertension and hypercholesterolemia.

In the first study [ 52 ], conducted with 7216 elderly participants (mean age = 67 years) of the Spanish PREDIMED (Prevención con Dieta Mediterránea) study, the authors assessed five levels of adherence to a provegetarian diet score by considering fruits, vegetables, nuts, cereals, legumes, olive oil, and potatoes as positive components, and animal fats, eggs, fish, dairy products, and meats or meat products as negative components. As the authors showed, this score was not correlated with the traditional MD score, supporting that both scores capture different dietary dimensions of a plant-based diet. This study used dietary data obtained through a validated 137-item FFQ, which was administered yearly during the follow-up. The outcome (death) was assessed through the review of clinical records and linkage to the mortality registries. During 4.8 years of follow-up, there were 130 cancer deaths documented, evenly distributed across the levels of adherence to the score. As a consequence, a nonsignificant association with cancer mortality was observed (HR high vs. very low score = 0.66; 95% CI: 0.35–1.24). The estimated HRs were adjusted for age, gender, smoking, physical activity, educational level, energy intake, and alcohol consumption. Additional adjustment for anthropometric measures and medical history, or accounting for variations in dietary habits over time, did not change the interpretation of the results.

Three different plant-based dietary indexes were evaluated in relation to cancer mortality in the study by Baden et al. [ 53 ]. The purpose of this study was to examine whether changes in adherence to these scores in 12 years were related to cancer mortality. A total of 75,314 men and women (mean age = 63 years) from the Nurses’ Health Study (NHS) and the Health Professionals Follow-up Study (HPFS) were followed-up for cancer deaths ( N = 4263). Dietary data were collected every four years with a validated semi-quantitative FFQ. The authors proposed an overall (standard) provegetarian plant-based score, as well as healthier and unhealthier versions of this score. These scores were based on the premise that plant-based foods have different diet quality, with foods such as fruit juices, refined grains, potatoes, sugar-sweetened beverages, and sweets and desserts presumed to be less healthy; whereas whole grains, fruits, vegetables, nuts, legumes, vegetable oils, tea, and coffee are considered the opposite. All plant-based foods were scored positively, while all other foods scored negatively. The change in adherence to the scores (at 8, 12, and 16 years) was evaluated in relation to risk of cancer death. Cancer deaths in the cohorts were identified through mortality registries and/or reported by the participant’s relatives. The authors found that an increase in adherence to the provegetarian standard score (vs. stable adherence in 12 years) was associated with a lower risk of cancer death. Concretely, 10 points increase in the adherence was associated with a 7% (95% CI: 2–11%) decrease of cancer mortality risk. No consistent associations were observed between the other provegetarian diet scores (healthy and unhealthy score) and overall cancer mortality in this study, which was an unexpected finding since decreases in consumption of healthy plant foods were associated with a higher risk of all-cause mortality. Neither were there statistically significant associations when assessing the association between 12-year change in adherence to these scores with cause-specific cancer mortality (lung, breast, and colon), except for prostate cancer (HR per 10 points increase in adherence = 0.73; 95% CI: 0.55–0.96). Adjustment variables included many potential confounders related with medical conditions, as well as body mass index (BMI).

It was not possible to meta-analyze the results of both studies due to differences in the assessment of the adherence to the plant-based score, either at baseline [ 52 ] or over time [ 53 ]. The comparison groups differed in both studies too, with regard to high vs. low adherence to the provegetarian diet score [ 52 ], and change in adherence to the score vs. stable adherence over time [ 53 ]. None of the studies accounted for cancer treatment or other prognostic factors.

3.3. Mediterranean Diet

Our literature search retrieved 13 studies on the association between the MD (either the original MD scores (MDS), or its derivatives including the alternate MD (aMED), the modified MD (mMSD), the MD pattern (MDP), or the relative Mediterranean diet (rMED) scores) and cancer mortality and other related-outcomes [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. Thus, the association between adherence to the MD and cancer mortality has been examined through five different a priori derived MD indexes. All studies were cohort studies, considering validated dietary assessment tools to evaluate the adherence to the MD, and all based the outcome assessment on reliable information sources (e.g., mortality and cancer registries). All studies controlled for relevant confounders (age, sex, region, energy intake, physical activity, education, and BMI), and several also considered comorbid conditions at baseline for adjustment [ 40 , 50 ]. Interestingly, none of the studies on the association between MD diet and overall cancer mortality risk controlled for prognostic-related factors in the analyses. More detailed characteristics of these studies are shown in Table 3 .

Characteristics of the studies evaluating the Mediterranean diet (high vs. low adherence) and cancer mortality including cause-specific cancer mortality.

HR = hazard ratio; SD = standard deviation; F = females; M = males; FFQ = food frequency questionnaire; 24-HR = 24 h recall; CRC = colorectal cancer; NA = not available; n = number of events; N = total population. Adjustments: 1: age; 2: smoking; 3: physical activity; 4: BMI; 5: education; 6: dietary score at baseline; 7: ethnicity; 8: total energy intake; 9: radiation treatment; 10: pack-years; 11: chemotherapy; 12: nonsteroidal anti-inflammatory drug (NSAID) use; 13: family history of colorectal cancer; 14: comorbidities; 15: marital status; 16: alcohol intake, 17: type 2 diabetes; 18: postmenopausal hormone replacement therapy; 19: race, 20: sex; 21: baseline examination year; 22: abnormal electrocardiogram; 23: dietary supplement use; 24: oral contraceptive use; 25: stage; 26: time since diagnosis; 27: age at first birth and parity; 28: menopausal status; 29: eeight change; 30: categories of treatment; 31: time between diagnosis and completion of the questionnaire; 32: socioeconomic status; 33: self-reported prevalent chronic diseases at baseline; 34: moderate to vigorous physical activity; 35: diet score; 36: family history of cancer in a first-degree relative; 37: sleep duration; 38: history of hypertension; 39: living alone; 40: Charlson comorbidity index; 41: aspirin use, 42: changes in smoking status; 43: family history of myocardial infarction; 44: family history of diabetes; 45: changes of physical activity; 46: changes in total energy intake; 47: weight change; 48: survey wave; 49: region; 50: nationality; 53: waist circumference; 54: potato intake; 55: egg intake, 56: polyunsaturated lipid intake; 57: sweet intake; 58: non-alcoholic beverage intake.

Among the studies analyzing the association between MDS adherence and cancer mortality risk [ 39 , 41 , 43 , 46 , 48 ], two studies showed a statistically significant protective association [ 39 , 41 ], but three studies did not support an association [ 43 , 46 , 59 ]. In this regard, Lassale et al. [ 39 ] studied this association in the European Prospective Investigation into Cancer and Nutrition (EPIC) study among 451,256 healthy participants, followed-up for 12.8 years. This study also evaluated adherence to rMED, and both MDS and rMED (high vs. low adherence) were associated with a statistically significant reduced overall cancer mortality risk (HR = 0.90; 95% CI: 0.88–0.92 and HR = 0.88; 95% CI: 0.86–0.90, respectively). The study by Vormund et al. [ 41 ], which included 17,861 Swiss men and women, also reported that a higher adherence to the MDS (vs. low) was associated with lower cancer mortality risk in both men and women (HR = 0.95; 95% CI: 0.92–0.99), although this association was stronger in men (HR = 0.92; 95% CI: 0.88–0.97) and absent in women. Furthermore, Lagiou et al. [ 43 ], in a cohort of 42,237 Swiss women, showed that a one-point increase in the MDS was not significantly associated with cancer mortality risk. On the other hand, a Spanish study conducted among 12,449 men and women [ 46 ], found that MDS was not significantly associated with cancer mortality risk. As for the MDP score, Cheng et al. [ 48 ] studied its association with cancer mortality risk within the prospective Iowa Women’s Health Study from the USA, which included 35,221 cancer-free women at baseline, of which 4665 died due to cancer during follow-up. The adjusted HR and 95% CI for all-cancer mortality among participants in the highest relative to the lowest quintile was 0.93 (95% CI: 0.84–1.03).

The characteristics of other MD indexes, such as the aMED [ 40 , 44 , 45 , 47 , 49 , 51 ], the mMED [ 50 ], and the rMED [ 39 , 42 ], are described in the original studies [ 30 , 63 ] and elsewhere [ 64 ]. In relation to their association with cancer mortality and other outcomes, the following results were reported: In general, the nine aMED studies suggested that high vs. low adherence to the aMED score was associated with a decreased cancer mortality risk [ 40 , 44 , 45 , 47 , 49 , 51 ]. It should be noted that HR and the corresponding 95% CIs were extracted from the figures in the study by Liese et al. [ 45 ]. This study analyzed the association within three cohorts in a standardized manner. Overall, the study showed the protective effects of the MD against cancer mortality. Two studies assessing the association with regard to the mMED score were included [ 50 ]. A study by Warensjö et al. [ 50 ] of 38,428 Swedish women, found that mMED was associated with a lower cancer mortality risk (HR high vs. low adherence = 0.81 95% CI: 0.69–0.94). Finally, the two rMED studies [ 39 , 42 ] showed diverging results: Lassale et al. [ 39 ] showed a statistically significant inverse association with cancer mortality risk (HR = 0.88; 95% CI: 0.86–0.90) when considering the entire EPIC study cohort, whereas Buckland et al. [ 42 ] concluded that a high compared with a low rMED score adherence was not associated with a significant reduction in mortality from overall cancer in the Spanish EPIC cohort (HR = 0.92; 95% CI: 0.75–1.12).

Pooled estimates of the effect size and 95% CIs for cancer mortality risk comparing high vs. low adherers to the MD (preferably for MDS and aMED) are depicted in Figure 2 and described below. Distinctions by type of MD score were not made. Only overall mortality could be assessed due to lack of studies on MD adherence and cancer-specific mortality (except one study on colorectal cancer mortality). There was no evidence for publication bias according to the funnel plot and Egger test (data not shown).

Meta-analysis under random-effects model (REM) with regard to adherers to the Mediterranean diet (MD) (high vs. low) and overall cancer mortality. F = females, M = males.

3.3.1. Overall Cancer Mortality

We included eight studies comparing extreme groups (high vs. low) of adherence to the MD [ 39 , 45 , 47 , 48 , 49 , 50 , 51 ]. There were two studies that did not report HR by these groups [ 41 , 43 ]. Moreover, the study by Buckland et al. [ 42 ] was also not considered for this meta-analysis due to population overlap with the study of Lassale et al. [ 39 ]. In addition, the study by Liese contributed only with women from the Women Health Initiative Observational Study (WHI-OS) [ 45 ]. The pooled analyses (combining 74,267 cancer deaths among 1,949,146 persons) revealed that overall cancer mortality risk was significantly reduced by 16% (95% CI: 11–30). However, there was significant heterogeneity between the studies (I2 = 84%, p < 0.001), which disappeared after excluding the study by Cuenca-Garcia et al. [ 46 ], while risk estimates remained the same.

3.3.2. Colorectal Cancer Mortality

The association between MD adherence and colorectal cancer mortality was assessed in one study [ 40 ]. In this study, conducted within the Multiethnic Cohort Study (MEC) with 4204 colorectal cancer events over follow-up and subsequent 1976 deaths (1095 were colorectal cancer-specific), a nonsignificant association was observed between high vs. low adherence to aMED and colorectal cancer mortality (HR = 0.90; 95% CI: 0.57–1.43). It is important to note that this was the only study controlling for cancer treatment variables in the analyses.

3.4. Post-Diagnosis Studies on Plant-Based Dietary Patterns and Cancer Outcomes

We identified six studies on this topic based on our literature search [ 54 , 55 , 56 , 57 , 58 , 59 ], complemented with studies included in previous reviews [ 11 , 12 ]. These studies were mostly focused on the MD as the reference dietary pattern, or other predefined PBDPs. Included outcomes were both cancer mortality and recurrence. A summary of the studies on post-diagnosis PBDPs and cancer mortality is shown in Table 4 .

Characteristics of the studies evaluating plant-based dietary patterns at post-diagnosis and cancer mortality and recurrence.

F = females; M = males; FFQ = food frequency questionnaire; 24-HR = 24 h recall; CRC = colorectal cancer; NA = not available. Adjustments: 1: antiestrogen use; 2: oophorectomy status; 3: tumor stage; 4: tumor site; 5: marital status; 6: age; 7: energy intake; 8: chemotherapy; 9: treatment; 10: smoking; 11: alcohol consumption; 12: multivitamin use; 13: physical activity; 14: menopausal status; 15: parity; 16: oral contraceptive use; 17: BMI; 18: weight change since diagnosis; 19: grade of tumor; 20: time since diagnosis; 21: chemotherapy; 22: year of diagnosis; 23: sex; 24 survival time from cancer diagnosis; 25: metastases; 26: occurrence of other cancers, 27: race; 28: socioeconomic status; 29: medical conditions; 30: hormone replacement therapy; 31: Gleason score; 32: pre-diagnostic Mediterranean diet.

Of the selected studies, there was only one intervention study [ 57 ]. This study was conducted within the Women’s Healthy Eating and Living (WHEL) Study, and included 3088 females with non-metastatic breast cancer, who were randomized into an intervention and control group. The intervention group received advice through telephone and cooking classes on how to adopt the plant-based diet defined as a low-fat and high-fiber diet, characterized by a daily intake of five vegetable servings, two glasses of vegetable juice, three fruit servings, 30 g of fiber, and 15% to 20% of energy intake from fat. The control group received advice on maintaining a healthy diet only. Compliance with the intervention was controlled through dietary records during the intervention phase, which lasted six months. Consideration was given to potential confounding of the association by prognostic factors of the disease, such as stage, and dietary habits at baseline. Results of this study did not support that a plant-based diet is associated with a reduced risk of cancer death or cancer recurrence. Although this was the only study available, the long follow-up, large sample size, low residual confounding risk, and the adequacy of procedures during the intervention and follow-up, makes the obtained results highly robust.

The remaining studies were cohort studies. Two of them were conducted within the NHS [ 54 , 55 ]. Both assessed the association between adherence to the MD by means of the alternate MD score (aMED) and cancer mortality with regard to breast cancer [ 55 ] or colorectal cancer [ 54 ]. Early-stage breast ( N = 2729) and colorectal cancer ( N = 1201) patients in these studies were females, who were diagnosed with the disease during the follow-up of the cohort and were followed-up thereafter for all-cause mortality and cancer cause-specific mortality. The cancer diagnosis and outcome was verified by reviewing clinical records and death certificates. Adherence to aMED was assessed by the use of the dietary information collected in two-yearly intervals with a FFQ. The aMED score, adapted from the traditional MD Trichopolou score, awards one point for intakes higher than the population median of vegetables, legumes, fruits, nuts, whole grains, fish, and monounsaturated:saturated fat ratio, and intakes less than the median in meat, and in alcohol if intake is outside a given range [ 64 ]. No statistically significant associations were found between high adherence to aMED (vs. low adherence) and breast or colorectal cancer mortality in this study, nor did the authors observe any association with regard to all-cause mortality in the breast or colorectal cancer survivors. All estimates were adjusted for relevant prognostic factors of the disease (e.g., tumor stage, cancer site, and treatment), as well as for the patient´s lifestyle or dietary habits before the cancer diagnosis. Another American cohort study, conducted within the HPFS, addressed the association between aMED and cancer mortality, particularly concerning prostate cancer mortality [ 58 ]. In this study, 4538 early-stage prostate cancer patients were followed-up since the cancer diagnosis until mortality or end of follow-up. Comparable methods were applied to collect dietary information and to ascertain cancer diagnosis and death. Likewise, by adjusting for prognostic factors of the disease and lifestyle/dietary habits before the diagnosis, a significant inverse association was observed between high vs. low adherence to the MD (both as aMED and MDS) and mortality from any cause (HR = 0.78; 95% CI: 0.67–0.90). However, statistical significance was not reached for mortality from prostate cancer.

Other post-diagnosis dietary indices that have been examined in relation to MD were the modified Mediterranean diet score (mMDS) and the traditional Mediterranean diet score (MDS), which are similar in their composition with respect to aMED [ 64 ]. The German PopGen Biobank Study [ 56 ], which included 1404 colorectal cancer patients (histologically confirmed), among which 204 died during six years of follow-up, reported that high vs. low adherence to the mMDS was associated with a reduced all-cause mortality risk among those patients. The study accounted for the influence of all possible prognostic factors and of pre-diagnostic adherence to the MD. Adherence to the MD by the mMDS at pre- and post-diagnosis was assessed using dietary information collected via FFQs administered in both time intervals. Lastly, within the National Health and Nutrition Examination Survey (NHANES) study [ 59 ], 240 participants who self-reported a cancer diagnosis of breast or gynecological cancers were followed-up during 10 years on average. Cancer mortality was assessed by death certificates from clinical records and mortality registers. Dietary information collected with a single 24 hour recall (HR) was used to score the participants into levels of adherence to the MDS. This study showed that high vs. low adherence to this score was not significantly associated with neither all-cause mortality nor breast cancer mortality.

It is worth noting that four of these studies also accounted for pre-diagnosis dietary information on PBDPs [ 55 , 58 , 59 , 63 ], which was considered for the adjustment of baseline dietary intake in some of these studies [ 58 , 63 ]. Only the study by Kim et al. [ 55 ] examined the effect of pre-diagnosis PBDPs (aMED score) on cancer mortality among cancer survivors. As a result, no association was reported for total mortality, breast cancer mortality, distant recurrences or non-breast cancer mortality.

Results of the meta-analyses of these studies by cancer site were only possible for breast and colorectal cancer. No study evaluated the association between post-diagnosis PBDPs and cancer mortality in relation to all-cause mortality. Publication bias was unlikely according to funnel plots and Egger test.

3.4.1. Breast Cancer Mortality

Of the three studies evaluating an association between a PBDP and cancer mortality, there were two cohort studies considering an MD score and breast cancer mortality [ 55 , 59 ]. The intervention study was not considered for the meta-analysis due to its different nature. Pooled estimates for the 2849 breast cancer patients revealed that adherence to the MD (high vs. low) was not associated with all-cause mortality among breast cancer survivors (RR = 0.87; 95% CI: 0.85–1.01). While heterogeneity between the studies was not apparent (I 2 = 0%, p = 1), the study by Karavisouglou contributed with a small number of breast cancer patients, whose diagnoses were not confirmed [ 59 ]. Moreover, in this study, breast and gynecological cancers were considered together, thus biasing the pooled analyses to some extent. The other two studies provided more robust results [ 55 , 57 ], and supported that a PBDP from breast cancer onset reduces all-cause or breast cancer mortality.

3.4.2. Colorectal Cancer Mortality

The studies by Fung [ 54 ] and Ratjen [ 56 ] were the only two studies available on the association between PBDP and cancer mortality in colorectal cancer survivors. The MD was considered as a plant-based pattern in both studies, either as aMED or mMDS. Together, these studies combined the results of 2605 colorectal cancer patients. The summary estimates for all-cause mortality revealed a nonsignificant association (HR = 0.66; 95% CI: 0.37–1.17); no other outcome events could be analyzed due to lack of data. However, there was heterogeneity between both studies (I 2 = 79%, p = 0.03), possibly driven by differences inherent to the study populations (country of recruitment, gender distribution, and clinical characteristics of the patients). It was not possible to examine the influence of these variables on the pooled results.

4. Discussion

The present review is the first to address the available evidence on the association between PBDPs and cancer-related outcomes, including overall cancer mortality, cause-specific mortality, and cancer recurrence. Plant-based diets have been traditionally regarded as vegetarian diets, but other definitions of plant-based foods can be considered by rating negatively some or all animal foods, and even accounting for the quality of plant-based foods in the diet. Only adherence of the MD was found to reduce overall cancer mortality risk, but none of the studies accounted for the influence of prognostic factors on this association; thus, the true independent effect of the MD with cancer mortality risk remains inconclusive. Studies on other PBDPs in relation to these cancer outcomes have provided nonsignificant or ambiguous results. The same was found for post-diagnostic studies on these associations. Therefore, for the considered cancer-related outcomes, there is still insufficient evidence for asserting that PBDPs help in reducing the risk of these outcomes.

Plant-based diets are dietary sources of several bioactive compounds such as fiber, phenol, polyphenol, and sulfur compounds, and other antioxidants compounds including vitamins [ 7 ]. In the literature, foods of vegetable origin have been associated with cancer mortality outcomes, although conflicting results have been reported. These include, for example, legumes [ 9 ], fruits and vegetables [ 15 ], nuts [ 16 ], whole grains [ 16 ], and olive oil [ 18 ]. Bioactive compounds in these foods, however, have been shown to have anti-carcinogenic effects in experimental models and epidemiological studies [ 7 ]. For instance, high intake of fiber and polyphenols from cereals have been shown to reduce cancer mortality risk [ 65 , 66 , 67 ]. As for cancer incidence, a two-tier mechanism could explain how these compounds could prevent cancer mortality risk: first, fiber, mostly soluble fiber, modulates the gut microbiota composition improving the colonic barrier functions, and second, substrates such as resistant starch, non-starch polysaccharides (β-glucans), and phenols are metabolized into active metabolites by the commensal microbiota [ 68 ]. The resulting metabolites have well known anti-cancer effects and could likewise prevent cancer mortality. For instance, phenolic acids are antioxidants with free-radical scavenging activity. Free radicals play an important role not only for cancer development but also in cancer treatment since anti-cancer drugs generate reactive oxygen species themselves [ 69 ]. Therefore, neutralizing their damaging effects is crucial to reduce mortality and secondary outcomes in cancer patients. Other metabolites, such as short-chain fatty acids are also major players in the maintenance of gut integrity and immune homeostasis, to prevent tumor development [ 68 ].

The drawbacks of individual nutrient or foods studies have been repeatedly described [ 19 ]. PBDPs or indexes based on the potential beneficial effects of various dietary factors, and accounting for the interaction of all phytochemicals contained in the diet, should be therefore a better approach to examine the association between plant-based diets and cancer mortality. PBDPs have been related to a low-risk immunological profile (lower C-reactive protein, fibrinogen, and leukocyte levels) [ 70 ], lower adiposity markers [ 62 ], and better antioxidant status [ 71 ]. These are probably the underlying mechanisms by which these diets could improve the immune response in cancer patients, while also slowing tumor growth and risk of developing subsequent events.

Vegetarian diets are based on the consumption of plant-based foods, namely vegetables, fruits, whole grains, legumes, nuts, and seeds, with the elimination of foods of animal origin such as meat, poultry, wild game, seafood, and their byproducts [ 72 ]. In a narrower sense, vegetarian diets may or may not include eggs and dairy products. Vegan diets, for instance, exclude these dietary factors, whereas lacto-, ovo-, and lacto-ovo vegetarians include either dairy products, or eggs and egg products, or both dairy products and eggs, respectively. Vegetarian diets comply with healthy diet recommendations and are therefore considered a healthful dietary pattern [ 72 ]. A negative side of this dietary pattern is the potential deficient intake of certain nutrients such as iron and vitamin B12. Long-chain n-3 fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), are lower in vegetarians and typically absent in vegans [ 73 ]. Decreased intakes of some of these nutrients have been related to a higher cancer incidence in some studies [ 74 ]. On the contrary, the reduced intake of heme-iron in vegetarians is presumed to be beneficial taking into consideration that high ferritin levels due to iron overload have been related to a higher risk of certain cancers [ 75 ]. However, it is considered that if vegetarian diets are well-planned, their naturally high content of beneficial bioactive compounds outweigh the negative sides [ 72 , 76 ]. Indeed, it has been reported that vegetarians have a lower cancer incidence than nonvegetarians [ 26 ], or at least regarding colorectal cancer for semi-vegetarians or pesco-vegetarians compared to nonvegetarians [ 77 ]. While the results of our review tended to suggest that vegetarians and vegans have a lower risk of cancer mortality and cancer-specific mortality when compared to nonvegetarians, we did not observe significant associations in meta-analyses combining results of the selected studies. By updating a previous review examining vegetarian diets regarding several health issues [ 26 ], we provide more consistent data, although it is still insufficient to provide meaningful conclusions. Indeed, vegetarians do not make up large population groups and are therefore difficult to ascertain in epidemiological studies. Moreover, as shown in the study by Key et al. [ 60 ], vegetarians or vegans often adopt a healthier lifestyle (e.g., non-smoking) as compared to omnivores, making it more difficult to establish comparisons between both groups. The sample size of the included studies was too limited to permit overall and cause-specific cancer mortality studies. Another important aspect as to why we probably did not observe a significant overall effect might be due to different considerations of vegetarians, vegans, and type of vegetarian diets among the studies. Based on the responses to a finite list of foods, participants were classified into vegetarians and vegans in most studies; only the study by Orlich based this classification upon a validated FFQ [ 61 ]. Misclassification of vegetarian diets was therefore likely in these studies. In addition, it was not possible to make any comparisons with regard to type of vegetarian diet since few studies attempted to link the type of diet with cancer mortality risk [ 38 ]. Thus, for example, whether pesco-vegetarian or lacto-vegetarian diets, rich in dietary factors with colorectal cancer prevention potential (e.g., calcium, fiber, n-3 fatty acids, and vitamin antioxidants), have a stronger prevention effect against mortality of this disease could not be confirmed.

Recently, a priori-defined plant-based scores have been proposed to allow comparisons between different levels of compliance with vegetable-derived foods against animal-derived foods [ 53 ]. Provegetarian food pattern scores are newly developed tools to assess the level of adherence to a vegetarian-like diet (i.e., preference of plant-based foods). Unlike the vegetarian dietary patterns, these scores consider moderate intakes of animal foods such as fish, poultry, and dairy in the assessment of the score, under the assumption that moderate intake of these foods may confer some health benefits [ 78 , 79 , 80 ]. Moreover, in this manner, these scores are presumed to overcome the potential nutrient intake deficiencies (e.g., vitamin B12), that a strict plant-based diet such as the vegetarian diet poses. Accordingly, they score vegetable-derived foods positively, while all animal-derived foods are scored negatively. However, with only two studies evaluating how these scores affect cancer mortality risk [ 52 , 53 ], it has been difficult to arrive at any concrete conclusion. The time point considered for the assessment of the provegetarian diet differed in both studies. Furthermore, both studies were not comparable in the sense that the provegetarian diet score was not defined in the same way. While both considered all animal foods, only the study by Baden et al. [ 53 ] distinguished the quality of plant-based foods in the scoring. This study reported associations for a standard provegetarian diet score, and for its healthy and unhealthy versions. Relative to the unhealthy score, the healthy one scored more negatively foods rich in refined carbohydrates, which are supposed to be implicated in cancer risk through energy metabolism, insulin, and insulin-like growth factor (IGF-1) upregulations. A major impact of the healthy provegetarian diet score for cancer mortality prevention was therefore expected. Nevertheless, significant associations were only observed for the standard score.

The MD encompasses the traditional dietary pattern found in the olive-growing regions of the Mediterranean basin in the 1960s [ 81 ], and is globally recognized as a healthy dietary model [ 82 ]. The traditional MD pattern is a well-defined plant-based dietary pattern characterized by: the daily use of olive oil; an abundance of plant foods such as fruits and vegetables, nuts and seeds, cereals and legumes; the consumption of fish and seafood especially in coastal regions; moderate-to-low intake of dairy products mostly from fresh cheese and yogurt; moderate alcohol mostly in the form of wine; and a less frequent consumption of meat and meat products [ 83 ]. Thus, the MD pattern is distinctively plant-based, and thus a valuable alternative to the vegetarian diet, as it provides a good supply of fiber, phytochemicals, vitamins, and minerals, even closing some nutritional gaps of the vegetarian diet [ 83 ]. For the reasons cited above, we considered the MD as a reference PBDP. Moreover, the MD is presumed to boost the endogenous antioxidant defense and the immune system to prevent cancer and, possibly, also fatal outcomes of this disease. With respect to cancer incidence, indeed, as is also the case with other dietary patterns, the MD is an established dietary pattern for cancer prevention [ 13 , 84 ]. Other dietary patterns resembling plant-based diets such as the very low ketogenic diet, seem to have cancer prevention potential through weight loss and related mechanisms, but have been scarcely examined with regard to cancer mortality [ 85 ].

Since the first definition of an MD index, multiple adaptations have been created to improve its conformity to the MD [ 64 ]. The MD scores used to assess the association between the MD and cancer mortality risk are the MDS, MDP, rMED, and aMED. Pooled analyses of the included studies revealed a significant inverse association between high vs. low adherence to the MD and overall cancer mortality risk. While we combined the studies regardless of the MD score, it has been reported that there is only moderate concordance between them [ 86 ]. Indeed, MD scores vary largely in the types of foods included and the intake levels in the population. Thus, variability in the scoring schemes of the MD scores might have affected our results. However, when restricting our meta-analysis to studies using the MDS (the predominant score), the result was maintained (data not shown). Despite the fact that our results support a beneficial effect of the MD for the prevention of cancer mortality, we have to be cautious in the interpretation of these results. No study considered the influence of relevant factors related with the disease outcomes, such as treatment, on the associations. In addition, there was scarce data on the association between MD and cancer mortality by type of cancer.

As outlined before, most of the studies on the association between PBDPs and cancer mortality considered the influence of such diets from pre-diagnosis states to the event outcomes. However, post-diagnosis diet might have a strong short-term or even long-term effect on cancer mortality and other related outcomes. Whether eating a PBDP after cancer diagnosis affects cancer outcomes has been addressed in several studies. Although these studies accounted for a large number of factors related to the outcome (all potential prognostic factors: clinical and pathological tumor characteristics, treatment, and comorbidities), their results must be interpreted with caution due to the limited sample sizes. Moreover, there was only one intervention study on the effect of a well-defined plant-based diet on cancer-related outcomes [ 57 ]. Importantly, this study did not address whether consuming the high vegetable/fruit/fiber and low-fat diet of the study intervention early in life would alter the risk of breast cancer and cancer mortality as well. The remaining studies were cohort studies assessing the association between adherence to the MD since the cancer diagnosis and cancer mortality [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 58 , 59 ]. Some [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 ], but not all [ 54 , 55 , 56 , 57 , 58 , 59 ], of the included studies also accounted for dietary behaviors before the cancer onset. In fact, as this review shows, dietary patterns over a lifetime are likely to influence cancer mortality. Future studies on the association between PBDPs and cancer mortality should therefore consider assessing adherence to PBDPs throughout the lifespan, to cover pre- and post-diagnostic influences of these dietary patterns on the associations.

Thus, while dietary patterns have been consistently associated with a reduction of cancer incidence, an association with cancer mortality has not been clearly established. This contradiction could be explained by the fact that cancer mortality is greatly influenced by the type and treatment regime. Indeed, cohort studies assessing the association from a pre-diagnosis state typically did not account for this factor. Rather, this was accounted for in studies evaluating how adherence to these plant-based diets since diagnosis impacts on these outcomes. On the other hand, the lack of statistical power to detect significant associations in cause-specific mortality studies is another important limiting factor that makes it impossible to draw any meaningful conclusions on this association. In fact, there are no studies on cause-specific cancer mortality other than lung cancer, breast cancer, colon cancer, and prostate cancer. Also of note, is the fact that studies on other cancer-related outcomes, such as secondary cancers, are unavailable. No study considered the interaction between dietary and genetic factors on these associations. Most studies relied on a single measurement of diet at baseline, but dietary patterns may change over time and the length of exposure to plant-based-like dietary patterns may account for the differences observed between results from different cohorts. Dietary assessment tools (3-day records, 24-h recalls or FFQs) also differed greatly between the studies, as well as the studied populations. For instance, there were both pre and postmenopausal breast cancer patients considered jointly in some studies. Moreover, while the definitions of vegetarians and vegans were well-defined, some studies seemed to mix up vegans and vegetarians. Further, in the case of the comparison groups, there were differences among the studies with regard to the definition of meat eaters (omnivores). The included studies considered either populations of adherers to a specific PBDP (for example, vegans and vegetarians) or cohorts with individuals classified into different levels of adherence to PBDPs (for example, MD adherence groups) according to their reported dietary data. Therefore, the possibility that some studies misclassified vegetarians, vegans or adherence to other types of plant-based diets cannot be ruled out. Finally, plant-based diets contain a huge variability of bioactive compounds depending on the dietary source, which makes the definition of a healthy plant-based diet particularly complex. Among the limitations of this review, there were also issues related to population overlap in the vegetarian studies [ 35 , 38 ], which could have introduced bias to our results. We could also not analyze whether duration of adherence to any PBDP had any influence on the results.

There are also strengths of this review worthy of consideration: this is the first review and meta-analysis that has analyzed all the available data on PBDPs in relation to cancer mortality and related outcomes. While a rapid review was conducted, we complemented the identification of studies with those included in other reviews related to our topic. However, a quality assessment of these studies was not performed. We were able to conduct analyses by some cancer types and to evaluate the influence of some study characteristics on the results. However, we were not able to meta-analyze studies according to all cancer types due to lack of studies.

5. Conclusions

This review and meta-analysis of the current available evidence on the association between PBDPs and cancer mortality show that there is limited evidence regarding the beneficial effects of vegetarian diets for the prevention of cancer-related outcomes in the general population and in cancer survivors. This review also shows that there is suggestive evidence regarding the association between the MD pattern and cancer mortality. There were very few studies evaluating how these dietary patterns influence cancer mortality after the cancer diagnosis. Breast cancer and colorectal cancer outcomes were by far the most studied cancer types, but the number of studies is small. Thus, there is an urgent need to increase our knowledge on the usefulness of plant-based diets for the prevention of cancer mortality. Well-designed studies, considering consensus definitions of PBDPs and all pertinent factors including prognostic factors of the disease, genomics, and others, are needed to determine the effect of plant-based diets on cancer survival and cancer recurrence, before and after the diagnosis of cancer.

Acknowledgments

The authors would like to acknowledge Dafina Petrova for her contributions in editing and proofreading the manuscript.

Search strategy applied to retrieve the studies (date: 20 April 2020)

#1) “Survival” OR “Progression-Free Survival” OR “Disease-Free Survival”

1217,476 results

#2) “Mortality”

1125,551 results

#3) “Cancer Survivors”

16.142 results

#4) #1 OR #2 OR #3

1984,149 results

#5) (“Diet, Vegetarian” OR “Diet, Paleolithic” OR “Diet, Ketogenic” OR “Diet, Healthy” OR “Diet, Mediterranean” OR “Diet, Protein-Restricted” OR “Diet, Macrobiotic” OR “Diet, Carbohydrate Loading” OR “Diet, Vegan” OR “Caloric Restriction” OR “Dietary Approaches To Stop Hypertension”)

23,070 results

#6) #4 AND #5

2234 results

B. Flow diagram illustrating the process of article selection.

Author Contributions

Conceptualization, E.M.-M., E.S.-F., B.G.-V., and M.J.S.; methodology, E.M.-M.; formal analysis, E.M.-M., E.S.-F., and B.G.-V.; data curation, E.S.-F.; writing—original draft preparation, E.M.-M. and E.S.-F.; writing—review and editing, E.M.-M., E.S.-F., B.G.-V., and M.J.S.; supervision, M.J.S. All authors have read and agreed to the published version of the manuscript.

This research was funded by CIBER Epidemiología y Salud Pública CIBERESP.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Plant-Based Foods May Reduce Risk of Prostate Cancer Progression

• A new study found that following a plant-based diet may slow the progression of prostate cancer in people recently diagnosed with the disease.
• A plant-based diet focuses primarily on fruits, vegetables, legumes, nuts, and whole grains.
• The study didn't establish causation, but one of the researchers said the association between plant-based diets and slower prostate cancer progression was "strong."

Following a plant-based diet—or one primarily filled with fruits, vegetables, legumes, and whole grains–has been associated with a reduced risk of everything from type 2 diabetes to premature death. Now, a new study suggests that the eating plan may slow the progression of prostate cancer . 

The research, published in the journal JAMA Network Open , found a link between a plant-based diet and a reduced risk of localized prostate cancer worsening over time. People with prostate cancer who ate the most plant-based foods had the lowest odds of cancer progressing.

The research indicates that “consuming a primarily plant-based diet—and less animal-based food—after a prostate cancer diagnosis may be associated with better prostate cancer-specific outcomes,” study author Stacey A Kenfield, ScD , a professor of urology at the University of California, San Francisco, told Health . “A plant-based diet is one promising diet that a clinician could recommend that has been associated with lower risk of prostate cancer progression, and numerous other outcomes.”

Kenfield said it’s important to note that this is a cohort study, meaning it can only show a connection between plant-based diets and better prostate cancer outcomes; it doesn’t establish causation. However, she added, the association her team found was “quite strong.”

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How Much May Following a Plant-Based Diet Reduce Prostate Cancer Progression?

The new study is a follow-up to research Kenfield and her colleagues conducted that found a link between a plant-based diet and a reduced risk of developing fatal prostate cancer, particularly in men under 65.

“The next step was to evaluate this dietary pattern, after diagnosis, in men with prostate cancer,” she explained. “These results complement the previous findings.”

Her team gathered data about 2,062 men with non-metastatic prostate cancer enrolled in the Cancer of the Prostate Strategic Urologic Research Endeavor (CaPSURE) study, a longitudinal observational study of more than 15,000 men with prostate cancer. The men were treated at 43 different urology practices nationwide from 1999 to 2018. Their median age was 65, and 95% of them were White.

Participants completed a diet and lifestyle questionnaire at some point—the median timeframe was 31 months—after their diagnosis, and researchers followed up with them for around 6.5 years. During that period, the cancer progressed for 190 people, and 61 people died from it. 

The team found that the men who ate the largest quantities of plant-based foods showed a 47% lower risk of prostate cancer progression. Among 680 men who had at least medium-grade cancer as determined by a measure known as the Gleason grading system, those who ate the most plant-based foods had a 55% reduced chance of cancer worsening. 

There was no association between diet and cancer progression, however, for people with low-grade prostate cancer. 

Samuel Haywood, MD , a urological oncologist at Cleveland Clinic, told Health that the connection between plant-based diets and slower cancer progression may in fact be due to other healthy habits people who tend to choose plant-based foods also practice. But, he said, the data adds to existing evidence suggesting that plant-based diets may benefit prostate cancer patients.

“My patients often are asking about dietary and exercise interventions when they are diagnosed with prostate cancer,” Hayward said. “For patients who are motivated to make dietary modifications, this may provide some inspiration to explore a plant-based diet.”

What’s Behind the Diet’s Possible Effect on Prostate Cancer?

Hayward said research hasn’t pinpointed the mechanism behind plant-based foods’ potential influence on prostate cancer progression, but that there are a few factors that could be behind the relationship.

Many plant-based foods contain antioxidants and anti-inflammatory compounds, which may have cancer-fighting properties. The plant-based diet may also cause beneficial changes to the microbiome that may affect prostate cancer progression. And, Hayward added, “these diets can often be healthier than a conventional diet, such as lower in fats or processed foods.”

Then there’s the high fiber content, which may regulate glucose levels and promote satiety.

“Managing weight or losing weight will improve insulin sensitivity and reduce chronic inflammation ,” Kenfield said. “Clinical evidence suggests an association between insulin resistance, chronic inflammation, and cancer.”

Which Foods Show Benefit?

While all plant-based foods pack a nutritional punch, Kenfield points to several that have been shown to be associated with better outcomes for prostate cancer patients. 

She said sources of vegetable fat, like nuts, avocado , and olive oil show promise, as does fish, cooked tomatoes, and cruciferous vegetables such as broccoli and cauliflower. 

Hayward said that while many foods have been suggested to be beneficial for prostate cancer, the “evidence for each individual component is not particularly strong, so it’s hard to push any individual food item for prostate cancer patients.”

Next Steps for Research

Kenfield acknowledged that future studies should examine the role plant-based diets may have on prostate cancer in a more racially and ethnically diverse population. She’d also like to see studies that focus on men with metastatic prostate cancer.

A randomized controlled trial—considered the gold standard in epidemiological research—would be necessary to assess whether plant-based diets cause better outcomes for cancer patients.

“In the meantime,” she said, “I hope we can confirm these findings in other ongoing observational studies, which would add to the limited data on this topic.”

Liu VN, Van Blarigan EL, Zhang L, et al. Plant-based diets and disease progression in men with prostate cancer . JAMA Netw Open . 2024;7(5):e249053. doi:10.1001/jamanetworkopen.2024.9053

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