Benefits and Drawbacks of Vegetarian and Vegan Diets: Key Points

The following are key points to remember from a state-of-the-art review on the benefits and drawbacks of vegetarian and vegan diets:

1. This review of plant-based diets provides an excellent reference for all who are providing nutrition advice and counseling, educating health care professionals, or who simply for personal reasons want to maximize their efforts to eat what might be a very healthy diet. The authors conducted an exhaustive search and included both basic and clinical research reports, epidemiologic studies, case-control studies, and randomized controlled trials.
2. Plant-based diets have become increasingly popular thanks to their purported health benefits espoused by health care professionals, well-known athletes, entertainers, and the media—and more recently for their positive environmental impact by reducing carbon emissions from farm/ranch animals. 
3. There are different types of plant-based diets including vegan (100% plant-based), lacto-ovo vegetarian (plant-based except for dairy products and/or eggs), and pesco-vegetarian or pescatarian (plant-based except for fish and seafood with or without eggs and dairy). All vegetarian diets exclude meat (e.g., beef, pork, lamb, venison, chicken, and other fowl) and related meat products.
4. In addition to epidemiologic studies, data from randomized clinical trials have confirmed a protective effect of vegetarian diets for the prevention of diabetes and reductions in weight, blood pressure, glycosylated hemoglobin, and low-density lipoprotein cholesterol (LDL-C), but to date, no data are available for cardiovascular (CV) event rates and cognitive impairment, and there are very limited data for cancer.
5. Importantly, not all plant-based foods are equally healthy. Unhealthy vegetarian diets are poor in specific nutrients (vitamin B12, iron, zinc, and calcium) and/or rich in highly processed refined flours, hydrogenated oils, high-fructose corn syrup, sucrose, artificial sweeteners, salt, and preservatives, each of which have been shown to increase morbidity and mortality. 
6. There are no data from mechanistic studies to help understand whether the advantages of healthy, minimally processed vegetarian diets represent an all-or-nothing phenomenon. Could it be that consuming primarily plant-based diets containing small quantities of animal products (e.g., pesco-vegetarian or Mediterranean diets) has beneficial, detrimental, or neutral effects on cardiometabolic health outcomes?
7. Among the potential mechanisms of vegan and well-balanced vegetarian diets for reducing the risk of coronary heart disease (CHD), cancer, and dementia include lipid lowering, glucose lowering and insulin sensitizing, antioxidative stress, anti-inflammatory, antihypertensive, and production of intestinal microbial metabolites influencing metabolic and immune health. Particularly important is the marked reduction in saturated fat.
8. Replacing saturated fat with refined carbohydrates increases the risk of CHD substantially but replacing saturated fat with vegetable polyunsaturated fatty acid (particularly seeds and nuts) decreases CHD by 30%, which is similar to statins. Foods rich in dietary fiber and with low glycemic index can lower insulin production and increase the levels of short-chain fatty acids produced by fiber fermentation, which have both been shown to inhibit cholesterol synthesis. Joint analysis of five prospective studies including 76,172 individuals has shown a lower CHD mortality in vegetarians than in omnivores: 34% less in lacto-ovo vegetarians and pesco-vegetarians and 26% lower in vegans. A meta-analysis of seven epidemiological studies (124,706 participants) found an 18% lower cancer incidence in vegetarians than omnivores. 
9. The high-fiber and water content and lower energy density of vegetables, legumes, and whole grains may in part explain this effect. Consumption of diets rich in fiber induces gastric distention, delays gastric emptying, and prevents large fluctuations in postprandial blood glucose. Similarly, short-chain fatty acids produced by the intestinal microbial metabolism of resistant starch and oligosaccharides of minimally refined plant foods markedly reduce blood glucose and body weight in randomized clinical trials.
10. Among the benefits of avoiding excessive calories is avoidance of central adiposity associated with insulin resistance and inflammatory and hormonal factors implicated in pathogenesis of atherosclerotic cardiovascular disease (ASCVD) and cancer including breast, endometrial, prostate, and colon. Interestingly, the vegetarian diet of low glycemic index load is associated with lower intake of protein intake than by omnivores who consume about 90-100 g of protein, of which 70% is animal. The latter is associated with increased prevalence and risk of pre-diabetes and type 2 diabetes mellitus (T2DM). Diabetes increases by 20-40% for every 10 g of protein in excess of 64 g per day, a finding that was found in animal but not plant protein. Not mentioned in the article is that while the keto diet can lower the hemoglobin A1c along with weight loss, after a year, it is associated with increased risk for diabetes. 
11. Much has been written about the importance of the gut microbiome. The DIRECT-PLUS trial showed that a calorie-restricted and (almost) red-meat-free version of the Mediterranean diet enriched in plant-based proteins (Green-MED diet) is superior to the classical Mediterranean diet in improving the 10-year Framingham risk score and in lowering waist circumference, intrahepatic fat, LDL-C, diastolic blood pressure, C-reactive protein, and HOMA insulin resistance. These cardiometabolic beneficial effects were partially mediated by a major shift in the composition and function of the gut microbiome. A growing body of evidence shows that reprogramming microbial functions through long-term adherence to healthier plant-rich diets has profound effects in shaping physiologic response to specific nutrients, to calorie restriction, and to other features of host biology that are instrumental in promoting health and longevity.

Summary Recommendations for Clinicians and Allied Health Practitioners:

1. For overweight men and women seeking weight loss and cardiometabolic improvement as a means of primary and secondary prevention of T2DM, hypertension, and CVD, well-balanced and supplemented vegetarian diets rich in minimally processed plant foods may be an option, especially when coupled with calorie restriction and regular exercise training, as recommended in the 2018 Physical Activity Guidelines Advisory Committee Scientific Report. Regular fish intake can provide additional CV health benefits. 
2. Additional trials are warranted to determine whether patients with CVD will ultimately benefit from consuming vegetarian and vegan diets and, if so, in what ways. As with any potential therapeutic strategy, the risks and benefits of vegetarian diets must be discussed with patients. There is evidence to suggest that some vegetarians, particularly those who follow restrictive diets such as vegans, may be at greater risk of hemorrhagic stroke and bone fractures if they do not carefully plan their diets and consume fortified plant-based foods or supplements. In addition, vegans and some vegetarians may be at risk of deficiencies in vitamins and minerals such as vitamin B12, riboflavin, iron, zinc, calcium, and omega-3 fatty acids. This can be particularly dangerous for pregnant and breastfeeding women and growing children, as these nutrients are crucial for fetal and child development.
3. Anyone considering a vegetarian or vegan diet should consult with a registered dietitian or health care provider to ensure that their diet is nutritionally adequate. Consuming vegetarian diets rich in refined grains, potatoes, high-sodium preserved vegetables, fried goods, sweets, juices, and sweetened beverages can increase the risk of developing T2DM and CVD morbidity and mortality. 
4. Finally, in the case of vegetarian diets and cancer, the benefits and risks are not well defined. As a weight loss strategy, this may be an option for some cancer patients, but there are currently no data to suggest that vegetarian or vegan diets in the absence of weight loss and/or changes in physical activity patterns will have a positive impact on cancer outcomes, including either recurrence or the development of metastatic cancers.

https://academic.oup.com/eurheartj/article-abstract/44/36/3423/7224412