Different groups have followed plant-based diets throughout human history. In ancient Egypt, meat consumption was seemingly reserved for wealthier Egyptians, including royalty; one analysis of hard and soft tissues from 5500–1500 BCE revealed that even middle-class Egyptians more commonly consumed plant-based foods such as wheat and barley than they did fish or meat.¹ In ancient India and China, emerging religions, including Hinduism, Jainism, and Buddhism, adopted vegetarianism as part of their practice of kindness and nonviolence toward living things (ahimsa).^2,3 Followers of Orphism in ancient Greece were banned from sacrificing animals or consuming animal products, to maintain their purity,³ and the Greek philosopher Pythagoras (ca. 570–ca. 490 BCE) taught his followers to abstain from consuming meat to facilitate reincarnation—in fact, for a time, vegetarianism was known as the Pythagorean diet.³ Although interest in vegetarianism was limited in Europe during the Middle Ages, it resurged during the Renaissance era and Age of Enlightenment.³ In September 1847, the first vegetarian society was established in Ramsgate, England, where the term vegetarian was also coined, and the American Vegetarian Society and the German Vegetarian Society were subsequently founded in 1850 and 1867, respectively.³ From 1863, the newly founded Seventh-Day Adventist Church in the United States also began encouraging vegetarianism among its members to improve health.⁴ 

Interest in plant-based eating appears to be growing worldwide: a 2020 Google Trends analysis of searches performed globally revealed that Google users were particularly interested in veganism and vegetarianism; among 59 countries, veganism was the most frequently searched diet type in 23 countries, and vegetarianism was the most frequently searched diet type in 14 countries.¹⁹ Here, we review the potential benefits and downsides to adhering to a vegetarian or vegan diet.

Benefits

Making the move from omnivore to vegetarian or vegan diet can result in an array of health benefits. Vegetarians and especially vegans show lower levels of both total and low-density lipoprotein cholesterol and apolipoprotein B (a protein that attaches to “bad” types of cholesterol that cause plaque build-up) than omnivores,^5–8 suggesting the potential of a plant-based diet to reduce the risk of cardiovascular disease. They also tend to have lower body mass index and blood pressure values than comparable individuals who consume meat.^5,6 Lower C-reactive protein levels have also been recorded in vegetarians and vegans compared to omnivores, suggesting less inflammation in the body.⁹ Both vegetarians and vegans tend to consume less saturated fat and sugar and more fiber due to their dietary choices,^5,6,8 which carry their own health benefits, and vegetarians and vegans may be more likely than omnivores to meet the recommended guidelines for certain nutrients (e.g., vitamin E, folate, magnesium) from diet alone.^8,10,11 Compared to omnivores, individuals following a vegetarian or vegan diet have higher levels of some antioxidants, including vitamins C and E and beta-carotene.^12,13 Research has also documented lower levels of certain oxidation stress biomarkers in vegans compared to omnivores.^8,14

To date, greater consumption of red and/or processed meat has been linked to increased risks for total mortality and a range of conditions, including cardiovascular disease, colorectal cancer, colon polyps, diverticular disease, pneumonia, and Type 2 diabetes.^15,16 Eating more poultry meat may similarly lead to gastro-esophageal reflux disease, gastritis, duodenitis, gallbladder disease, and diabetes.¹⁵ In contrast, the risk of many of these conditions and certain others, such as cataracts, and possibly some cancers may be reduced in vegetarians and vegans.^5,10,17 Moreover, when comparing vegetarian and vegan diets, the latter may offer additional degrees of protection against obesity, hypertension, Type 2 diabetes, and mortality from cardiovascular causes.¹⁸ 

Downsides

Newbies and long-term practitioners of vegetarianism/veganism alike should keep in mind that, in excluding some or all animal products from their diets, they may be leaving behind certain essential nutrients that must be accounted for in other ways. Meat, especially red meat, provides an array of micronutrients like iron, zinc, and vitamins B6 and 12 and functions as a “complete” source of high-quality dietary protein.²⁰ Indeed, one study reported that the intake concentrations of nine amino acids, including seven essential amino acids, were significantly lower (by up to 57%) among vegans compared to omnivores despite both groups exhibiting nearly the same energy intake (kcal/day).²¹ Other research has suggested that broader nutrient inadequacy may exist among vegetarians and vegans compared to omnivores. For example, while omnivores in the NuEva study failed to meet the daily recommendations per the German Society of Nutrition for the intake of carbohydrates (particularly dietary fiber), polyunsaturated fatty acids (PUFAs) (mainly omega-3 PUFAs), potassium, and vitamins D and E through diet alone, vegans and vegetarians had inadequate dietary intake levels of total protein, PUFAs (mainly omega-3 PUFAs), pantothenic acid, vitamins B12 and D, calcium, iron (in women), potassium, and zinc, and vegans additionally had inadequate dietary intake levels of vitamins A and B2.⁸ Even with supplementation, vegans may be at particular risk of missing certain nutrition recommendations.²²

Individuals who are new to vegetarianism or veganism also face the potential for increased carbohydrate intake, as many plant-based protein sources, such as legumes, tend to be high in carbohydrates.²³ In some cases, the same nutrient found in both meat and plant sources may not be equally available; research shows that heme iron, found exclusively in animal products, is more bioavailable, more easily absorbed, and does not require absorption-enhancing cofactors, compared to the non-heme iron found in plant sources, which is less bioavailable and its absorption depends on the balance between dietary enhancers and inhibitors and body iron stores.²⁴ 

Increased consumption of certain foods containing so-called “antinutrients” as part of a vegetarian or vegan diet may also influence the absorption of certain nutrients. For example, glucosinolates and goitrogens in cruciferous vegetables, such as Brussels sprouts and kale, can reduce the uptake of iodine by the thyroid,²⁵ while oxalates found in green leafy vegetables, tea, beans, and nuts bind to calcium in the stomach and gastrointestinal tract and prevent it from being absorbed.²⁶ However, food-preparation practices, such as soaking and cooking, may help to reduce antinutrient contents.²⁶ Tailoring the intake of other nutrients may also help to limit their effects; for instance, although phytic acid, found in whole grains, seeds, beans, and legumes, limits the absorption of non-heme iron,²⁷ other research has determined that the intake of vitamins A and C and beta-carotene can improve non-heme iron absorption.^27,28 

Finally, although switching to a vegetarian and vegan diet can reduce the risk of many diseases,^5,10,17,18 it can raise the risk of certain ones if the nutritional inadequacies often associated with a plant-based diet are not addressed: B12 levels that are too low may increase the risk of stroke,³⁰ and inadequate calcium and vitamin D intake may facilitate bone fracture and osteoporosis.³¹ An increase in other nutrients as part of a plant-based diet can also cause problems: for example, greater oxalate intake may facilitate kidney stone formation.²⁶ The quality of a plant-based diet may also matter in determining health: compared to healthier or higher-quality plant-based diets, unhealthy/low-quality plant-based diets have been linked to hypertension,³² metabolic syndrome,³³ and higher total and disease-specific mortality rates.³⁴ 

What About Processed Plant-based Meat Substitutes?

The introduction of new plant-based meat alternatives complicates things further. According to one study, aside from the traditional vegan diet, all examined diets with traditional minimally processed plant-based meat substitutes (pulses, legumes, and vegetables) were lower in saturated fat, sodium, and sugar than the reference diet and met daily requirements for nutrients such as calcium, potassium, magnesium, phosphorus, zinc, iron, and vitamin B12, while diets including novel highly processed plant-based meat substitutes (e.g., Impossible™ Burger [Impossible Foods, Redwood City, CA, USA], Beyond Burger® [Beyond Meat, El Segundo, CA, USA]) exceeded reference levels for saturated fat, sodium, and sugar and had below daily requirements for nutrient levels of calcium, potassium, magnesium, zinc, and vitamin B12.²⁹

Bottom Line

Vegetarianism and veganism have been practiced for centuries. Evidence supports that consuming a primarily or exclusively plant-based diet confers many health benefits, but also increases the risk of becoming deficient in some key nutrients, which may cause health issues. Like any highly processed food, consumption of commercially processed meat alternatives should be moderated. Consult with your primary care physician or a licensed dietitian or nutritionist to determine the best diet plan for you. 

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