Plant-Based Diets: A Complete Science-Based Guide

Plant-based eating went from niche to mainstream in the last decade. The Academy of Nutrition and Dietetics, the world's largest organization of food and nutrition professionals, stated in its 2016 position paper that appropriately planned vegetarian and vegan diets are healthful, nutritionally adequate, and may provide health benefits for prevention and treatment of certain diseases. The position is supported by the EPIC-Oxford cohort, the Adventist Health Studies, and a substantial randomized trial literature. But "plant-based" is a slippery term that includes everything from a strict whole-food vegan diet to a flexitarian pattern that occasionally includes meat. This guide covers the definitions, the evidence, the nutritional considerations, and a practical transition plan.

What "plant-based" actually means

The term "plant-based" has no formal definition, which is part of its appeal and part of its problem. In practice, four patterns are usually meant:

• Vegan: excludes all animal products, including meat, dairy, eggs, honey, and gelatin
• Lacto-ovo vegetarian: excludes meat, poultry, and fish, but includes dairy and eggs
• Pescatarian: excludes meat and poultry but includes fish, dairy, and eggs
• Flexitarian: predominantly plant-based but occasionally includes meat or fish (sometimes defined as meat 2 or fewer days per week)
• Whole-food plant-based (WFPB): vegan but emphasizes minimally processed whole foods (whole grains, legumes, vegetables, fruits, nuts, seeds) and minimizes refined oils, added sugars, and processed vegan products

The nutritional and health implications differ meaningfully across these patterns. A WFPB diet is nutritionally dense and consistently associated with health benefits. A vegan diet that relies heavily on processed meat substitutes, refined grains, and sweets is essentially an ultra-processed diet without animal products, and the health effects are much less favorable.

What the major cohort studies show

The Adventist Health Studies

The Adventist Health Studies, conducted in a population of Seventh-day Adventists in California (a population with unusually healthy behaviors including low rates of smoking and alcohol use), have produced some of the strongest evidence on vegetarian patterns. The Adventist Health Study 2 (AHS-2), with about 96,000 participants, has shown that vegetarians have lower BMI, lower rates of type 2 diabetes (about half the rate of non-vegetarians), lower rates of hypertension, and lower all-cause mortality. Vegans had the lowest BMI (average 23.6 vs 28.8 for non-vegetarians) and the lowest rate of type 2 diabetes.

Within the AHS-2, the healthiest subgroup is the "Blue Zone" of Loma Linda, California, one of the world's longest-living populations. Their pattern is largely plant-based, with regular nuts, beans, whole grains, and water, and minimal meat.

EPIC-Oxford

The European Prospective Investigation into Cancer and Nutrition-Oxford cohort follows about 65,000 people in the UK, including a large vegetarian subgroup. EPIC-Oxford has shown that vegetarians have lower rates of ischemic heart disease (about 32 percent lower than non-vegetarians in a 2013 follow-up) but similar or slightly higher rates of stroke. The stroke finding is interesting and consistent with concerns about low B12 status affecting vascular health in some vegetarians.

The pooled evidence

A 2019 systematic review and meta-analysis by Dinu and colleagues in Critical Reviews in Food Science and Nutrition pooled data from observational studies on vegetarian and vegan diets. Vegetarians had significantly lower BMI, lower total cholesterol, lower LDL cholesterol, and lower rates of ischemic heart disease and cancer than non-vegetarians. Vegans had the most favorable cardiometabolic profile, but also the highest risk of inadequate B12, iron, and zinc intake.

What the randomized trials show

Randomized trials have tested plant-based diets for specific conditions with mostly positive results.

A 2016 randomized trial by Yokoyama and colleagues in Nutrition Research found that a WFPB vegan diet improved cardiometabolic parameters in overweight adults over 6 months more than the American Heart Association diet. A 2017 randomized trial by Macknin and colleagues in the Journal of Pediatrics found that a WFPB vegan diet in adults with cardiovascular risk factors improved BMI, blood pressure, and lipid profiles over 4 weeks more than the American Heart Association diet.

The landmark BROAD study, published in 2017 by Wright and colleagues in Nutrition & Diabetes, randomized 65 adults with overweight or obesity and at least one chronic condition to a WFPB diet or normal care for 12 months. The WFPB group lost significantly more weight (average 12 kg vs 1.5 kg), improved HbA1c, and reduced LDL cholesterol, with high adherence rates.

For type 2 diabetes specifically, the work of Neal Barnard and the Physicians Committee for Responsible Medicine has produced multiple randomized trials showing that low-fat WFPB vegan diets improve glycemic control and reduce medication needs in type 2 diabetes, often comparably to or better than conventional diabetes diets.

Nutritional considerations

A poorly planned plant-based diet can be deficient in several nutrients. The Academy of Nutrition and Dietetics position paper emphasizes "appropriately planned." The nutrients that need the most attention are:

Vitamin B12

B12 is produced by bacteria and is found almost exclusively in animal products. Vegans who do not supplement will develop deficiency, which causes megaloblastic anemia and irreversible neurological damage. The Cochrane review and multiple case series document B12 deficiency in unsupplemented vegans, with rates approaching 50 percent in some populations. The Endocrine Society recommends 250 to 500 micrograms per day of supplemental B12 for vegans. The RDA for adults is 2.4 micrograms per day, but absorption from supplements is dose-dependent (about 1 percent of an oral dose is absorbed via passive diffusion), so larger doses are needed. Test your B12 status with the B12 Interpreter and track intake with the Vitamin B12 Calculator.

Iron

Plant iron (non-heme) is absorbed at about one-third the rate of animal iron (heme). Vegetarians need about 1.8 times the standard RDA. The Institute of Medicine set the RDA for vegetarians at 14 mg per day for men and 32 mg per day for premenopausal women. Practical strategies include combining iron-rich plant foods with vitamin C, separating iron-rich meals from coffee, tea, and dairy, and cooking acidic foods in cast iron. See our Iron Deficiency Anemia Guide for details.

Zinc

Zinc is less bioavailable from plant foods due to phytates, which bind zinc and inhibit absorption. Vegetarians need about 1.5 times the standard RDA (11 mg per day for men, 8 mg per day for women, scaled up). Practical strategies include soaking and sprouting legumes and grains to reduce phytate, choosing zinc-rich foods (pumpkin seeds, cashews, chickpeas, oats, tofu), and considering a low-dose supplement. Track with the Zinc Calculator.

Omega-3 fatty acids

Plant sources of omega-3 (ALA from flax, chia, walnuts, and hemp) convert to EPA and DHA inefficiently (5 to 10 percent for EPA, less than 1 percent for DHA). Vegans and vegetarians have lower blood EPA and DHA levels than omnivores. The International Society for the Study of Fatty Acids and Lipids recommends that vegetarians and vegans consume algae-derived DHA supplements (200 to 300 mg per day). See our Omega-3 EPA and DHA Science Guide for details.

Calcium and vitamin D

Vegans who do not consume dairy need to ensure adequate calcium from fortified plant milks (most are fortified to match cow's milk), tofu made with calcium sulfate, leafy greens (collards, kale, bok choy), and almonds. The RDA for adults is 1,000 to 1,200 mg per day. Vitamin D status depends on sun exposure and fortified foods. See our Vitamin D Testing and Supplementation Guide.

Protein

The "where do you get your protein" question is overblown. The Academy of Nutrition and Dietetics position paper confirms that vegetarian and vegan diets can meet protein needs easily. Legumes (beans, lentils, chickpeas, peas), soy products (tofu, tempeh, edamame), nuts, seeds, and whole grains all contribute substantial protein. The protein RDA is 0.8 grams per kg of body weight per day, but most adults benefit from 1.0 to 1.6 g per kg, particularly older adults and athletes. A varied plant diet typically hits 12 to 15 percent of calories from protein without effort. Track with the Protein Calculator.

Iodine

Iodine is found primarily in dairy, seafood, and iodized salt. Vegans who do not use iodized salt and do not eat sea vegetables may have low intake. The American Thyroid Association recommends that pregnant and lactating women take a prenatal vitamin with 150 micrograms of iodine. Track with the Iodine Calculator.

Vitamin K2

Vitamin K2 is found primarily in animal products (especially natto, a fermented soy product, and certain cheeses). Plant foods are rich in K1 (phylloquinone, found in leafy greens) but not K2 (menaquinone). The body converts some K1 to K2, but the conversion is inefficient. Vegans can get K2 from natto if they tolerate it. The clinical significance of low K2 intake in vegans is debated.

What the evidence actually supports

The strongest evidence for plant-based diets is for cardiovascular disease prevention. The EPIC-Oxford and Adventist data, plus the LDL-lowering effects demonstrated in randomized trials, support that vegetarians and vegans have lower LDL cholesterol and lower rates of ischemic heart disease. The effect is partly due to lower saturated fat intake and higher fiber intake, and partly to the displacement of red and processed meats.

For type 2 diabetes, the evidence is strong. Adventist data shows about half the rate of type 2 diabetes in vegetarians compared with non-vegetarians. Randomized trials of WFPB vegan diets in people with type 2 diabetes show improved glycemic control and reduced medication needs.

For cancer, the evidence is more modest. Vegetarians have slightly lower rates of overall cancer in the Adventist cohort, with the strongest reductions for colorectal cancer. The EPIC-Oxford data is similar. The mechanism is partly the displacement of red and processed meats (classified as carcinogens by the WHO IARC), partly the higher fiber intake, and partly the lower obesity rates.

For all-cause mortality, the Adventist data shows lower mortality in vegetarians (about 10 to 15 percent lower), but the EPIC-Oxford data shows a smaller and sometimes null effect. The difference may relate to the overall health behaviors of the Adventist population (no smoking, no alcohol, regular exercise) versus the more general UK vegetarian population.

For bone health, the data is mixed. Some studies show higher fracture rates in vegans, particularly vegans with low calcium intake. A 2020 EPIC-Oxford follow-up found that vegans had higher rates of total and hip fracture than meat-eaters, but this was attenuated when calcium and protein intakes were adequate. The implication: vegans must pay attention to calcium and protein, not just B12.

For cognitive function, the data is limited and mixed. There is no clear evidence that plant-based diets harm cognition in adults with adequate B12, but deficiency of B12 is itself associated with cognitive decline, so vigilance is needed.

What plant-based does not automatically mean

Plant-based does not automatically mean healthy. A diet of french fries, soda, white bread, and vegan cookies is technically vegan. The Beyond Burger has similar calorie and saturated fat content to a beef burger. Coconut oil is highly saturated. Many processed vegan products are ultra-processed.

The NOVA classification of food processing classifies ultra-processed foods as a category associated with obesity, type 2 diabetes, and cardiovascular disease regardless of whether they are plant-based or animal-based. A 2024 randomized trial by Hall and colleagues at the NIH found that adults eating an ultra-processed diet ate about 500 more calories per day and gained weight compared with adults eating a minimally processed diet, even when matched for macronutrients.

The implication: a "whole-food plant-based" diet is well-supported by evidence. A "plant-based" diet that includes lots of processed foods is not. See our Ultra-Processed Foods Guide for more on this.

A practical transition plan

For most people, a gradual shift is more sustainable than an overnight change. A reasonable 6-week transition:

Week 1: Audit

Keep a food diary for a week. Identify your current meat, dairy, and egg intake. Note where plants already appear in your diet.

Week 2: Add, don't subtract

Add one new plant-based meal per day. Try oatmeal with nuts and berries for breakfast, or a bean salad for lunch. Don't try to eliminate anything yet.

Week 3: Swap the easy ones

Swap cow's milk for fortified soy or pea milk (these have the most similar protein profile). Swap beef tacos for black bean tacos. Swap chicken stir-fry for tofu stir-fry.

Week 4: Tackle dinner

Make 3 to 4 dinners per week vegetarian. Try lentil soup, chickpea curry, tofu stir-fry, or pasta with white bean sauce. Cook extra for leftovers.

Week 5: Take stock

Assess how you feel. Energy, digestion, satiety. Adjust portion sizes if hungry. Ensure you are getting enough protein (legumes, soy, whole grains) and supplement with B12.

Week 6: Decide your pattern

Choose the pattern that fits your life. Flexitarian (3 to 4 plant-based days per week), vegetarian, or fully plant-based. The best pattern is the one you can sustain.

Use the Macro Calculator and Fiber Calculator to track your nutrition during transition.

Common pitfalls

• Not supplementing B12. This is the single most important supplement for vegans. No exceptions.
• Relying on processed meat substitutes. Beyond Burgers, Impossible Burgers, and similar products are ultra-processed and should be occasional, not daily.
• Eating too little protein. Plant proteins are slightly less bioavailable, so total intake needs to be a bit higher. Aim for 1.0 to 1.6 g per kg of body weight.
• Forgetting about omega-3s. Algae-derived DHA is the most reliable vegan source.
• Going too low-fat. Very low-fat plant-based diets can be unsatisfying and hard to sustain. Include nuts, seeds, avocado, and olive oil.
• Not getting enough calcium. Fortified plant milks and calcium-set tofu should be regular parts of the diet.

Special populations

Children and adolescents

The Academy of Nutrition and Dietetics and the American Academy of Pediatrics state that well-planned vegetarian and vegan diets are appropriate for all stages of life, including childhood and adolescence. Children need careful attention to calories, protein, iron, zinc, calcium, vitamin D, and B12. Vegan diets for infants and toddlers require particular care, including B12 supplementation from birth for breastfed infants of vegan mothers.

Pregnancy and lactation

Vegetarian and vegan diets can support healthy pregnancy, but require careful planning. The American College of Obstetricians and Gynecologists notes the importance of B12, iron, vitamin D, omega-3, and iodine. Most prenatal vitamins cover these, but vegans should confirm adequate B12 (250 to 500 micrograms per day) and consider algae-derived DHA.

Older adults

Older adults on plant-based diets need extra attention to protein (1.0 to 1.5 g per kg), B12 absorption (which declines with age), calcium, and vitamin D. Plant proteins may need to be slightly higher than animal proteins to support muscle protein synthesis in older adults.

Athletes

Plant-based athletes can perform at elite levels (the documentary "The Game Changers" showcased several, though the science is more nuanced). The main considerations are adequate total protein, leucine intake (which is lower in plant proteins), iron status, and B12. A 2020 review in Nutrients concluded that well-planned plant-based diets can support athletic performance, with some evidence for faster recovery from reduced inflammation.

The bottom line

Appropriately planned plant-based diets are healthful, nutritionally adequate, and supported by major nutrition and dietetics organizations for all stages of life. The strongest evidence is for cardiovascular disease prevention and type 2 diabetes management. The Adventist Health Studies and EPIC-Oxford cohort, plus a substantial randomized trial literature, support that vegetarians and vegans have lower BMI, lower LDL cholesterol, lower blood pressure, and lower rates of ischemic heart disease and type 2 diabetes. The catch is that "appropriately planned" matters: vegans must supplement B12, pay attention to iron, zinc, calcium, iodine, and omega-3s, and emphasize whole foods over ultra-processed vegan products. A flexitarian pattern that includes modest amounts of high-quality animal foods is also supported by evidence and may be easier for most people to sustain. The best pattern is the one you can maintain for decades, not the one that produces the most dramatic short-term result.