While a well-planned, balanced vegan diet is suitable to meet all recommendations for nutrients in every stage of life, improperly-planned vegan diets may be deficient in vitamin B12, vitamin D, calcium, iodine, iron, zinc, riboflavin (vitamin B2), long-chain fatty acids EPA and DHA, and omega-3 fatty acids.
Doctors Dean Ornish, T. Colin Campbell, John A. McDougall, Michael Klaper, Caldwell Esselstyn, Michael Greger, Joel Fuhrman, and Neal D. Barnard conclude that high animal fat and protein diets, such as the standard American diet, are detrimental to health. They also state that a lifestyle change incorporating a vegan whole foods diet could not only prevent various degenerative diseases, such as coronary artery disease, but reverse them. A number of documentary films, such as Fat, Sick and Nearly Dead, Planeat and Forks over Knives, focus on the purported health benefits of plant-based diets. Although there is general consensus amongst doctors advocating plant-based diets, Joel Fuhrman and Michael Greger have disagreed with Campbell and Esselstyn on the use of nuts and seeds.
Some athletes, such as Brendan Brazier, follow a vegan diet, including raw veganism. Other examples of endurance and strength include ultramarathon runner Scott Jurek, who has set several records, and has won two dozen ultramarathons on a vegan diet, and strongman competitor Patrik Baboumian, who has set 4 strongman world records while vegan.
Vegan diets tend to be higher in dietary fibre, magnesium, folic acid, vitamin C, vitamin E, iron, and phytochemicals, and lower in calories, saturated fat, cholesterol, long-chain omega-3 fatty acids, vitamin D, calcium, zinc, and vitamin B12. Because uncontaminated plant foods do not provide vitamin B12 (which is produced by microorganisms such as bacteria), researchers agree that vegans should eat foods fortified with B12 or take a supplement.
The American Academy of Nutrition and Dietetics (formerly known as the American Dietetic Association), the American Dietetic Association and Dietitians of Canada and the British Dietetic Association state that well-planned vegan diets can meet all nutrient requirements and are appropriate for all stages of the life cycle, including during pregnancy, lactation, infancy, childhood, and adolescence, while the German Society for Nutrition does not recommend vegan diets for children, adolescents, or during pregnancy and breastfeeding. The Academy of Nutrition and Dietetics adds that well-planned vegan diets are also appropriate for older adults and athletes, and that vegan diets can reduce the risk of certain health conditions, including ischemic heart disease (coronary artery disease), type 2 diabetes, hypertension, certain types of cancer, obesity, and chronic disease.
Special attention may be necessary to ensure that an all-plant (vegan) diet will provide adequate amounts of vitamin B12, omega-3 fatty acids, vitamin D, calcium, iron, zinc, and iodine. These nutrients may be available in plant foods, with the exception of vitamin B12, which can only be obtained from B12 fortified vegan foods or supplements. Iodine may also require supplements (iodized salt).
Because natural food sources of vitamin B12 are limited to foods that come from animals, the UK Vegan Society, the Vegetarian Resource Group, and the Physicians Committee for Responsible Medicine, among others, recommend that every vegan consume adequate B12 from either fortified foods or by taking a supplement.
Vitamin B12 deficiency is potentially extremely serious, leading to megaloblastic anemia, nerve degeneration and irreversible neurological damage. Because B12 is stored in large amounts in the liver, deficiency in adults may only begin years after moving to a diet which lacks B12. For infants and young children who have not built up these stores, onset of B12 deficiency can be faster and supplementation for vegan children is thus crucial.
Evidence shows that vegans who are not taking vitamin B12 supplements do not consume sufficient B12 and often have abnormally low blood concentrations of vitamin B12. This is because, unless fortified, plant foods do not contain reliable amounts of active vitamin B12. Vegans are recommended to do one of the following dietary options:
- Consume fortified foods 2-3 times per day to get at least 3 micrograms of vitamin B12,
- or take 10 micrograms of B12 as a supplement once per day
- or take 2000 micrograms of B12 as a supplement once per week
B12 is more efficiently absorbed in small regular doses, which explains why the quantity required rises so quickly as frequency goes down.
The US National Institutes of Health recommends B12 intake in a range from 0.4 micrograms a day for infants, to 2.4 micrograms for adults, and up to 2.8 micrograms for nursing mothers.  The European Food Safety Authority set the Adequate Intake at 1.5 micrograms for infants, 4 micrograms for children and adults, and 4.5 and 5 micrograms during pregnancy and nursing. 
These amounts can be obtained by eating B12 fortified foods, which include some common breakfast cereals, soy milks, and meat analogues, as well as from common multivitamins such as One-A-Day and Flintstones. Some of the fortified foods require only a single serving to provide the recommended B12 amounts. 
Other B12 fortified foods include: some almond milks, coconut milks, other plant milks, nutritional yeast, vegan mayonnaise, tofu, and various types and brands of vegan deli slices, burgers, and other veggie meats.
Proteins are composed of amino acids and essential amino acids cannot be synthesised by the human body. A 1994 study found a varied intake of plant sources can be adequate. Foods high in protein in a vegan diet include beans, lentils, soy products (such as Tofu, Edamame, and Tempeh), nuts, seeds, grains and grain products (such as seitan).
Omega-3 fatty acids
Vegan diets can be low in omega-3 fatty acids (O3FA). Major vegan sources of O3FA include algae, hempseeds and hempseed oil, walnuts, flaxseeds and flaxseed oil, olive oil, canola (rapeseed) oil, avocado and chia seeds. However, diets lacking generous amounts of sea vegetables (seaweed) generally lack a direct source of long-chain O3FA such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Vegan diets, like the standard American diet, may also have a high ratio of O6FA to O3FA, which inhibits the conversion of short-chain fatty acids such as alpha-Linolenic acid (ALA), found in most vegan O3FA sources, to EPA and DHA. Short-term supplemental ALA has been shown to increase EPA levels but not DHA levels, suggesting poor conversion of the intermediary EPA to DHA. DHA supplements derived from DHA-rich microalgae are available, and the human body can also convert DHA to EPA.
There is only weak evidence that omega-3 benefits cardiovascular health, and although omega-3 has previously been thought useful for helping alleviate dementia, as of 2016[update], there is no good evidence of effectiveness.
While there is little evidence of adverse health or cognitive effects due to DHA deficiency in adult vegetarians or vegans, fetal and breast milk levels remain a concern. EPA and DHA supplementation has been shown to reduce platelet aggregation in vegetarians, but a direct link to cardiovascular morbidity and mortality, which is already lower for vegetarians, has yet to be determined.
The most concentrated sources of these fatty acids are seeds, such as flax, chia, camelina, canola, and hemp, as well as walnuts, and their oils. Supplements are also available.
It is recommended that vegans eat three servings per day of a high-calcium food, such as fortified soy milk, other plant based milks, almonds, hazelnuts, kale, collard greens, Chinese greens, etc., and take a calcium supplement or other calcium-fortified foods as necessary.
Many studies have examined possible correlation between veganism, calcium intake, and bone health. The EPIC-Oxford study suggested that vegans who consumed 525 mg or less of calcium per day have an increased risk of bone fractures over meat eaters and vegetarians, but that vegans consuming more than 525 mg/day had a risk of fractures similar to other groups. Overall, the entire group of vegans had a higher risk of fractures. A 2009 study of bone density found the bone density of vegans was 94 percent that of omnivores, but deemed the difference clinically insignificant. Another study in 2009 by the same researchers examined over 100 vegan post-menopausal women, and found that their diet had no adverse effect on bone mineral density (BMD) and no alteration in body composition. Biochemist T. Colin Campbell suggested in The China Study (2005) that osteoporosis is linked to the consumption of animal protein because, unlike plant protein, animal protein increases the acidity of blood and tissues, which is then neutralized by calcium pulled from the bones resulting in hypercalciuria. Campbell wrote that his China-Oxford-Cornell study of nutrition in the 1970s and 1980s found that, in rural China, "where the animal to plant ratio [for protein] was about 10 percent, the fracture rate is only one-fifth that of the U.S."
Calcium is one component of the most common type of human kidney stones, calcium oxalate. Some studies suggest that people who take supplemental calcium have a higher risk of developing kidney stones, and these findings have been used as the basis for setting the recommended daily intake (RDI) for calcium in adults.
One study reported a "potential danger of iodine deficiency disorders due to strict forms of vegetarian nutrition, especially when fruits and vegetables grown in soils with low [iodine] levels are ingested." Vegan diets typically require special attention for iodine, as it is generally only available from sea vegetables and iodized salt and supplements. The iodine content of sea vegetables varies widely and may provide more than the recommended upper limit of iodine intake.
It is recommended for vegans to eat iron-rich foods and vitamin C daily. In several studies, vegans were not found to suffer from iron-deficiency any more than non-vegans. However, due to the low absorption rate on non-heme iron it is recommended to eat dark leafy greens (and other sources of iron) together with sources of Vitamin C.
Due to lack of evidence, no country has published a recommended daily intake for choline. The Australian, New Zealand, and European Union national nutrition bodies note there have been no reports of choline deficiency in the general population. There are, however, Adequate Intakes such as the European Union's number of 400 mg/day for adults, and the US's number of 425 mg/day for adult women and 550 mg/day for adult men.
Choline deficiency, as created in lab conditions, can lead to health problems such as liver damage, a result of liver cells initiating programmed cell death (apoptosis), as well as an increase in neural tube defects in pregnant women. In a study, 77% of men, 44% of premenopausal women, and 80% of postmenopausal women developed fatty liver or muscle damage due to choline deficiency, showing that subject characteristics regulate the dietary requirement. There is also some evidence that choline is an anti-inflammatory as well, but further studies are needed to confirm/refute findings. It is worth noting that many multivitamins do not contain the Adequate Intake of choline.
Although many animal products, like liver and egg, contain high amounts of choline (355 mg/3 oz and 126 mg/large egg, respectively), wheat germ (172 mg/cup), brussel sprouts (63 mg/cup), and broccoli (62 mg/cup) are also good sources of choline. Other sources include soy lecithin, cauliflower, spinach, wheat germ, firm tofu, kidney beans, quinoa and amaranth.
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- For vitamin D and calcium: Appleby, P; Roddam, A; Allen, N; Key, T (2007). "Comparative fracture risk in vegetarians and nonvegetarians in EPIC-Oxford". European Journal of Clinical Nutrition. 61 (12): 1400–6. doi:10.1038/sj.ejcn.1602659. PMID 17299475.
- For iron: "Iron deficiency—adults". Better Health Channel. Government of Victoria, Australia. Retrieved February 4, 2011.
High-risk groups such as vegetarians, adolescent girls and women athletes need to eat iron-rich foods each day (combined with foods that are high in vitamin C). …Vegetarians who exclude all animal products from their diet may need almost twice as much dietary iron each day as non-vegetarians. Sources include dark green leafy vegetables—such as spinach—and raisins, nuts, seeds, beans, peas, and iron-fortified cereals, breads and pastas.[unreliable medical source?]
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- Also see Nijjar, Raman. "From pro athletes to CEOs and doughnut cravers, the rise of the vegan diet", CBC News, June 4, 2011.
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The key role of ascorbic acid for the absorption of dietary nonheme iron is generally accepted. The reasons for its action are twofold: (1) the prevention of the formation of insoluble and unabsorbable iron compounds and (2) the reduction of ferric to ferrous iron, which seems to be a requirement for the uptake of iron into the mucosal cells.
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Although choline is essential, there appear to have been no reports of deficiency in the general population. Deficiencies have been seen in experimental situations and also in total parenteral nutrition (Buchman et al. 1992, 1993, 1995, Chalwa et al. 1989, Shapira et al. 1986, Sheard et al. 1986).
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