Vegan nutrition refers to the nutritional and human health aspects of vegan diets. A well-planned, balanced vegan diet is suitable to meet all recommendations for nutrients in every stage of human life. Improperly planned vegan diets may be deficient in vitamin B12, vitamin D, calcium, iodine, iron, zinc and riboflavin (vitamin B2). Preliminary evidence from epidemiological research indicates that a vegan diet may lower the risk of cancer.
Vegan diets, compared to standard diets, tend to be higher in dietary fibre, magnesium, folic acid, vitamin C, vitamin E, and iron, and lower in calories, saturated fat, cholesterol, long-chain omega-3 fatty acids, vitamin D, calcium, zinc, and vitamin B12. Because plant foods do not provide vitamin B12 (which is produced by microorganisms such as bacteria), researchers agree that those on a vegan diet should eat foods fortified with B12 or take a dietary supplement.
The American Academy of Nutrition and Dietetics (formerly known as the American Dietetic Association), Dietitians of Canada and the British Dietetic Association state that well-planned vegan diets can meet all human nutrient requirements and are appropriate for all stages of life, 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 American Academy of Nutrition and Dietetics adds that well-planned vegan diets are also appropriate for older adults and athletes, and that vegan diets may reduce the risk of certain health conditions, such as cancer.
Special attention may be necessary to ensure that a 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 supplementation, such as using iodized salt.
Vitamin B12 is not made by plants or animals, but by bacteria that grow in soil, feces, dirty water or laboratories, so plant foods are not reliable sources of B12. 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. Some of the fortified foods require only a single serving to provide the recommended B12 amounts. 
Other B12 fortified foods may 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.
Upon digestion, all protein foods supply amino acids. Varied intake of plant foods can meet human health needs for protein and amino acids. Foods high in protein in a vegan diet include legumes (such as beans and lentils), nuts, seeds, and grains (such as oats, wheat, and quinoa).
Omega-3 fatty acids
Diets without seafood are lower in non-essential long-chain omega-3 fatty acids like DHA and EPA. Short-term supplemental ALA has been shown to increase EPA levels, but not DHA levels, suggesting limited 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. 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.
It is recommended that vegans eat three servings per day of a high-calcium food, such as fortified plant milks, green leafy vegetables, seeds, tofu, or other calcium-rich foods, and take a calcium supplement 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 he believed was 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, for which the only substantial and reliable vegan sources are sea vegetables, 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. Iron supplementation should be taken at different times to other supplements with a 2+ valence (chemistry) such as calcium or magnesium, as they inhibit the absorption of iron.
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Due to lack of evidence, no country has published a recommended daily intake for choline, which is a vitamin-like essential nutrient. 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 non-pregnant 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.
The main function of vitamin D in the body is increased absorption of calcium, therefore it is good for the bones. It also connects with receptors in the prostate, the heart, blood vessels, muscles, endocrine glands and others. 
Sunlight, fortified foods, and supplements, are the main sources of vitamin D for vegans. Humans produce vitamin D naturally in response to ultraviolet light (UV). Complete cloud cover reduces UV penetration by up to 50%, and outdoor shade reduces UV penetration by 60%. UV light penetrates the skin at wavelengths between 290 and 320 nanometers, where it is then converted into vitamin D3.  Vitamin D2 can be obtained from fungi, such as mushrooms exposed to sun or industrial ultraviolet light, offering a vegan choice for dietary or supplemental vitamin D.
Although vitamin D3 is produced in small amounts by lichens or algae exposed to sunlight, industrial production in commercial quantities is limited, and there are few supplement products as of 2019.
Reports on exactly how much vitamin D is produced by exposure to sunlight are wild and varied, across the board.
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