Vegetarian nutrition

Red beans and rice

Despite the long-standing, widespread belief that vegetarians must consume grains and beans within a short time to make a complete protein that contains all 9 essential amino acids that must be supplied through diet, this has never been substantiated by research. The protein-combining theory was brought to popular attention after being promoted in Frances Moore Lappé's 1971 bestselling book Diet for a Small Planet. In later editions of the book, starting in 1981, Lappé withdrew her contention that protein combining is necessary.[6]

Plant foods rich in protein include soy beans and soy products such as tofu, veggie burgers, and soy milk; other legumes; nuts and seeds; and cereal grains.[7]

Vitamin B₁₂ deficiency can be extremely serious and lead to megaloblastic anemia, nerve degeneration and irreversible neurological damage.[8]

Vegetarians may get vitamin B₁₂ from eggs and dairy products (milk, cheese, etc.);[9] for some, this is adequate, while others may still remain B₁₂-deficient.[1] More broadly, according to the Academy of Nutrition and Dietetics, even the form of vitamin B₁₂ sourced from animal products is protein-bound and not as easily digested as supplements, especially as people age, and therefore B₁₂ supplementation is recommended for everyone over the age of 50.[1] Pregnant and lactating vegetarian mothers—and breastfed infants if the vegetarian mother's diet is not supplemented—should also use supplements, whether B₁₂-pills, B₁₂-injections, or B₁₂-fortified foods, if they don't get adequate vitamin B₁₂ from animal products like eggs or dairy.

Eggs are a source of vitamin B₁₂ for vegetarians.

Generally, humans need 2.4 to 3 micrograms of vitamin B₁₂ each day.[9] There are cases to suggest that vegetarians and vegans who are not taking vitamin B₁₂ supplements or food fortified with B₁₂ do not consume sufficient servings of B₁₂ and have abnormally low blood concentrations of vitamin B₁₂.[10][11] This is because, unless fortified, plant foods do not contain reliable amounts of active vitamin B₁₂.[1]

It is essential, therefore, that vegetarians consume adequate amounts of dietary supplements or foods that have been fortified with B₁₂, such as vegetable stock, veggie burger mixes, textured vegetable protein, soy milks, vegetable and sunflower margarines, and breakfast cereals.[8] B₁₂ used in these foods or supplements is typically grown from vegan sources (such as bacteria). Soybeans and barley seeds from plants grown in soils amended either with cow dung (which is rich in B₁₂) or with pure B₁₂ had a higher B₁₂ content than those grown without this supplementation.[12] There is a patent for cultivating plants in a solution containing vitamin B₁₂ so that the plant contains more B₁₂.[13]

Flaxseeds are a rich source of ALA, but contains negligible amounts of DHA and EPA, the long-chain omega-3 fatty acids the FDA considers to be healthful.

Vegetarian diets can be low in omega-3 fatty acids (O3FAs). Major vegetarian O3FA sources include algae, hempseeds and hempseed oil, walnuts, flaxseeds and flaxseed oil, olive oil, canola oil, avocado, and chia seeds.

A potential problem is that vegetarian diets lacking eggs or generous amounts of edible seaweed generally lack a direct source of long-chain O3FAs such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Vegetarian diets may also have a high ratio of omega-6 fatty acids to O3FAs, which inhibits the conversion of short-chain fatty acids such as alpha-Linolenic acid (ALA), which is found in most vegetarian O3FA sources, to EPA and DHA.[1] Short-term supplemental ALA has been shown to increase EPA levels but not DHA levels, suggesting poor conversion of the intermediary EPA to DHA.[14] To remedy this, DHA and EPA supplements derived from microalgae are available.[1]

Walnuts are a source of omega-3 fatty acids.

There is only weak evidence that O3FAs benefit cardiovascular health,[15] and although O3FAs have been thought useful for helping to alleviate dementia, as of 2016 there is no good evidence for their effectiveness in this regard.[16]

While there is little evidence of adverse health or cognitive effects caused by DHA deficiency in adult vegetarians, fetal and breast milk levels remain a concern for pregnant or lactating mothers.[14] EPA and DHA supplementation has been shown to reduce blood platelet aggregation in vegetarians, but a direct link to cardiovascular morbidity and mortality, which is already lower for vegetarians, has yet to be determined as of 2000.[17]

In several studies, vegetarians were not found to suffer from iron (Fe) deficiency more than those who ate meat.[18][19][20][21] However, while one study agreed that iron-deficiency anemia is not more common among vegetarians, they found "vegetarian children had ... reduced levels of haemoglobin and iron compared to omnivores" due "to the absence of animal iron sources with high utilizability".[22] Another study done in India found that "strict vegetarian mothers as well as their newborns have a greater incidence and risk of anemia and iron deficiency."[23]

Tofu, a soy product, can be a valuable source of not only iron, but also protein, zinc and calcium for vegetarians.

The recommended iron intake for vegetarians is 180% that of nonvegetarians,[1] because plants, dairy, and eggs contain only non-heme iron, and this is absorbed less efficiently than heme iron.[22] Although a lower percentage of non-heme iron is absorbed by the body, greater total amounts of non-heme iron are concentrated in many non-meat sources of iron, and therefore breakfast cereals, eggs, nuts, seeds, and legumes (including soy foods, peas, beans, chickpeas, and lentils) are significant sources of iron,[24] and a well-planned vegetarian diet should not lead to iron deficiency, but fruitarianism and raw foods diets should not be pursued for infants or children.[1]

Meat, including fish and poultry, and not dairy or eggs, is the only source of heme iron; intake of heme iron may be associated with colon cancer.[25] Non-heme iron is more sensitive to both inhibitors and enhancers of iron absorption: Vitamin C is an iron absorption enhancer; the main inhibitors for most people are phytates (e.g. legumes and cereal grains), but other inhibitors include tannins (from tea and wine), calcium, and polyphenols.[1]

Iron is an integral part in the chemical structure of many proteins and enzymes, which maintain good health. In humans, iron is an essential component of proteins involved in the transport of oxygen of red blood cells. Iron also helps regulate cell growth and cellular differentiation.[26]

A 2013 review found that zinc intake and serum zinc concentrations were significantly lower in populations that follow vegetarian diets compared with non-vegetarians.[27]

Phytates in many whole grains, and dietary fiber in many plant foods may interfere with zinc absorption, and marginal zinc intake has poorly understood effects. Vegetarians may need more than the US Recommended Daily Allowance of 15 mg of zinc each day to compensate if their diet is high in phytates.[1] Major plant sources of zinc include cooked dried beans, edible seaweed, fortified breakfast cereals, soy products, nuts, peas, and seeds.[1]

One study reported a "potential danger of [iodine] (I) deficiency disorders due to strict forms of vegetarian nutrition, especially when fruits and vegetables grown in soils with low [iodine] levels are ingested."[28] Iodine, however, is usually supplied by iodized salt and other sources in first world countries. Other significant vegetarian sources of iodine include edible seaweed and bread made with dough conditioners.[1]