Vegetarian nutrition is the set of health-related challenges and advantages of vegetarian diets.
If well-planned and fortified to balance their deficiencies, vegetarian and vegan diets can become nutritionally adequate and can be appropriate for all stages of the life cycle, including during pregnancy, lactation, infancy, childhood, and adolescence. When fortified, a vegetarian diet can provide adequate protein, iron, zinc, vitamin B12, and calcium intake. However, in non fortified vegetarian diets, and when not enough calories are consumed, these nutrients can be dangerously low and may compromise children's health and development.
Evidence suggests that vegetarians have lower rates of coronary heart disease, obesity, hypertension, type 2 diabetes, and osteoporosis. Vegetarian diets tend to be rich in carbohydrates, omega-6 fatty acids, dietary fibre, carotenoids, folic acid, vitamin C, vitamin E, potassium and magnesium. They are generally low in saturated fat, cholesterol, and animal protein.
The Oxford Vegetarian Study showed that the health of vegetarians compares favourably with that of meat-eaters (excluding pescetarians). British vegetarians have lower death rates than non-vegetarians, although this is at least partly due to non-dietary lifestyle factors, such as a low prevalence of smoking and the generally high socioeconomic status of vegetarians, or to aspects of the diet other than the avoidance of meat and fish.
Loma Linda University School of Public Health has conducted three cohort studies that identify the health benefits of a vegetarian diet. The University is a Seventh-day Adventist health science institution. The first study, funded by the US Public Health Service in 1958 and limited to Adventists in California, included many vegetarians. The next cohort of California Adventists, the Adventist Health Study-1 (AHS-1), collected data from 1974 to 1976. From 2002 to 2007 the Adventist Health Study-2 (AHS-2) collected dietary data from 96,000 church members from the United States and Canada. Many scientific articles have been published on the health and nutrition properties of a vegetarian diet from these cohort studies. The most recent AHS-2 study includes findings on metabolic syndrome, Vitamin D absorption and type-2 diabetes
In prospective studies of adults, compared to non-vegetarian eating patterns, vegetarian-style eating patterns have been associated with improved health outcomes—lower levels of obesity, a reduced risk of cardiovascular disease, and lower total mortality. Several clinical trials have documented that vegetarian eating patterns lower blood pressure.
On average, vegetarians consume a lower proportion of calories from fat (particularly saturated fatty acids); fewer overall calories; and more fiber, potassium, and vitamin C than do non-vegetarians. Vegetarians generally have a lower body mass index. These characteristics and other lifestyle factors associated with a vegetarian diet may contribute to the positive health outcomes that have been identified among vegetarians.
Vegetarians may avoid the negative health effects of processed red meat: A 1999 meta-study of five studies comparing vegetarian and non-vegetarian mortality rates in Western countries found that in comparison with regular meat eaters, mortality from ischemic heart disease was 34% lower in people who ate fish but not meat (pescetarians), 34% lower in ovo-lacto vegetarians, 26% lower in vegans and 20% lower in occasional meat eaters. A 2010 study found that heart disease is not linked with unprocessed red meat.
Studies of cancer have not shown clear differences in cancer rates between vegetarians and non-vegetarians. There is evidence that vegetarians tend to have a lower body mass index, lower risk of obesity, lower blood cholesterol levels, lower homocysteine levels, lower risk of high blood pressure, and lower risk of type 2 diabetes.
Despite a widespread belief that vegetarians must eat grains and beans within a few hours of each other to make a 'complete' protein which contains all 9 "essential amino acids", this has never been substantiated by research. The protein-combining theory was brought to popular attention in Frances Moore Lappé's 1971 bestseller Diet for a Small Planet. In later editions of the book, as early as 1981, Lappé withdrew her contention that protein 'combining' is necessary. Plant sources of protein include soy beans (and soy products such as tofu, veggie burgers, soy milk, etc.), other legumes, nuts and seeds, and grains.
In several studies, vegans and other vegetarians were not found to suffer from iron-deficiency more than non-vegetarians. 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". Another study, in India, found that "strict vegetarian mothers as well as their newborns have a greater incidence and risk of anemia and iron deficiency."
The recommended iron intake for vegetarians is 1.8 times that of nonvegetarians, because plants, dairy, and eggs contain only non-heme iron, and this is absorbed less efficiently than heme iron. Although a lower percentage of non-heme iron is absorbed, greater amounts of non-heme iron are concentrated in many non-meat sources of iron (than the amount of iron per serving in meats), and therefore, cereals, eggs, nuts, seeds, and legumes (including soy foods, peas, beans, chickpeas, and lentils) are significant sources of iron, 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. Meat, fish, and poultry (not dairy or eggs) are the only sources of heme iron; intake of heme iron may be associated with colon cancer. 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 grains), but other inhibitors include tannins (tea, wine), calcium and polyphenols.
Iron is an integral part of many proteins and enzymes which maintain good health. In humans, iron is an essential component of proteins involved in red blood cells' oxygen transport. Iron also helps regulate cell growth and differentiation.
Western vegetarians and vegans have not been found to suffer from overt zinc deficiencies any more than meat-eaters. However, phytates in many whole-grains and fiber in many foods may interfere with zinc absorption and marginal zinc intake has poorly understood effects. Vegetarians may need more than the US RDA (15 mg) of zinc daily if their diet is high in phytates.
Lacto-ovo-vegetarians may get vitamin B12 from eggs and dairy products (milk, cheese, etc.); for some this is adequate but some may still remain B12-deficient. According to the Academy of Nutrition and Dietetics, the form of vitamin B12 sourced from animal-products is protein-bound and not as easily digested, especially as people age, and therefore recommends B12 supplementation for everyone over the age of 50. Pregnant and lactating vegetarians (and breastfed infants if the mother's diet is not supplemented) should also use supplements, whether pills, injections, or B12-fortified foods, if they don't get adequate B12 from animal-products like eggs or dairy.
Generally, humans need 2.4-3 micrograms of B12 each day. Although some argue that developing a disease from B12 deficiency by following a sensible diet is extremely rare—less than one chance in a million, 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. This is because, unless fortified, plant foods do not contain reliable amounts of active vitamin B12.
It is essential, therefore, that vegetarians consume adequate amounts of dietary supplements or foods that have been fortified with B12 (such as nutritional yeast or other yeast extracts, vegetable stock, veggie burger mixes, textured vegetable protein, soy milks, vegetable and sunflower margarines, and breakfast cereals). B12 that is to be used in these supplements is typically grown from vegan sources (such as bacteria). Soybean and barley seeds grown in soils amended with cow dung (which is naturally rich in B12), or with pure B12 had a higher B12 content than those grown without this supplementation. There is a patent for cultivating plants in a solution containing vitamin B12 so that the crop contains more B12.
Omega-3 fatty acids
Vegetarian diets can be low in omega-3 fatty acids (O3FA). Major vegetarian 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 fish, eggs, or generous amounts of sea vegetables (seaweed) generally lack a direct source of long-chain O3FA such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Vegetarian diets 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 vegetarian 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 and EPA supplements derived from microalgae are available.
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 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.
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." Iodine, however, is usually supplied by iodized salt and other sources in first world countries. Other significant sources of iodine include sea vegetables (seaweed) and bread made with dough conditioners.
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