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If well-planned and fortified to balance possible deficiencies, vegetarian diets can become nutritionally adequate and can be appropriate for all stages of the human 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, or when not enough calories are consumed, these nutrients can be dangerously low and may compromise children's health and development.
Evidence suggests that vegetarians generally 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 possibly low in saturated fat, cholesterol, and animal protein.
The Oxford Vegetarian Study showed that the health of vegetarians compares favourably with those who eat meat (except for pescetarians). British vegetarians have lower death rates than those who eat meat, although this is at least partly due to lifestyle factors beyond diet, such as a low prevalence of smoking, and the generally high socioeconomic status of vegetarians, or to certain aspects of the diet other than the specific avoidance of meat and fish.
The School of Public Health at Loma Linda University 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. Then, 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 red meat in the form of processed meats: 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, however, evidence that vegetarians tend to have some positive health attributes: a lower body mass index (BMI), 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 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.
Vegetarians may get vitamin B12 from eggs and dairy products (milk, cheese, etc.); for some, this is adequate, while others may still remain B12-deficient. 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. 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.
Generally, humans need 2.4 to 3 micrograms of vitamin 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 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. There is a patent for cultivating plants in a solution containing vitamin B12 so that the plant contains more B12.
Omega-3 fatty acids
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. Short-term supplemental ALA has been shown to increase EPA levels but not DHA levels, suggesting poor conversion of the intermediary EPA to DHA. To remedy this, DHA and EPA supplements derived from microalgae are available.
There is only weak evidence that O3FAs benefit cardiovascular health, and although O3FAs have been thought useful for helping to alleviate dementia, as of 2016[update] there is no good evidence for their effectiveness in this regard.
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. 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[update].
In several studies, vegetarians were not found to suffer from iron (Fe) deficiency more than those who ate meat. 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 done 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 180% 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 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, 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, 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. 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.
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.
Western vegetarians and vegans have not been found to suffer from overt zinc (Zn) deficiencies any more than those who eat meat. However, 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.
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." 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.
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