Vegetarianism is considered a healthy, viable diet. The American Dietetic Association and the Dietitians of Canada have found a properly-planned vegetarian diet to satisfy the nutritional needs for all stages of life, and large-scale studies have shown vegetarianism to significantly lower risks of cancer, ischaemic heart disease, and other fatal diseases. Necessary nutrients, proteins, and amino acids for the body's sustenance can be found in green leafy vegetables, grains, nuts, and fortified juices or soymilk.
Vegetarian diets can aid in keeping body weight under control and substantially reduce risks of heart disease and osteoporosis. Non-lean red meat, in particular, has been found to be directly associated with dramatically increased risk of cancers of the lung, oesophagus, liver, and colon. Other studies have shown that there were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, breast cancer, or prostate cancer.
The American Dietetic Association and Dietitians of Canada have stated: "Vegetarian diets offer a number of nutritional benefits, including lower levels of saturated fat, cholesterol, and animal protein as well as higher levels of carbohydrates, fiber, magnesium, potassium, folate, and antioxidants such as vitamins C and E and phytochemicals."
Nutrition
A fruit stall in Barcelona
Western vegetarian diets are typically high in carotenoids, but relatively low in long-chain n-3 fatty acids, and vitamin B12. Vegans can have particularly low intake of vitamin B and calcium if they do not eat enough items such as collard greens, leafy greens, tempeh and tofu. High levels of dietary fibre, folic acid, vitamins C and E, and magnesium, and low consumption of saturated fat could all be beneficial aspects of a vegetarian diet.
Protein
Protein intake in vegetarian and vegan diets is only slightly lower than in meat diets and can meet daily requirements for any person, including athletes and bodybuilders. Studies by Harvard University as well as other studies conducted in the United States, Great Britain, Canada, Australia, New Zealand and various European countries, have confirmed that vegetarian diets provide more than sufficient protein intake as long as a variety of plant sources are available and consumed. Proteins are composed of amino acids, and a common concern with protein acquired from vegetable sources is an adequate intake of the "essential amino acids", which cannot be synthesised by the human body. While dairy and egg products provide complete sources for lacto-ovo vegetarians, the only vegetable sources with significant amounts of all eight types of essential amino acids are lupin, soy, hempseed, chia seed, amaranth, buckwheat and quinoa. It is not necessary, however, to obtain protein from these sources — the essential amino acids can also be obtained by eating a variety of complementary plant sources that, in combination, provide all eight essential amino acids (e.g. brown rice and beans, or hummus and whole wheat pita, though protein combining in the same meal is not necessary). A varied intake of such sources can be adequate.
Iron
Vegetarian diets typically contain similar levels of iron to non-vegetarian diets, but this has lower bioavailability than iron from meat sources, and its absorption can sometimes be inhibited by other dietary constituents. Vegetarian foods rich in iron include black beans, cashews, kidney beans, lentils, oatmeal, raisins, black-eyed peas, soybeans, many breakfast cereals, sunflower seeds, chickpeas, veggie burgers, tomato juice, tempeh, molasses, and whole-wheat bread. Vegan diets can often be higher in iron than vegetarian diets, because dairy products are low in iron. Iron stores often tend to be lower in vegetarians than non-vegetarians and iron deficiency is thus more common in vegetarian and vegan women and children (adult males are rarely iron deficient), however, iron deficiency anaemia is rare.
Vitamin B12
Plants are not generally significant sources of Vitamin B12, However, vegetarians can obtain B12 from dairy products, eggs, fortified foods and dietary supplements. Since the human body preserves B12 and reuses it without destroying the substance, clinical evidence of B12 deficiency is uncommon. The body can preserve stores of the vitamin for up to 30 years without needing its supplies to be replenished.
The recommendation of taking supplements has been challenged by studies indicating that exogenous B12 may actually interfere with the proper absorption of this vitamin in its natural form. The research on vitamin B12 sources has increased in the latest years and researchers at Hiroshima University have developed methods for growing plants rich in vitamin B12.
Fatty acids
Fish is a major source of Omega 3 fatty acids, although some plant-based sources exist such as soy, walnuts, pumpkin seeds, canola oil and, especially, hempseed, chia seed, flaxseed, and purslane. Purslane contains more Omega 3 than any other known leafy green. Plant foods can provide alpha-linolenic acid but not the long-chain n-3 fatty acids EPA and DHA, which are found in low levels in eggs and dairy products. Vegetarians, and particularly vegans, have lower levels of EPA and DHA than meat-eaters. While the health effects of low levels of EPA and DHA are unknown, it is unlikely that supplementation with alpha-linolenic acid will significantly increase levels. Recently, some companies have begun to market vegetarian DHA supplements containing seaweed extracts. Similar supplements providing both DHA and EPA have also begun to appear. Whole seaweeds are not suitable for supplementation because their high iodine content limits the amount that may be safely consumed. However, certain algae such as spirulina are good sources of gamma-linolenic acid (GLA), alpha-linolenic acid (ALA), linoleic acid (LA), stearidonic acid (SDA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (AA).
Calcium
Calcium intake in vegetarians is similar to non-vegetarians. Some impaired bone mineralisation has been found among vegans who do not consume enough leafy greens, which are sources of abundant calcium. However, this is not found in vegetarians.
Vitamin D
Vitamin D levels do not appear to be lower in vegetarians (although studies have shown that much of the general population is deficient). Vitamin D needs can be met via the human body's own generation upon sufficient and sensible UV sun exposure. Products including milk, soy milk and cereal grains may be fortified to provide a good source of Vitamin D and mushrooms provide over 2700 IU per serving (approx. 3 oz or 1/2 cup) of vitamin D2, if exposed to just 5 minutes of UV light after being harvested;for those who do not get adequate sun exposure and/or food sources, Vitamin D supplementation may be necessary.
