According to reputable sources such as the American Dietetic Association, American Heart Association, National Institutes of Health, British Medical Association, and the Mayo Clinic, vegetarian diets offer a number of health benefits compared to non-vegetarian diets. For example, vegetarians have lower body mass indices, lower levels of cholesterol, lower blood pressure, and less incidence of heart disease, hypertension, some forms of cancer, type 2 diabetes, renal disease, osteoporosis, dementias such as Alzheimer’s Disease, and more. The Journal of the ADA also reports that children raised as vegans have approximately 20 more points of IQ than average. (source: JADA 1980; 76:142-7)

See also: ADA's position, with many citations of third-party research, Mayo Clinic's synopsis.

As with any diet, vegetarians should have a diverse and well-planned diet for optimal health. In addition to the specific-nutrient information in the above links, the following is a list of some nutrients which are, were thought to be, or are still alleged to be of particular concern for vegetarians, and especially for vegans, fruitarians, raw-foodists, macrobiotics, or any more-restrictive form of vegetarianism.

Protein

The typical vegetarian, and the typical vegan, gets adequate protein so long as caloric intake is adequate and a variety of foods are eaten. (source: ADA) However, the typical vegetarian does get less protein than the typical non-vegetarian, which can be beneficial because:

• Due to excess protein intake, people in affluent societies commonly lose about 30 percent of their kidney function by their 80’s (J Gerentol 31:155, 1976), and the amount of protein typically consumed in the American diet, 12% to 15% protein, is probably a partial cause (New Eng Jrnl Med 307:652, 1982); by contrast, some high-protein diets, such as the Atkins diet recommend 30%;
• Excessive protein intake -- particularly sulfurous amino acids which predominate in animal-proteins -- causes systemic acidity, and to counter this acidity, the body then leaches calcium from bones, causing osteoporosis. (sources: Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Milk, dietary calcium, and bone fractures in women: a 12-year prospective study. Amer Jrnl Public Health 1997;87:992-7. See also follow-up in February, 2003 issue of the American Journal of Clinical Nutrition (Vol. 77, No. 2, 504-511), which includes 72,000+ people and 18 years of data. See also Cumming RG, Klineberg RJ. Case-control study of risk factors for hip fractures in the elderly. Amer Jrnl Epidemiology 1994;139:493-503.). The Rural Chinese Diet described in the below link -- low-protein, no milk, and nearly vegan -- actually results in less osteoporosis than the average Westerner's diet, as well as other health benefits similar to vegetarianism: www.msnbc.com/news/916555.asp.

Protein is essential to both the structure and function of all living cells. Some soy-based meat analogues contain even more protein per pound than beef (e.g. soy concentrate, which is similar to TVP). Sea vegetables also can provide even more protein per pound than beef. Wheat, rice, beans, and nuts are also good sources of protein.

Combining Proteins

A widely-held myth about protein -- and vegetarianism itself -- is that vegetarians must carefully 'combine' various plant foods in their diets by consuming them within a few hours of each other in order to make a 'complete' protein which contains all 8 essential amino acids. While this myth has been extensively published, it has never been substantiated by research, and ADA cites research which refutes this myth. The only plant known to contain a ‘complete protein,’ i.e., protein which contains all of the essential amino acids, is the soybean plant, which is processed into many foods. However, even vegans who eat no soyfoods do not need to plan for 'complementary proteins,' so long as their diet is diverse. (source: ADA) It is recommended that all diets, from veganism to non-vegetarianism, should contain such diversity anyway.

The protein combining myth was given wings in Frances Moore Lappe's 1971 runaway besteller Diet for a Small Planet. In later editions of her book, as early as 1981, Lappe reversed her opinion that protein 'combining' is necessary. John McDougall, M.D., in The McDougall Plan, uses research to concur with Lappe's 1981 conslusion. As McDougall says, "Many people believe that animal foods contain protein that is superior in quality to the protein found in plants. This is a misconception dating back to 1914, when Osborn and Mendel studied the protein requirements of laboratory rats. (11) ... It has since been shown that the initial premise that animal products supplied the most ideal protein pattern for humans, as it did for rats, was incorrect."

Even today, McDougall still aggressively challenges those who perpetuate the protein combining myth by demanding that they produce evidence to support their claims. [McDougall vs. the American Heart Association]

Iron

Even vegans seem not to suffer from iron-deficiency any more than non-vegetarians do (source: Am J Clin Nutr. 1994;59 (suppl):1233S-1237S), but meat is the only source of heme iron; plants contain non-heme iron. The human body absorbs non-heme iron less efficiently. However, heme iron has been shown to increase colon cancer risk due to cytotoxic factors.(source: ADA) Non-heme iron is also more sensitive to both inhibitors and enhancers of iron absorption. Vitamin C is an iron absorption enhancer. Inhibitors include tannins (tea, wine), phytates (legumes, grains), calcium and polyphenols. Also, excess amounts of iron can be toxic and even result in death.

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 transportation. Iron also helps regulate cell growth and differentiation. Some vegetarian sources of iron are tofu, potato skins, nuts & seeds, some beans & peas, green leafy vegetables, sea vegetables, whole-grain and/or enriched breads, miso, raisins, wheat germ, and foods cooked in cast-iron.

Zinc

Vegetarians and vegans also seem not to suffer from zinc deficiencies any more than non-vegetarians. (source: Freeland-Graves JH, Bodzy PW, Epright MA. Zinc status of vegetarians. J Am Diet Assoc. 1980;77:655-661.) However, phylate in many whole-grains and fiber in many foods may interfere with zinc absorption. High doses of zinc may cause cancer.

Zinc is essential for building many enzymes, protein synthesis, reproductive health, immune system, speeds healing as a topical application, and may be an anti-oxidant. Plant sources of zinc include cooked dried beans, sea vegetables, fortified cereals, soyfoods, nuts, peas, and seeds.

Vitamin B₁₂

Vitamin B-12 is produced only by bacteria, which are found in animals and plants alike. (source: Mozafar, A. 1994. Enrichment of some B-vitamins in plants with application of organic fertilizers [and/or manufacture by bacteria in healthy, cobalt-replete soil], Plant and Soil 167:305-311.) However, pesticides can kill these bacteria when applied to plants (Ibid.). If animals eat these B12-deficient plants, they may become B12-deficient themselves, and if we eat these B12-deficient animals (or if we eat the non-organic plants, directly), we may also become B12-deficient. And indeed, even non-vegans have prevalent B12-deficiency (source: USDA-ARS Human Nutrition Research Center on Aging at Tufts University, and Framingham (Massachusetts) Offspring Study; as cited in: "Are You Vitamin B-12 Deficient?"; Aug. 2000 issue of Agricultural Research mag.), so there is no reliable source of B-12 without dietary supplementation; B12-supplementation is recommended for people of any diet. Although we are able to absorb B12 from the beneficial bacteria within our intestines, and re-absorb nearly 100% of that which is digested, then excreted back into our intestines for re-usage (sources: Nature 1980 Feb 21;283(5749):781-2, Vitamin B12 synthesis by human small intestinal bacteria. Albert MJ, Mathan VI, Baker SJ. See also [1]), animals, including humans, must also ingest a minuscule amount of B-12 to maintain proper B-12 levels. (source: Callender ST, Spray GH. Latent pernicious anemia. Br J Haematol 1962; 8: 230-240) However, someone of any diet, starting with a healthy level of B-12, typically lasts for decades without running critically deficient in this important nutrient. (source: Am J Clin Nutr 1988; 48: 852-858. Herbert V.; Vitamin B-12: Plant sources, requirements, and assay.)

Vitamin B-12 is necessary for DNA replication, so deficiencies can prevent rapid cell division, particularly of blood cells, which causes anemia. The human body also uses B-12 in the natural protein sheaths around nerves, so deficiencies can lead to potentially irreversible nerve degeneration. Vegetarian sources of Vitamin B-12 include eggs, dairy products, tempeh, some sea-vegetables, foods and pills fortified with synthetic supplements of Vitamin B-12.

Omega-3 fatty acids

Vegetarian sources of omega-3 fatty acids include flaxseeds and flaxseed oil, walnuts, and canola (rapeseed) oil, and eggs. A well-known source for nonvegetarians is fish, yet one need only consume 1/3 the weight of walnuts compared to salmon to get the same amount of omega-3's; and flax seeds have even more highly-concentrated omega-3's than walnuts. (source: WebMD/MSN Health) However, vegetarian sources of omega-3 fatty acids are likely to have lower concentrations of the particular essential fatty acids (EFA's), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The body can synthesize small quantities of EPA and DHA from other omega-3 fatty acids, such as alpha-linolenic acids, which are present in vegetarian sources of omega-3 fatty acids. The body can also convert DHA into EPA. DHA supplements derived from DHA-rich microalgae are available.

While there is no scientific consensus on the role of omega-3 fatty acids, it is generally believed that they may reduce the risk of coronary heart disease, lower triglycerides, stabilize mood and prevent depression, prevent ADD, reduce joint pain and other rheumatoid problems, and reduce the risk of dementia in older age.

Vitamin D

The human body can synthesize Vitamin D when skin is exposed to ultraviolet radiation from the sun. Vegans who do not eat foods or pills fortified with synthetic vitamin D and with little exposure to the sun's ultraviolet radiation, e.g., who don’t expose their extremities for at least 15-30 minutes per day, or those living at latitudes close to the poles, are vulnerable to Vitamin D deficiencies.

Vitamin D acts as a hormone, sending a message to the intestines to increase the absorption of calcium and phosphorus, which produces strong bones. Vitamin D also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization . Research also suggests that vitamin D may help maintain a healthy immune system and help regulate cell growth and differentiation.

Iodine

It is very rare, but some vegetarians as well as non-vegetarians, especially those who eschew iodized salt, do not get enough iodine (source: WebMD/MSN Health). Two groups who often minimize salt intake are naturalists and those fearful of the fact that excessive salt contributes to hypertension, and both groups are commonly associated as some of those whom adopt a vegetarian diet. Another good iodine source is sea vegetables, but many people, vegetarian and non-vegetarian alike, eat few or no sea vegetables. Some vegetarians and a few non-vegetarians are further at risk due to eating too much soy or other foods containing goitrogens; goitrogens interfere with iodine uptake.

Lack of iodine in any diet may cause hypothyroidism, but excessive salt, iodized or not, may cause hypertension. But moderate amounts of iodized salt, an iodine supplement, or other iodine intake usually avoids iodine deficiency. Until recent years, iodine used as a disinfectant on bread, meat, and dairy processing equipment also provided iodine.(Ibid)

More external links: Vegetarian Resource Group Veg News American Heart Association