While this point may seem obvious, it should be noted that if a vegetarian is living on tofu dogs, "vice cream," and french fries, this will negate many potential health benefits of a plant-based diet. Ideally, the removal of meat or other animal products from one's diet would be paired with increased consumption of fruits, vegetables, nuts, seeds, whole grains, and legumes; wholesome foods with health-promoting properties. Even if all-out vegetarianism is not one's cup of tea, simply reducing meat intake is probably good idea for one consuming the Standard American Diet (so appropriately abbreviated as SAD).
A whole lot of fat
While a certain amount of fat is necessary for a healthful diet, saturated fat and cholesterol -- both of which abound in flesh, eggs, and dairy products -- are not the kinds of lipids that we should be seeking out. (The flesh of aquatic creatures tends to contain more of the "good fats" than the flesh of terrestrial livestock, but there are still significant concerns about the "healthfulness" of fish and other sea life; see "toxins and pathogens" below.) Saturated fat in particular is linked to high blood cholesterol, particularly LDL or "bad" cholesterol, which is a major risk factor in developing cardiovascular disease. In contrast, unsaturated fatty acids found in plant foods may help to decrease levels of "bad" cholesterol in the blood. Diets high in saturated fat have also been linked to certain kinds of cancer, notably breast, prostate, and colon cancers.
Toxins and pathogens
Even if one chooses leaner cuts of meat, fish, egg whites, and low-fat dairy alternatives, other nasty surprises may lurk in products of animal origin. The "mad cow" scare, for example, has had many a consumer swearing off beef. E. Coli bacteria, including the deadly strain O157:H7, may be found in beef or chicken as a result of contamination on the farm or on the slaughterhouse floor. Salmonella is found in raw or undercooked eggs, meats, poultry, dairy products, and shrimp. Camphylobacter bacteria is also frequently found in foods of animal origin. Pork products are frequently infected with parasites including Trichinella spiralis, which causes trichinosis. Undercooked or raw shellfish can also harbor the hepatitis A virus.
Environmental toxins such as dioxins and drug and pesticide residues also tend to accumulate in animals' bodies; as much as 90% of North Americans' exposure to commercial pesticides comes not from conventionally grown produce, but from consuming flesh foods. Milk and other dairy products often contain remnants of bovine growth hormones which may have their own detrimental health effects. Mercury, PCBs, DDT, and other toxins can also be found in supposedly healthy fish and other sea life, particularly bottom feeders like catfish and oysters, and carnivorous fish such as salmon, shark, and swordfish.
Milk, meat, and your bones
The United States, Israel, Sweden, and Finland are the world's top per capita consumers of dairy products -- supposedly the richest sources of calcium available. Yet interestingly enough, these same nations also have the world's highest rates of osteoporosis. The dairy industry's "3-a-day" campaign notwithstanding, milk is not the best food source of calcium; calcium is actually better absorbed from plant sources such as leafy greens. And while calcium is one important mineral in bone health, it is certainly not the only factor.
Too much dietary phosphorus or protein -- which abound in animal products -- can lead to increased urinary excretion of dietary calcium and contribute to bone breakdown and osteoporosis. Meat is also particularly heavy in the sulfur-containing amino acids, which cause bodily acidification and an increased need to "buffer" with calcium taken from the bones. This is why too much protein from animal products is more likely to be detrimental to bone health than protein from plant sources. Pre-formed vitamin A in large amounts -- as is found in milk and organ meats -- may also contribute to bone breakdown. Bear in mind that carotenoids, the vitamin A precursors found in plant foods, do not have this same effect.
Other health issues
Meat eaters are statistically far more likely than vegetarians or vegans to suffer from stroke, cardiac arrest, coronary artery disease, angina, diverticulosis, ulcers, irritable bowel syndrome, high blood cholesterol, hypertension, gout, arthritis, osteoporosis, obesity, kidney stones, gall stones, kidney disease, liver disease, diabetes, and prostate, colon, and breast cancers. Vegetarians are also less likely to suffer from Alzheimer's disease and related dementias.
The American Dietetic Association has stated emphatically that vegetarian and vegan diets are healthy for all people, including children, athletes, and pregnant or lactating women. For a person eating a reasonably varied diet, it is difficult not to obtain all of the essential nutrients (yes, even protein) from grains, fruits, vegetables, and legumes. In fact, only vitamin B12 -- which should be obtained from supplements or other fortified sources -- is not found naturally in plant foods. In contrast, animal foods lack dietary fiber, vitamins C and E, beta-carotene, and numerous cancer-fighting antioxidants and phytonutrients, all of which are only found in plants. Flesh is not a significant source of the mineral magnesium, and unless you are eating liver or giblets, you won't find much folic acid in meat, either.
It bears mention that while some natural hygienists used to prescribe vegetarianism as a means of taming the "passions," there is simply no medical evidence that cutting out meat puts the brakes on sexual appetite or prowess. If anything, a healthy vegetarian diet will help keep the "animal" in you alive, and possibly even improve one's physical relationships. After all, a healthy body is more conducive to satisfying sexual experience -- and the popular saying goes that "vegetarians taste better!"
Folks who are familiar with the USDA's Food Pyramid may wonder why, with all of the medical evidence to the contrary, does the Department of Agriculture continue to advocate 2-3 servings of meat and 3 servings of dairy per day. This has more to do with politics than sound nutritional advice. The USDA is charged with not only setting dietary guidelines, but at the same time must promote agriculture, in particular the meat and dairy industries. For more information, I recommend Marion Nestle's outstanding book, Food Politics.
Bibliography and suggestions for further reading:
Cousens, Gabriel -- Conscious Eating
Hill, John Lawrence -- The Case for Vegetarianism: Philosophy For a Small Planet
Lyman, Howard -- Mad Cowboy: Plain Truth from the Cattle Rancher Who Won't Eat Meat
Marcus, Erik -- Vegan: The New Ethics of Eating
Nestle, Marion -- Food Politics
Quinn, Daniel -- Ishmael
Reinhardt, Mark Warren -- The Perfectly Contented Meat Eater's Guide to Vegetarianism
Rifkin, Jeremy -- Beyond Beef: The Rise and Fall of Cattle Culture
Robbins, John -- The Food Revolution
Saunders, Kerrie -- The Vegan Diet As Chronic Disease Prevention
Schlosser, Eric -- Fast Food Nation: The Dark Side of the All American Meal
Stepaniak, Joanne -- Being Vegan