The shift of interest to a plant-based diet in the United States happened quietly and without much fanfare in response to all the talk about lowering the fat content in our diets and eating more whole grains, legumes, fruits and vegetables.

Those people who have been practicing vegetarians for a long time and who have put up with the scorn of meat-eaters for so many decades are now firmly in the health mainstream.

When compared to the average American, they seem to be at lower risk for heart disease, diabetes and certain types of cancers (especially of the colon, breast and lung). Their blood pressure and cholesterol levels tend to be lower, and they are much less likely to suffer from obesity. They also tend to have fewer digestive system problems such as constipation and diverticulosis.

There are several different types of vegetarian diets.

Vegans are total vegetarians. These individuals do not eat any animal foods at all. And it's the most difficult type of vegetarian diet to follow and to balance nutritionally.

Lacto vegetarians include dairy products as protein sources in their diets. In order to keep this type of diet low in fat and cholesterol, low-fat or skim milk products must be used.

Lacto-ovo vegetarians add eggs and dairy foods to their diets. Most American vegetarians fall into this category.

Pesco vegetarians include fish, as well as eggs and dairy products.

Semi-vegetarians are people who eat small amounts of meat, fish or chicken. In fact, most low-fat, "heart-healthy" plans suggest using lean animal products as side dishes instead of entrees. Many ethnic cuisines fall into this grouping. This may be the easiest type of diet to use as a transition if you are trying to change your eating habits.

Twenty or 30 years ago, the chief worry of vegetarians was whether they were getting enough protein and particularly whether the proteins were "complete" or complementary. Now we know that the proteins in vegetables and grains are not entirely lacking any essential amino acids, but some just have more than others.

For example, bread, which has long been a staple of every human diet, is rich in the amino acid methionine, but is low in lysine. On the other hand, legumes are rich in lysine, but poor in methionine, so if you eat legumes and bread together, you will have a complete protein. If you think that sounds like a peanut butter sandwich, you're right.

Contrary to popular belief, you don't have to combine proteins in the same meal. You can eat the foods three or four hours apart and their amino acids will still complement each other in useful ways. The important thing to remember is that if you eat a wide variety of foods, you'll probably absorb a full complement of amino acids.

There has been a kind of instinctive blending of dishes in various cuisines. Without really understanding the biochemistry behind it, cooks have for centuries put together lentils and rice, beans and brown bread or cornbread, corn and lima beans. Most of the diets in the world contribute adequate combinations of amino acids and protein, so the worry about protein is pretty much unfounded. In addition, we know that most Americans eat far too much protein, so this is certainly no reason to shun a vegetarian diet, especially a lacto or lacto-ovo vegetarian regime.

Strict vegans may have reason to be concerned about certain vitamins. Some of the potential deficiencies that could exist are in vitamins B-12 (riboflavin) and D which are found only in animal products. Although a B-12 deficiency can bring on anemia or certain changes in the nervous system, there is no evidence that vegans can become so severely depleted that they develop these symptoms. However, if you are not eating any meat, dairy products or eggs, you'll either have to take supplements or eat fortified foods such as soy milk or cereals (be sure to check the labels carefully -- not all fortified cereals include B-12).

B-12 is one of those nutrients that we don't need in huge quantities -- 3 millionths of a gram a day will do. Most plants have little or no vitamin B-12 and the foods that contain the most are liver, beef, eggs, milk and shellfish, which also sounds very much like a list of the foods highest in fat and cholesterol.

One of the products currently being sold (by the ton) to serve as a B-12 supplement is spirulina, a type of blue-green fresh-water algae. It has received a great deal of attention and has spawned a flourishing market in the health food stores and through individual distributorships. Make no mistake about it, spirulina is now a very big business.

But is it really the answer for vegetarians who want to forestall any deficiencies of B-12? Unfortunately, there is no evidence that it does.

Vitamin D is needed for calcium absorption, and a deficiency can cause rickets in children. However, you really don't need very much vitamin D, and if you get some sun every day your body will probably be able to synthesize all you require.

Calcium, iron and zinc are essential minerals that may be more difficult to come by in a strictly vegetarian diet. Although vegetable products do contain these and other minerals, they are often in a form that is less absorbable than in animal products. Women, especially, need to be very careful to get enough iron and calcium if they are strict vegans. However, since the typical vegetarian diet contains large quantities of foods that are good sources of these minerals, there is usually no reason to be concerned.

Peas, lentils and wheat germ are good sources of zinc while broccoli, kale, collard greens, mustard greens and tofu contain calcium. Beans, potatoes, dried fruit and fortified cereals and breads supply iron. You can increase your absorption of iron from these vegetable sources if you eat plenty of foods rich in vitamin C (berries, citrus fruits, tomatoes, broccoli) at the same time as you eat your iron sources.