— Merrill Carey, Brooklyn, New York
A: A food earns complete protein status if it contains the nine essential amino acids the body needs to build the proteins that help maintain muscle, bone, and organs. Unlike most amino acids, these nine can't be generated by our bodies, so they have to come from food. (You need to get about .4 grams of protein daily per pound of body weight, though Americans get about double that, on average.) The foods we consider complete proteins tend to come from animal sources—meat, fish, and dairy—while fruits, vegetables, and grains tend to be incomplete proteins.
Tofu is an exception. It is complete, though it's not considered as high-quality as animal sources. (Quality is judged by the proportion of amino acids in the food.) Oddly enough, soybeans, from which tofu is made, are considered very high-quality protein; tofu, however, is made from only the curds of soybeans. But even if you're a vegetarian, you don't have to spend much time worrying about your protein intake, provided you eat a variety of vegetables and grains. It's a myth that people need to get complete proteins in every meal. For example, if you eat beans—an incomplete protein—at one meal and a tortilla—also incomplete, but complementary to beans—at the next, your body will be able to get the essential amino acids from both that it needs.
While there may be some variation in the nutrient content of fresh versus packaged tofu—omega-3 fatty acids, for instance, can degrade with exposure to light and air—the quantity and quality of amino acids should be the same. Using soy products as a source of protein lets you reduce your intake of meat, which may well confer a net health benefit—especially if the meat being replaced is high in saturated fat.
David L. Katz, MD, is director of the Yale-Griffin Prevention Research Center and president of the nonprofit Turn the Tide Foundation.