According to the Centers for Disease Control and Prevention, complementary proteins are two or more incomplete protein sources that together provide adequate amounts of all the essential amino acids. Examples include grains and legumes or legumes, seeds and nuts.
People following a vegetarian diet are advised to include a variety of foods that have complementary proteins so that the essential amino acids missing from one protein source can be supplied by another. According to Nutrition 411, most non-animal sources of proteins, aside from soybeans, lack one or more of the essential amino acids that are needed to keep the body functioning properly. Legumes such as lentils, beans and peas lack adequate methionine and tryptophan, while whole grains such as brown rice, oats and whole wheat lack isoleucine and lysine. A combination of these two food groups yields "complete" proteins that provide the body with the full range of essential amino acids.
Research shows that it is not a requirement to eat complementary proteins at each meal, although numerous cultures combine these foods naturally in meals such as corn and beans, falafel and pita, and rice and lentils. Combinations of food available to vegetarians and vegans include dairy and grains, dairy and seeds, dairy and legumes, grains and seeds, grains and legumes, and legumes and seeds.