Some of the protein you eat contains all the amino acids needed to build new proteins. This kind is called complete protein. Animal sources of protein tend to be complete. Other protein sources lack one or more “essential” amino acids—that is, amino acids that the body can’t make from scratch or create by modifying another amino acid. Called incomplete proteins, these usually come from fruits, vegetables, grains, and nuts.
Vegetarians need to be aware of this. To get all the amino acids needed to make new protein—and thus to keep the body’s systems in good shape—people who don’t eat meat, fish, poultry, eggs, or dairy products should eat a variety of protein-containing foods each day.