What is Protein? Proteins are found in the cells and tissues of all living things. They are chains of amino acids, molecules that are involved in a variety of biological functions. There are 20 amino acids, nine of which cannot be synthesized in the human body and must be acquired through diet. These are known as essential amino acids. Animal sources of protein (and a select few plant proteins including soy and quinoa) are considered “complete” in that they contain adequate amounts of all the essential amino acids the human body needs. Most plant foods are considered “incomplete” proteins, because they typically have low levels of, or are missing, one or more of the essential amino acids. For example, grains are low in the amino acid lysine, but have adequate methionine. Legumes (beans, lentils, chickpeas, peas, and peanuts), on the other hand, contain adequate lysine, but are low in methionine. Thus, a dietary pattern that includes both whole grains and legumes will provide a sufficient amount of all essential amino acids. Although it was once thought that complementary foods like these needed to be consumed at the same time, it is now understood that eating a variety of plant foods throughout the day can provide all the amino acids the body needs.