Protein supplies the body with essential amino acids, which are used for growth and tissue repair. When whole food sources of protein are broken down during digestion, they create components called amino acids. A complete protein is a whole protein that contains sufficient amounts of all nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine) in the ratios needed by humans for good health. In other words, it's a perfect balance of these key nutrients. Essential amino acids, unlike non-essential ones, cannot be made by the human body and must come from an outside source such as food or supplements.