The classification of proteins as complete or incomplete helps in choosing a diet that provides all the essential amino acids. When two incomplete proteins are combined in one meal, they may produce complete protein if the right foods are chosen and are eaten in the right amounts. Red kidney beans and corn, when combined, supplement each other in lysine and methionine.
Not all incomplete protein combinations are this effective, however, and careful, informed selection is needed to assure an adequate intake of protein. Nutrition scientists use another classification of protein foods – one that assigns them a chemical value based on how closely they match the exact pattern of amino acids needed for protein synthesis within the human body. Although your body is able to take proteins from any source and use them to reconstruct the proteins it needs, some foods provide a combination of amino acids in proportions very close to your body’s exact needs. In this classification a whole egg gets the highest rating (100 percent), whereas foods that are lacking in one or more amino acids are rated lower.
Yet another system of protein evaluation a biological one – rates not only amino acid content but efficiency of use; how much of the protein is actually digested by the body. This standard of protein quality is called net protein utilization.
The fourth, and most widely used, standard is the “protein efficiency ratio,” which uses the relationship between intake and growth to compare the protein values of individual foods. By this system, whole egg, again, and milk proteins are given a value of 100, whereas wheat gluten is given a value of 20, signifying that five times the amount of wheat gluten would need to be consumed to achieve the same growth rate and gain in body weight as are achieved by consuming an egg, under the same conditions. Experts stress that there is a great deal of complex information to be considered in determining the value of protein foods to your body, of which these four classifications form only a small part. Even more important than ratings of individual foods, as well as the way in which the foods are prepared (which affects their digestibility), is a person’s full diet with its complex interactions. All these factors are considered in establishing the Recommended Dietary Allowances for protein.