Each food has a different mix of amino acids. Therefore, it is important to have an understanding of protein composition and to apply it to our diet. The meat foods (including fish and poultry), dairy foods, and eggs almost all have sufficient quantities of amino acids to sustain life; that is, they are complete proteins. When we eat these foods daily, we do not really need to worry about amino acid complementarity; in fact, there are concerns that overconsumption of protein foods (particularly meat and milk) in many societies contributes to some major illnesses, so we may not wish to consume these foods daily, or at all.

Vegetarians or other people on diets that limit certain foods may need to be more knowledgeable about combining food. Lacto-ovo-vegetarians, who eat eggs and dairy foods—both complete proteins—need have less concern than the pure vegetarian, or vegan. Of the essential amino acids, we have seen that lysine, methionine, and tryptophan are the deficient. They are present in all vegetable proteins, but at lower levels than other amino acids. Since they are not all low in the same foods, it is not as difficult as many think to obtain a good protein balance from vegetables, grains, nuts, and legumes. The simplest idea is to eat grains with some beans or seeds, for example, millet and aduki beans or brown rice and sunflower seeds. Other complete-protein combinations of vegetable sources include soybeans and rice or soybeans with sesame, corn, wheat, or rye; peanuts with grain or coconut; grain with legumes or leafy greens; beans and corn or rice (South American diet); and peas and wheat. The Nutrition Almanac, written by Nutrition Research, Inc. (McGraw-Hill, 1984) has a food-by-food breakdown of amino acid content that is very helpful in creating a diet to achieve proper protein intake.

The body will make protein only as long as it has sufficient levels of all necessary amino acids in the cellular "storage" pool. When one amino acid is deficient, we will not be able to produce most proteins, and then either muscle protein will be catabolized to obtain adequate amounts of the needed amino acid(s) or the metabolism will use protein for energy. The body breaks down an average of about 300 mg. (maybe much more under many stressful conditions) of protein per day, which it replaces if there are sufficient nutrients. If there are not, however, we experience net protein loss; thus the importance of consuming all the amino acids through a daily intake of 50–60 grams of "balanced" protein in forms that are easily digested and assimilated.

Protein foods have been classified according to their ability to be digested and used by the body; that is, their biological availability, or value. The measurement of this ability is termed net protein utilization (NPU); it is also called biological value (BV). Chicken eggs are considered to have the protein (ovalbumin) of highest known NPU. Following eggs, in descending order, are fish, cow's milk and cheese, brown rice, red meat, and poultry (Garrison and Somer, Nutrition Desk Reference, Keats Publishing, 1985). Again, this is not based on protein content but on biological value, how efficiently the body utilizes the protein in the food. Clearly, the amino acids in brown rice do not make it as complete a protein as the others, though the protein in it is readily usable. Here is where the vegetarian complementing with legumes may help as long as digestion and assimilation are functioning properly.