Proteins form the building blocks of most body structures. They are necessary for building and recovery of the muscles, organs, nervous system and blood. Proteins are composed of amino acids which are linked in long chains. The body contains 21 amino acids. Twelve of these amino acids are nonessential and the body can synthesize these for itself. There are nine amino acids the body either cannot make at all or cannot make in sufficient quantity to meet its needs. These nine amino acids must be supplied through diet; they are essential. When eating a normal, balanced diet, these amino acids will be supplied without any problem.
It is important to combine different protein foods. Human beings require complete dietary proteins that contain all essential amino acids in relatively the same amounts. Generally, proteins derived from animals (meat, fish, poultry, cheese, eggs, milk) are complete. Plant derived proteins (vegetables, grains, legumes) have more diverse amino acid patterns, and some tend to be limiting in one or more essential amino acids. It is important to eat a variety of grains, legumes, seeds, nuts and vegetables, especially for vegetarians. To yield complementary proteins that contain all the essential amino acids, combine plant-protein foods with different but complementary amino acid patterns. This strategy is called mutual supplementation. It is, however, not necessary to balance amino acid intake at each meal when protein intake is varied and energy intake is sufficient.