Approximately 80 amino acids are found in nature, but only 20 are necessary for human metabolism or growth. These are histidine, isoleucine, leucine, lysine, methionine, cysteine, phenylalanine, tyrosine, threonine, tryptophan, and valine. The nonessential amino acids are alanine, aspartic acid, arginine, citrulline, glutamic acid, glycine, hydroxyglutamic acid, hydroxyproline, norleucine, proline, and serine. Oral preparations of amino acids may be used as dietary supplements. Arginine, while nonessential for the adult, cannot be formed quickly enough to supply the demand in infants and thus is classed as essential in early life.
Some proteins contain all the essential amino acids and are called complete proteins.
Examples are milk, cheese, eggs, and meat. Proteins that do not contain all the essential amino acids are called incomplete proteins. Examples are vegetables and grains. Amino acids pass unchanged through the intestinal wall into the blood, then through the portal vein to the liver and into the general circulation, from which they are absorbed by the tissues according to the specific amino acid needed by that tissue to make its own protein. Amino acids if not otherwise metabolized may be converted into urea.