A group of organic compounds identified by the presence of both an amino group(NH2) and a carboxyl group(COOH). They are the building blocks for protein and are essential to life. Although around 80 amino acids are found in nature, only 22 are needed for human metabolism. The ones that cannot be produced by the body, and must be supplied by food, are called essential amino acids . The essential amino acids are histidine, isoleucine, leucine, lysine , methionine, cysteine, phenylalanine, tyrosine , threonine ,tryptophan, and valine. The non- essential amino acids (which the body can manufacture itself), are alanine, aspartic acid, arginine, citrulline, glutamic acid, lycine, hydroxyl glutamic acid ,hydroxyproline, norleucine, praline and serine. Arginine can be essential in certain states or age groups because the body cannot make it fast enough to supply the demand.