essential amino acid

An essential amino acid is an amino acid that an organism cannot synthesize and therefore has to be ingested as part of the diet. Of the 20 amino acids used in the human body, eight of them are "essential" in adults. They are lysine, threonine, methionine, isoleucine, leucine, valine, phenylalanine, and tryptophan. Two others, arginine and histidine, are essential only in children.