Amino Acid (Essential)

An amino acid that cannot be synthesised by the human body in sufficient quantities and must be obtained through diet or supplementation. There are nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine). Contrast with non-essential amino acids (synthesised endogenously) and conditionally essential amino acids (essential only under physiological stress).