An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo (from scratch) by the organism, but must be supplied in its diet. The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine (i.e., F V T W M L I K H).