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).