essential amino acid [L, essentia, quality] , an organic compound not synthesized in the body that is essential for protein synthesis in adults and optimal growth in infants and children. Adults require isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Infants need these amino acids plus arginine and histidine. Cysteine and tyrosine are derived from methionine and phenylalanine, respectively, and are considered semiessential. See also amino acid, nonessential amino acid.
