This is the preferred term for amino acids that are essential for the body's normal growth and development, but which the body is unable to produce. Nine such amino acids exist – HISTIDINE, ISOLEUCINE, LEUCINE, LYSINE, METHIONINE, PHENYLALANINE, THREONINE, TRYPTOPHAN, and VALINE – and they are present in foods rich in protein: dairy products, eggs, meat, and liver.