an amino acid that is nutritionally important and occurs in small amounts in proteins. It is an essential amino acid, meaning that humans and certain other animals cannot synthesize it and must obtain it from their diets. Infants require greater amounts of tryptophan than adults to ensure normal growth and development. Tryptophan is used by the body to manufacture several important substances, including the neurotransmitter serotonin and the vitamin niacin. Diets poor in tryptophan can lead to pellagra (q.v.), a disease resulting from niacin deficiency; however, this disease is now rare in developed countries. In 1901 the English biochemist Frederick G. Hopkins isolated tryptophan from casein, the major protein found in milk.