Abstract
L-tryptophan (tryptophan; Trp) is a large neutral amino acid essential to human metabolism because it is the metabolic precursor of serotonin (a neurotransmitter), melatonin (a neurohormone), and niacin (vitamin B3). As a component of dietary protein, tryptophan is particularly plentiful in chocolate, oats, bananas, dried dates, milk, cottage cheese, meat, fish, turkey, and peanuts. Approximately 300 mg Trp is available in three ounces of turkey, lamb, beef, tuna, or peanuts.1 Relative to other amino acids, small amounts are needed to have a therapeutic effect, which is fortunate because Trp is the least abundant amino acid in the diet.2 In 1989, the importation of L-tryptophan was banned in the United States after cases of a deadly autoimmune illness called eosinophilia-myalgia syndrome were traced to an improperly-prepared batch of tryptophan.3 Although the tryptophan was isolated to a single Japanese factory that allowed a toxic bacterial metabolite through the purification process, the ban was maintained and Trp availability was limited to the prescription drug (Tryptan), infant formulas, and enteral feeding products. Since 1994 tryptophan has been available and marketed as a dietary supplement in the United States, while imported product remains limited by special regulations.