L-tryptophan and depressive illness: a valuable adjunct to therapy?

Abstract

L-tryptophan is an essential amino acid in human nutrition. The minimum daily requirement for adults is in the range of 175 to 250 mg daily and this is normally exceeded in the average western diet which contains 600 to 1000 mg. Excess tryptophan is normally metabolised through the kynurenine pathway and only 1–2% of tryptophan in the diet is converted to 5-HT. The concept that 5-HT had a part to play in depressive illness evolved after the original observation by Ashcroft & Sharman in 1960 that patients with severe depressive illness had lower levels of the metabolite of 5-HT in cerebrospinal fluid compared with controls. In addition, early papers on the therapeutic efficacy of tryptophan suggested that it was potentially as successful as ECT.