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June 1981

Plasma Tryptophan and Five Other Amino Acids in Depressed and Normal Subjects

Author Affiliations

From the Department of Psychiatry, Institute of Psychiatric Research (Drs DeMyer and Hendrie), and the Departments of Neurobiology and Biochemistry (Dr Shea), Indiana University School of Medicine, Indianapolis; and the Department of Metabolic Research, American McGaw Corp, Irvine, Calif (Dr Yoshimura). Dr Hendrie is Albert E. Sterne Professor of the Department of Psychiatry, Indiana University School of Medicine.

Arch Gen Psychiatry. 1981;38(6):642-646. doi:10.1001/archpsyc.1981.01780310042003

• Decreases in brain serotonin levels have been implicated in the pathogenesis of human depression. While circulating levels of tryptophan (TRP) might possibly reflect concentration of brain serotonin, a more relevant peripheral measure may be the ratio of plasma TRP to five other neutral amino acids (TRP/5aa ratio). In depressed subjects and normal controls, plasma TRP (free and total), phenylalanine (PHE), tyrosine (TYR), leucine, isoleucine, and valine were measured on three days. When depression was most severe, depressed patients had lower TRP/5aa ratios and total TRP levels and higher PHE and TYR levels. After the patients' conditions improved, these differences disappeared. As Hamilton depression scores improved, the plasma TRP/5aa ratios increased significantly. This finding tends to support the idea that changes in brain serotonin level reflect changes in depression severity.