Serum tryptophan ratio and suicidal behavior in adolescents: a prospective study.

A lower ratio of tryptophan to other amino acids in serum (tryptophan ratio), established as an index of serotonin precursor available to the brain, has been shown to be associated with increased suicidal behavior in cross-sectional studies. The purpose of this study was to conduct the first prospective examination of the utility of the tryptophan ratio along with clinical variables in predicting suicidal behavior in high-risk and reference adolescents. Adolescents with alcohol use disorders (AUDs) and prior suicidal attempts (n=20), adolescents with AUDs without suicide attempts (n=20), and community controls with neither of these characteristics (n=20) were matched on demographic variables. These groups were not significantly different on the tryptophan ratio at baseline. Of 56 subjects who completed follow-up assessments, six had suicidal behavior in the follow-up period. The tryptophan ratio, along with demographic and clinical variables, was examined using Cox regression with a backward stepwise variable elimination procedure. In the final model, the tryptophan ratio and major depressive disorder both significantly contributed to the prediction of suicidal behavior. This finding suggests that the tryptophan ratio may improve the identification of adolescents at high risk for suicidal behavior.