Oral administration of 5-hydroxytryptophan (5-HTP) impairs decision making under ambiguity but not under risk: evidence from the Iowa Gambling Task

Objective Although the serotonin precursor 5-hydroxytryptophan (5-HTP) is marketed as a psychoactive nutritional supplement, knowledge is limited regarding the effects of exogenous 5-HTP on brain activity. This study examined if oral administration of 5-HTP to healthy adults impacted: (1) mood states, as measured by the Profile of Mood States (POMS); and (2) performance on the Iowa Gambling Task (IGT), a measure sensitive to alterations in frontocortical serotonin levels. Methods A sample of 46 undergraduates participated, and each received either two 50 mg 5-HTP capsules or placebos, and completed the IGT and POMS following an absorption period. Results 5-HTP did not significantly alter mood states, but did impair performance on the IGT. Specifically, the 5-HTP group performed more poorly than the placebo group during the first 20 trials of the IGT but did not differ from the placebo group on trials 21-100. This suggests that oral 5-HTP specifically impaired decision making under ambiguity but not under risk. Males also performed more poorly on the first 20 trials of the IGT, regardless of treatment group. Conclusions Oral 5-HTP is psychoactive at low doses. Decisions made under ambiguity may be differentially sensitive to increased serotonin release or associated reductions in frontocortical dopamine activity. Copyright (C) 2010 John Wiley & Sons, Ltd.