Cognitive effects of acute tryptophan depletion in the healthy elderly.

BACKGROUND Studies investigating the cognitive effects of serotonin depletion, using the technique of acute tryptophan depletion (ATD) by dietary means, have generally suggested that ATD impairs delayed verbal recall and recognition. In two previous studies in the elderly, this result has not been replicated and ATD impaired working memory. These results may be susceptible to type II error but a similar testing schedule in the individual studies allows data to be pooled in a larger analysis. METHODS Data from two separate double-blind placebo-controlled studies of the effects of ATD on cognitive function in the elderly were combined. In one study, a low dose and in the other a high dose of amino acids was used. In a repeated measures analysis of variance, the effects of ATD and the interaction of this with the other factors (age, gender and dose) on cognitive measures was examined. RESULTS Data from 31 healthy subjects aged between 60 and 81 years were analysed. There were no main effects of ATD or consistent interactions between ATD and age, gender or dose. There were significant interactions between ATD, gender and dose. When tryptophan depleted, females having the higher dose drink had reduced scores on Digit span and immediate recall on the Rey Auditory Verbal Learning Test. CONCLUSION The enlarged data set did not confirm an overall effect of ATD on working memory or on delayed word recall but does suggest an effect of ATD on encoding or registration in the subgroup of females receiving a higher strength drink.