iCASE

Recognition and understanding of intentions are key processes in social cognition. An important neuromodulator regulating social cognition is serotonin, the precursor of which is the essential amino acid tryptophan (TRP). Altered serotonin availability and metabolization is hypothesized for several mental problems, physical illnesses and further also for elderly individuals. Reduced TRP availability leads to weaker impulse control ability and higher aggression, while TRP supplementation promotes confidence.

We investigated the influence of TRP on social cognition with two social-cognitive tasks. In the moral task, participants rated the reprehensibility or permissibility of protagonists’ behaviors in short stories. In the other task, participants determined emotions from pictures of persons eye regions. Participants were given 5-hydroxytryptophan (5-HTP) or TRP and filled in food diaries or recalls. Further, participants filled in questionnaires e.g. on mood and empathy. 5-hydroxyindoleacetic acid (5-HIAA) was measured from urine samples and blood samples were taken for genetic analyses.

With 5-HTP or a TRP rich diet, immoral behavior without negative consequences for others was rated as more reprehensible, and during story reading activation in insula and supramarginal gyrus was increased after a 4-week TRP intake. An important covariate was the 5-HIAA as an indicator of serotonin status. The supramarginal gyrus is associated with empathy, pain and related information integration. Consistently, participants showed a stricter evaluation of bad behavior possibly due to better integration of information. No significant effects of TRP on emotion recognition were identified when including all age groups. Emotion recognition ability decreased with age, but not for positive emotions when consuming TRP, possibly indicating a compensatory effect. Likewise, TRP seemed to improve recognition of positive emotions in S´S´ allele carriers of the serotonin transporter polymorphism, additionally, better mood was found. The monoamine oxidase-A high activity allele (relevant for serotonin catabolism), together with the degree of methylation at a promoter CpG site on the tryptophan hydroxylase-2 gene (relevant for serotonin synthesis) explained significant proportions of variance in emotion recognition performance. Some other nutrients in the diet seemed to affect social cognition, e.g. cholesterol was negatively associated with perspective-taking.

In healthy individuals who have a lower TRP availability or an altered serotonin metabolism (e.g. older age, higher BMI, risk allele carriers, degree of methylation, natural nutrition), TRP supplementation could have a positive influence on social cognition. Besides the above mentioned factors, the 5-HIAA level might be an additional indicator for individualized diet recommendations. Our results regarding TRP may help to ameliorate interventions in healthy people at risk and initiate future research in this field.