We therefore studied in a double-blind randomized controlled pilot trial whether orally administered capsules of butyrate compared to a single infusion of donor feces derived from post-RYGB patients affects brain SERT and DAT binding, whole body serotonin metabolism, and small intestinal tryptophan hydroxylase 1 (TPH1) gene expression as well as insulin sensitivity in subjects with metabolic syndrome. The gene discussed is TPH1; the disease is metabolic syndrome.