“…SCFAs, including acetate, propionate, and butyrate, have been shown to induce intestinal production of 5-hydroxytryptophan (5-HTP) and serotonin (5-hydroxytryptamine, 5-HT) by stimulating tryptophan hydroxylase (TpH1) expression in the enterochromaffin cells [ 49 ]. 5-HTP, a precursor of 5-HT, crosses the blood–brain barrier, unlike 5-HT; therefore, gut-derived 5-HTP could contribute to the synthesis of 5-HT in the brain [ 50 ]. 5-HTP and its metabolites, 5-HT and melatonin, have been reported to influence gut motility, mood, and sleep/arousal [ 51 ].…”