In healthy young adults, an eight-day period of short-term overfeeding significantly reduces fecal serotonin levels compared to control and underfeeding diets, and this reduction is associated with decreased abundance of Bacteroides and its tryptophanase enzyme, indicating a link between dietary energy load and gut microbial metabolism of serotonin precursors.

When a person eats more food than usual, the gut bacteria called Bacteroides decrease in number. These bacteria make an enzyme that breaks down a chemical needed to produce serotonin. With fewer bacteria and less enzyme, less serotonin ends up in the stool. The faster movement of food through the gut also reduces the time bacteria have to make serotonin, further lowering its levels in feces.