More than 95% of all serotonin in the human organism is produced and stored in the gut in specialized cells called Enterochromaffin cells, making the gut the largest store of this mood-modulating molecule.

Serotonin is synthesized in the gut from precursors that come from ingested food, and microbes in the gut can produce chemicals that stimulate serotonin production.

Many patients with depression complain of constipation and gastrointestinal dysfunction, and medications like SSRIs (Prozac and similar drugs) often cause transient gastrointestinal side effects, indicating a clinical connection between gut and mood.