The human gastrointestinal system principally refers to the stomach and intestine, but can encompass the entire gastrointestinal tract, including all structures and organs from the mouth to the anus.

Most of us have experienced the effects of moods and emotions on our gastrointestinal tract, from "butterflies" in the stomach caused by nervousness to a loss of appetite when we're feeling blue.

In animal studies, boosting serotonin in the cells that line the gut reduced anxious and depressive-like behaviors without causing cognitive or gastrointestinal side effects.