The gut contains more serotonin than the brain

About 95% of the body's serotonin — the neurotransmitter associated with mood, wellbeing, and sleep — is produced in the gut, not the brain. Enterochromaffin cells in the intestinal lining produce serotonin in response to food and gut bacteria, where it regulates intestinal movement. The gut-brain axis — a bidirectional communication network via the vagus nerve — means gut serotonin indirectly influences mood and anxiety. This is part of why gut microbiome health correlates with mental health outcomes: the bacteria that help produce serotonin precursors are directly affected by diet, antibiotics, and stress.

Source: Yano et al., Cell (2015)