The gastrointestinal tract contains the largest peripheral reservoir of serotonin — approximately 95% of the body's serotonin is produced by enterochromaffin cells in the gut — and bidirectional communication between the enteric nervous system and the CNS occurs via the vagus nerve, immune signaling, and microbial metabolites; gut microbiome composition influences CNS serotonergic signaling, neuroinflammation, and HPA axis function through these pathways

Serotonin is produced exclusively in the brain by raphe nuclei neurons; the gut contains no serotonin and has no neural connection to the central nervous system

The gut microbiome has no influence on brain function or psychiatric symptoms; the co-occurrence of IBS and depression in this patient is entirely coincidental

The vagus nerve only transmits motor signals from the brain to the gut; it cannot carry sensory information from the gut to the brain, so the gut cannot influence mood or cognition