Gut-brain axis

The gut-brain axis is the bidirectional communication network linking the enteric nervous system, vagus nerve, hypothalamic-pituitary-adrenal (HPA) axis, immune signalling and blood-borne microbial metabolites between the gastrointestinal tract and the central nervous system. The gut microbiota participates in this network by producing neuroactive compounds — including serotonin precursors (roughly 90 % of whole-body serotonin is synthesised in enterochromaffin cells, stimulated partly by microbial metabolites), gamma-aminobutyric acid (GABA), short-chain fatty acids and secondary bile acids — that can reach the brain directly or modulate afferent vagal signalling. Animal studies have provided compelling evidence that germ-free or antibiotic-treated mice show altered stress responses, anxiety-like behaviour and neuroinflammation that can be partly reversed by specific microbiota reconstitution or probiotic treatment. Human evidence is more limited: proof-of-concept trials with specific psychobiotics have shown modest anxiolytic or antidepressant-like signals in some cohorts, but effect sizes are small and replication has been inconsistent, meaning clinical translation remains premature.