# Gut-brain axis

The gut-brain axis is the two-way communication line between your gut and your brain. It runs over several channels: the gut's own nervous system, the vagus nerve, the stress-hormone (HPA) axis, immune signals, and microbial chemicals carried in the blood. Your gut bacteria join the conversation by making brain-active compounds. These include serotonin building blocks (about 90% of your body's peripheral serotonin is made in gut cells, partly nudged by microbial signals), GABA, short-chain fatty acids, and secondary bile acids. They can reach the brain directly or tweak the signals traveling up the vagus nerve. Animal studies make a strong case: germ-free or antibiotic-treated mice show altered stress responses, anxiety-like behavior, and brain inflammation, which can be partly reversed by restoring specific microbes or giving probiotics. The human evidence is thinner. Trials of specific 'psychobiotics' show small calming or mood signals in some groups, but the effects are modest and do not replicate consistently, so clinical use is still premature.

## Sources

- Mayer EA, Tillisch K, Gupta A. (2015). Gut/brain axis and the microbiota. Journal of Clinical Investigation. https://doi.org/10.1172/JCI76304

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