The microbiome: your second brain

The gut-brain axis is one of the fastest-growing research fields in neuroscience and medicine over the last fifteen years. The accumulated evidence shows that the gut and the brain communicate bidirectionally through multiple channels: the vagus nerve, the endocrine system and the immune system. The bacteria living in the gut — around 100 trillion in total from more than a thousand different species — are not neutral passengers but active participants in that communication.

Gut bacteria produce or influence the production of neurotransmitters that the brain uses: around 95% of the body's serotonin is produced in the gut, not in the brain. GABA, the main inhibitory neurotransmitter of the central nervous system, is also synthesised by certain gut bacteria. That the bacteria living in the gut influence the levels of neurotransmitters the brain uses to regulate mood, anxiety and sleep is a plausible biological mechanism with growing experimental support.

The connection to diet is direct. Beneficial gut bacteria thrive on fibre — complex polysaccharides that humans cannot digest but that bacteria ferment and use. A high-fibre diet of fruits, vegetables and legumes feeds beneficial bacteria and maintains microbiome diversity. A diet high in simple sugars and ultra-processed foods favours less beneficial bacteria and reduces microbial diversity — a marker that researchers associate with worse metabolic health and higher risk of mood disorders. Dietary intervention studies focused on the microbiome are still an emerging area, but preliminary results are consistent enough to justify clinical attention.