The enteric nervous system of the gut contains approximately 500 million neurons, more than the spinal cord. It communicates with the brain through the vagus nerve, the hypothalamic-pituitary-adrenal axis, the immune system, and through the production of neurotransmitters including approximately 90 to 95 percent of the body's serotonin. The gut is not merely responsive to the brain. It is in constant bidirectional dialogue with it.
The Microbiome and Mood
Gut bacteria produce and modulate the metabolites that influence brain function. Short-chain fatty acids produced by bacterial fermentation of fibre influence microglia activity, the immune cells of the brain. Serotonin precursor availability depends on tryptophan metabolism by gut bacteria. GABA, the primary inhibitory neurotransmitter, is produced by specific Lactobacillus species. A disrupted microbiome alters the availability and balance of these neuroactive compounds.
The Clinical Evidence
Multiple randomised controlled trials have demonstrated that probiotic supplementation with specific Lactobacillus and Bifidobacterium strains reduces self-reported anxiety and depression scores in healthy adults. Dietary interventions that improve gut microbiome diversity are associated with improved mood outcomes. The Smiles trial found that a Mediterranean dietary intervention produced significantly greater reductions in depression scores than social support alone in adults with major depression.
Practical Support for the Gut-Brain Axis
- Mediterranean dietary pattern: the most comprehensively evidenced dietary approach for gut microbiome diversity and mood
- Regular exercise: increases microbiome diversity and promotes the growth of butyrate-producing bacteria
- Stress management: chronic psychological stress disrupts the microbiome directly through HPA axis activation and altered gut motility
- Sleep optimisation: the microbiome has its own circadian rhythm, and sleep disruption alters its composition and function