The gut produces approximately 95 percent of the body's serotonin. It contains 500 million neurons embedded in the gut wall, communicating bidirectionally with the brain via the vagus nerve in what neuroscientists now call the enteric nervous system, or the second brain. The bacteria in the gut produce neurotransmitters and neuroactive compounds including GABA, dopamine precursors, short-chain fatty acids that cross the blood-brain barrier, and immune signalling molecules that directly affect brain function and mood.
The Diet-Depression Connection
The SMILES trial, the first randomised controlled trial to test a dietary intervention for clinical depression, found that patients with major depressive disorder who adopted a Mediterranean dietary pattern for 12 weeks showed significantly greater reductions in depression symptoms than those in a social support control group. At 12 weeks, 32 percent of the dietary intervention group were in clinical remission compared to 8 percent in the control group. Food as medicine for mental health is not a metaphor. It is an evidence-based intervention.
Inflammatory Foods and Depression
Approximately 30 percent of depression is characterised by elevated inflammatory markers, a pattern referred to as inflammatory depression. For this subgroup, anti-inflammatory dietary interventions are likely to produce the greatest mental health benefit. Reducing ultra-processed food, refined carbohydrates, and omega-6 vegetable oils while increasing oily fish, vegetables, and olive oil directly reduces the neuroinflammatory burden that maintains this form of depression.
Feeding the Right Bacteria for Mood
Lactobacillus and Bifidobacterium species produce GABA locally in the gut. GABA produced in the gut does not cross the blood-brain barrier in significant amounts directly, but these bacteria influence vagal signalling and the systemic GABA balance. Multiple clinical trials of probiotic interventions in healthy and depressed populations show improvements in self-reported anxiety, perceived stress, and quality of life, particularly with multi-strain formulations containing these genera.