Is there a real connection between your gut microbiome and your mood or brain?
There is a biologically plausible and, in animal research, robustly supported connection between your gut microbiome and your brain, but causality has not yet been proven in healthy humans. A varied, fibre-rich diet is the most concrete step you can take right now.
About 90 percent of the serotonin in your body is produced in the gut, not in the brain. The bacteria in your gut influence that production. This is one of the most concrete pieces of evidence for a biological link between your gut microbiome and your mood.
Gut bacteria communicate with your brain through multiple pathways simultaneously: via the vagus nerve (a long nerve that runs directly from the brainstem to the abdomen), via the immune system, via hormones, and via bacterial waste products called short-chain fatty acids. This interplay has been convincingly demonstrated in animal research. In humans, observational studies show that a disrupted gut microbiome -- caused by a poor diet, antibiotic use, or unhealthy lifestyle habits -- is associated with higher levels of depressive complaints. Whether the disruption is the cause or a consequence of those complaints has not yet been definitively established in humans.
A small randomised trial in 63 healthy adults aged 65 and over showed, after twelve weeks of probiotic use, a noticeable improvement in mental flexibility and stress score, plus an increase in a substance in the blood that supports brain cell growth. This is a positive signal, but the study is too small to draw firm conclusions from. Larger trials are needed.
The link is clearest in people with severe liver disease: a disrupted gut microbiome is associated there with a markedly higher risk of cognitive problems and low-mood complaints. However, this is a specific situation and says little about healthy people.
Psychobiotics -- bacteria or substances intended to influence the brain via the gut microbiome -- are being extensively investigated as a possible treatment for depression. It is an active area of research, but the clinical evidence is currently too thin for concrete treatment recommendations. What you can do yourself: a varied, fibre-rich diet supports a diverse gut microbiome, and observational studies consistently link healthy eating to better mental health.
Based on multiple reviews and a small RCT (n=63). Animal research is robust; evidence in healthy humans is promising but largely observational and mechanistic. Causality in humans has not yet been definitively demonstrated.