Human study maps region-specific links between gut microbes and brain chemistry

A study of 61 healthy women has identified region-specific associations between the gut microbiome’s metabolic potential and levels of key chemical messengers in the brain. The findings, published in Molecular Psychiatry and reported by Medical Xpress, add human evidence to a gut–brain connection previously studied largely in animals and other preclinical models.

Researchers from the University of Surrey and the University of Roehampton combined stool analysis, brain imaging and psychological measures in participants aged 17 to 25. Proton magnetic resonance spectroscopy was used to measure gamma-aminobutyric acid (GABA) and glutamate in three brain regions. GABA generally dampens neural activity, while glutamate generally promotes it; their balance is often described as excitatory/inhibitory balance.

The stool samples underwent shotgun metagenomic sequencing, which identified the genetic capacity of participants’ gut microbes to perform metabolic processes involving GABA, glutamate, short-chain fatty acids and other neuroactive compounds. This approach assessed what the microbial community could potentially do genetically, rather than directly measuring the metabolites it produced.

The associations differed by brain region. The inferior occipital gyrus, included as a visual-processing comparison area, showed links with the broadest range of microbial pathways, including those related to glutamate degradation, GABA metabolism, short-chain fatty acids, inositol and p-cresol. The anterior cingulate cortex showed a different pattern involving microbial glutamate and propionate pathways. In the dorsolateral prefrontal cortex, the researchers observed a more selective association involving a pathway linked to microbial GABA production.

The team also examined psychological measures. A GABA-related microbial pathway was associated with trait and social anxiety, while pathways involved in tryptophan metabolism were associated with depressive symptoms and social anxiety. A pathway involved in producing the short-chain fatty acid propionate was associated with poorer self-reported sleep quality.

The results do not show that specific bacteria cause anxiety, alter brain chemistry or affect sleep. Because the study was cross-sectional, it cannot establish whether gut-related differences influence the brain, whether brain or behavioral factors influence the gut, or whether another biological process affects both. The researchers also measured genetic potential rather than directly observing microbial production of the relevant compounds.

The study’s authors said possible communication routes could include the vagus nerve, immune signaling, the intestinal barrier and other gut-derived metabolites, rather than neurotransmitters traveling directly from the gut to the brain. Larger longitudinal and intervention studies will be needed to test whether changing the identified microbial pathways can alter human brain chemistry.