The connection between the gut microbiome, mood health, and brain function is referred to as the gut-brain axis. It is a bidirectional signaling pathway between the central and enteric nervous system where the gut microbiome communicates with the brain, impacting certain aspects of mood and brain health. The gut-brain axis is thought to influence cognitive functioning and mood through mechanisms related to the immune system, inflammation modulation, and cellular metabolism. A recent review by Liu and colleagues explored the potential link between mood health and the microbial composition of the gut. 

Specific microbial species in the gut have been associated with mood health, according to preclinical studies. Some species in the Bacteroidaceae family including Bacteroides thetaiotaomicron, B. fragilis, and B. uniformis have been linked to a potentially increased risk of depressive-like mood changes. 

Both animal and clinical studies have investigated the potential link between microbial dysbiosis and changes in mood health. A 2023 animal study performed a fecal transplant from individuals with major depressive disorder (MDD) to healthy murine populations. The study results indicated that depressive-like symptoms were induced in the previously healthy animals, suggesting that microbial dysbiosis in the gut may impact overall mood balance. Recent research also suggests that there may be a feedback loop between gut dysbiosis and depression-like changes. A population study from 2021 linked the increased presence of certain commensal species in the Bacteroides genus with improved anxiety and depression scores.

Certain microbial metabolites may also influence the gut-brain axis. Reduced levels of short-chain fatty acids (SCFAs) and changes in bile acid metabolism have been observed in individuals with MDD. In addition, the administration of SCFAs has been shown to help support improvements in depressive-like symptoms, intestinal permeability, and hypothalamic-pituitary-adrenal (HPA) axis reactivity. 

Liu and colleagues discuss potential lifestyle interventions that may influence gut microbial composition. The Mediterranean diet has been shown to help support many aspects of health and research demonstrates adopting the diet may help promote a healthy gut microbiome and a positive mental outlook. It has also been linked to intestinal barrier health, increased gut microbial diversity, and improvements in the gastrointestinal inflammatory response. 

Furthermore, research shows that certain probiotics help support the gut-brain axis. Clinical studies have demonstrated that decreased plasma levels of interferon-ɣ, tumor necrosis factor-ɑ, and cortisol, along with improvements in certain parameters related to memory and cognition, have been observed in the presence of L. plantarum.

Similarly, a systematic review and meta-analysis of clinical trials related to probiotics and mood health further explored the gut-brain connection. Compared to placebo, the treatments groups analyzed from 23 trials who received probiotic supplements experienced fewer depression symptoms. Anxiety symptoms were also lower in the treatment arms from twenty-two trials that included probiotics compared to the placebo groups. 

More research is needed, particularly in clinical studies with a higher number of study participants. However, evidence suggests that lifestyle changes and certain probiotics may help support gut health, cognitive function, and mood health. 

By Dr. C. Ambrose, ND, MAT