Can gut bacteria influence your mental health?

It is commonly known that maintaining mental and physical health are crucial to your overall wellbeing, but what we didn’t realise is that they may be linked. Two studies have contributed to a growing body of scientific research that supports the connection between the human gut microbiome and brain function, particularly post-traumatic stress disorder (PTSD) and Alzheimer's disease.

A study published in Scientific Reports compared the gut microbiome of 25 human subjects with Alzheimer's disease to 25 cognitively healthy participants.

"By using DNA sequencing to take a 'snapshot' of gut bacterial composition, we found that individuals with dementia had decreased microbial richness and diversity in their gut microbiome compared to people without a diagnosis of dementia," explained first author of the study Nicholas Vogt.

"We were able to identify broad taxonomical changes in gut bacterial composition, as well as changes in abundance of a number of bacterial groups, some of which were more abundant in people with dementia due to Alzheimer's disease and some of which were less abundant."

Conducted by researchers from the University of Wisconsin School of Medicine and Public Health, the study took cerebrospinal fluid samples from participants which indicated that the abundance of these bacterial groups was associated with the pathologies of Alzheimer's disease.

"A profile of gut bacteria that looked more similar to what we observed in dementia was associated with Alzheimer's disease pathology in the brain," said Dr Barbara Bendlin, Associate Professor of Medicine (geriatrics) at UW. "This was even the case among people who were cognitively healthy, suggesting a link between gut bacteria and the brain even in the absence of dementia."

The second study, carried out by Stellenbosch University researchers, suggested three types of bacteria present in the gut microbiome might indicate whether a person will develop PTSD.

"Our study compared the gut microbiomes of individuals with PTSD to that of people who also experienced significant trauma, but did not develop PTSD (trauma-exposed controls)," said lead researcher Stefanie Malan-Muller. "We identified a combination of three bacteria (Actinobacteria, Lentisphaerae and Verrucomicrobia) that were different in people with PTSD."

Gut microbes are known to metabolise food and medicine and fight infections, and this study indicates that the microbiome can also produce neurotransmitters/hormones, immune-regulating molecules and bacterial toxins. When an individual is stressed or emotional, the composition of the microbiome is altered and this can lead to inflammation in the brain.

Participants with PTSD had significantly lower levels of all three bacteria compared to the control group which suggests the three bacteria could result in heightened levels of inflammation, impacting the brain and contributing to the symptoms of PTSD.

However, it remains unclear whether gut bacteria impacts the disease or whether they are a consequence of the disease.

While living conditions, childhood experiences and genetic make-up are believed to influence a person’s susceptibility to PTSD, all contributing factors are not fully understood. Research into the connection between the gut and brain continues to be of paramount importance as it may help the development of future treatments.

This was especially true as Malan-Muller noted: “The microbiome can easily be altered with the use of prebiotics (non-digestible food substances), probiotics (live, beneficial microorganisms) and synbiotics (a combination of probiotics and prebiotics), or dietary interventions."

The study was published in the journal Psychosomatic Medicine.