Psychology of the brain-gut-microbiome axis

"Excretion is a universal part of the human experience, but it is veiled in taboo. Psychologists have torn the veil off other taboos, such as sex and death, but they have largely ignored elimination. Nevertheless, it is linked to a rich assortment of intense emotions, mental disorders, personality traits, social attitudes and linguistic practices. From psychoanalysis to neurogastroenterology...the psychology of the toilet offers surprising insights into mind–body connections, culture and gender."

Nick Haslam (2012), The Psychologist magazine

"And I remember...my first memory...I was four years old, and I was standing in front of my parents' house and I was shitting in my pants. I was just shitting a massive, terribly painful shit...the centre of this shit was so wide that I actually came online as a result of the anal pain that I was experiencing. It actually awakened me into AAAAAAAA this stream of consciousness I am now living"

Louis C.K., stand-up comedian

Those interested in the human mind have been interested in how it interacts with our guts since at least the days of Freud and colleagues, although as Haslam implies, interest in this area may have waned with the decline of psychoanalysis within academic and research-driven psychology. However, with an increasing interest in the body within psychology and cognitive neuroscience, research is starting to address how gastrointestinal factors may play a role in human psychology.

The nature of this interaction likely goes well beyond the psychology of elimination habits. Although our central nervous systems may do the cognitive heavy lifting available to consciousness, we have an enteric nervous system within our gastrointestinal tracts. Bacteria can produce neurochemicals that impact upon receptors within this nervous system. Ted Dinan, professor of psychiatry and mentor of mine at UCC, refers to it as a form of "collective unconscious". Although the Human Genome Project has mapped out the genes of human cells, there are a huge host of bacterial cells within us; furthermore, different types of bacteria will appear in different people, so this area is opening up a whole industry of gene sequencing of our microbial tenants. The whole area of gene X environment interactions just became more complex.

A major topic in this area is stress; intuitively, you may have noticed changes in your bowel habit while going through periods of heightened stress. One of the major research findings in this area is evidence of alteration in the gut microbiota in stress-related psychological disorder. Irritable bowel syndrome is the most obvious example, but studies from Ireland, China and Norway have indicated alteration of the microbiota in major depression. However, although these studies use healthy adults as controls, it is still too early to comment on what "the" healthy human microbiome looks like. A greater level of diversity is generally seen as a good thing, but as I mentioned above there is likely to be considerable variation in microbiota between different individuals who are generally healthy, so what "the right mix" is is still up for debate.

Despite this interesting evidence in stress-related disorder, there has generally been a lack of research looking at how chronic levels of stress per se can alter the microbiota. This is regrettable when one considers that following the same individuals over periods of greater or lesser stress could get around the issue of how much difference there is between different individuals.

Conversely, could tweaking the microbiota affect stress? A small study I was working on indicated that administration of a probiotic over four weeks could reduce reported daily stress as well as an acute stress response in healthy volunteers. This would suggest that manipulation of the microbiota can potentially affect psychological outcomes. These effects did not occur using the same assessments with a different probiotic; one would expect that different strains will have different effects, but even trying to combine research on a given strain can be fraught, as pointed out in a recent editorial.

Perhaps one of the more tractable questions in this area is how changes in dietary behaviour may impact upon our microbiota at a relatively broad level. There have been studies that compare a contemporary Western diet to groups of people relatively untouched by such dietary trends, such as the Hazda of Tanzania and children from rural Burkina Faso. Although one might think the easy availability of foods from around the world might increase the Westerner's microbial diversity, some findings suggest that it's actually the other way round. We could, of course, speculate that differences in levels of stress in different populations could also have some impact upon microbial differences (and indeed, our diets may become somewhat more processed during stressful times!).

Needless to say, issues such as diet and stress play out in a broader cultural context. It would be interesting to see more research being done on the social psychology surroundings the taboos and neuroses of our toilet habits and the kind of mishaps described by Louis C.K. However, unravelling the question of just how, and to what extent, the microbiota interact with an embodied psychology is going to be keeping people busy for some time.

Allen, A. P., Dinan, T. G., Clarke, G., & Cryan, J. F. (2017). A psychology of the human brain–gut–microbiome axis. Social and Personality Psychology Compass, 11(4).

Image is a detail adapted from Fig. 1 of the paper cited above. See full text here.

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