For a long time, the intestine was dismissed as a mere digestive tube. But now researchers are dedicating themselves to this eight-metre-long organ. Located at the body’s core, the “intestinal brain” extends to influence both our emotional world and our thinking. The relationship between the metabolism of the microbiome (intestinal flora) and the psyche is certainly one of the most fascinating topics in microbiome research. In recent years, increasing evidence has shown that mental illnesses are associated with altered intestinal flora. This has now been confirmed by a study on individuals suffering depression published in the Nature Microbiology Journal in 2019.
The relationship between the metabolism of our intestinal flora and our mental health is one of the most gripping yet controversial topics in microbiome research. Although only a fraction of intestinal microorganisms have been researched, we do know about the close interaction between the microbiome, the intestine, and the brain. Intestinal bacteria not only influence our emotions and thoughts to a certain extent but probably also influence some diseases (from depression to autism). These, in turn, are reflected in an altered composition of the microbiome. However, it is not yet clear what came first: the chicken or the egg. In other words: whether changes in intestinal flora trigger the disease or the disease changes the intestinal flora. While the relationship between intestinal flora, intestine, and brain has already been investigated in several animal models, specific human studies on the subject are still lacking.
Study: Altered intestinal flora in depression
In the current study, a 15-member research team compared the microbiome of 1,054 patients diagnosed with depression with the microbiome of 1,070 healthy people. It was shown that the butyrate-producing bacteria Faecalibacterium and Coprococcus were associated with a higher quality of life. However, the bacterial counts of Coprococcus and Dialister was decimated in the sample of patients suffering depression – even after the depression had been treated with antidepressants.
A metabolic analysis of the micro-organism mixtures present in the subjects also showed a positive correlation between the bacterial metabolite 3,4-dihydroxyphenylacetic acid (an intermediate product of the endogenous happiness hormone, dopamine) and mental quality of life and suggests a possible role of the microbial production of γ-aminobutyric acid (an important endogenous messenger substance) in depression.
A relationship between a disturbed intestinal flora and depression has long been suspected. The current study results once again confirm that the intestinal flora and metabolic products of patients with depression differ significantly from those of healthy individuals.
Valles-Colomer, M. et al. 2019. The neuroactive potential of the human gut microbiota in quality of life and depression. Nat Microbiol. 4(4):623–32.