Mental illness and the gut microbiota: the end of a riddle?

Many studies have been conducted throughout the world recently to identify the special characteristics of disturbances in the gut microbiota of people suffering from mental illnesses. Is their intestinal flora less rich than that of people in good health? Less diversified? Are some species of micro-organisms very well represented? Or conversely are they missing? The stakes are high because if specific characteristics associated with one or more mental illnesses are found in different studies, they could serve as useful markers for the diagnosis of patients, the treatment strategy or the assessment of the response to treatments. However, up to now, these studies provide results that are still contradictory.

Imbalances common to several psychiatric illnesses 

A publication in JAMA (sidenote: JAMA Journal of the American Medical Association   )  Psychiatry is gaining ground by reviewing nearly 60 studies performed on this subject. The objective of its authors is to confirm that mental illnesses are indeed associated with disturbances of the gut microbiota and to determine if these are specific to each illness:

• depression,
• bipolar disorders,
• OCD,
• schizophrenia,
• psychosis,
• anorexia,
• anxiety,
• etc.

Scientists have observed a significant reduction in the richness of the gut microbiota of patients with mental disorders, but little difference in the diversity of species in comparison with the microbiota of participants in good health. Instead of demonstrating specific characteristics for each illness, these studies rather show similar imbalances of the intestinal flora shared by several mental disorders. In particular, these disturbances result in the increase in certain species promoting inflammation and the reduction of other species with an anti-inflammatory action in bipolar disorders, schizophrenia and anxiety.

Confusion factors to be taken into consideration

Finally, the review has made it possible to determine the factors responsible for the variations of results between the studies. On the one hand, the geographical area: diet, so the microbiota and the imbalances in the microbiota are not the same in China as they are in Western countries. On the other hand, the taking of medicines: psychotropic drugs seem to favour dysbioses. Researchers must, therefore, keep these parameters in mind to be able to unveil all the mysteries of the link between the gut microbiota and mental illnesses, for the benefit of patients. 

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