Chinese Study Shows a Connection Between the Intestinal Flora and Depression

In addition to their known role in digestion, intestinal bacteria also influence brain activity and mood. Chinese researchers have observed changes in the intestinal flora in people suffering from depression.

Depression

The intestinal microbiota is a collection of bacteria, viruses, and fungi that live between the villi of our intestines. These microorganisms are not harmful but actively participate in the proper functioning of the human body by synthesizing essential metabolites or training our immune system, among others. Unbalance of the intestinal microbiota has been observed in certain diseases, particularly mental illnesses. According to various hypotheses, the microbiota affects the brain via the axis “microbiota brain intestines”. An experiment with mice supports these hypotheses: transplantation of the microbiota from a depressed patient to mice without intestinal flora led to the development of depressive disorders in the rodents.

A publication at the beginning of December in Science Advances also supports this idea. Scientists from various Chinese medical universities have analyzed the bacterial and metabolic composition of various fecal samples from patients with severe depressive disorders who are not taking drugs that can alter the intestinal flora. Their analysis revealed a significantly different composition than that of healthy patients.

Observable differences in the stools of depressed patients

The scientists analyzed 311 stool samples, first under the prism of metagenomics and then under the prism of metabolomics. The metagenomics allows an accurate knowledge of the bacterial species present in the sample and their proportions. As a result, the scientists were able to show that the intestinal microbiota of depressed patients is enriched with bacteria of the genus Bacteroides, and poor in bacteria of the genus Blautia, Eubacterium and Clostridium compared to the microbiota of healthy humans.

They also investigated the composition of bacteriophages, the bacteria-specific viruses, but no significant difference was observed between depressed and healthy patients.

The metabolic analysis also revealed significant differences. In the feces of depressed patients, compounds involved in the metabolism of amino acids were depleted, especially arginine, proline, and a neurotransmitter responsible for the inhibition of the nervous system, GABA.

Biomarkers for diagnosing diseases

The scientists believe that these lower levels of GABA are directly related to the modification of the microbial composition of the microbiota in depressed patients. A gene involved in the synthesis of GABA also shows lower activity in depressed patients. Previous research has already associated major depressive disorders with reduced GABA production.

Eventually, scientists were able to identify a patient with a major depressive disorder by analyzing the combination of these two biomarkers. Currently, depression is diagnosed after an interview by a mental health professional, a method that contains many biases and can lead to a misdiagnosis. Scientists hope that these results will serve as a basis for the development of new and more reliable diagnostic techniques.