"The belly, our second brain". This adage has been getting a lot of press since researchers started looking at the microbial life in our gut lumen. When we know that our intestines contain as many neurons as the brain of a small dog, it makes us think about the importance of this organ in terms of its influence on our behavior.
At a time when France is one of the largest consumers of anti-depressants, researchers are interested in the possible effects of certain strains of bacteria in our microbiota on our mental equilibrium, particularly in the case of depression.
In this article, we will look at where the research stands and what links may have been established or suspected between the microbiota and depression.
The mechanisms of depression
The mechanisms of depression are complex, multifactorial, and interrelated. To simplify and to remain in the theme of our article, we will stop at the definition proposed by the INSERM which exposes it thus: "Depression is an illness that affects all ages, from childhood to late in life. Its many symptoms including sadness or loss of pleasure, are very disabling and increase the risk of suicide. The disease can be effectively treated with medication antidepressants and psychotherapy "
Antidepressants target their action on neurotransmitters, which are kinds of chemical messengers that allow proper communication between neurons. In the case of a depressionThis communication is strongly altered. Thus, some of these treatments aim to reuptake serotonin (among other things), commonly known as the good mood hormone.
Research has shown that 90% of this serotonin is produced by the intestines because, it is a derivative of tryptophan, an amino acid found only inthe diet: banana, brown rice, dark chocolate, eggs, legumes.
In addition, we must not neglect the role of of the liver which also synthesizes serotonin through a process called methylation. If this is disrupted by a clogged liver, depression is all the more likely to set in.
Gut and brain are connected by the vagus nerve
The communication between the intestinal neurons and those of the central nervous system is then direct, and this, in both directions. In fact, it is the vagus nerve that controls the pylorus, a small valve that prevents stomach acids from entering the intestines except during the passage of the food bolus. A state ofchronic stress can disrupt the proper functioning of this little doorway, allowing stomach acids to spill out washing away all the microbial flora that is so beneficial.
The flora is disturbed which leads to an inflammatoryprocess of the intestinal walls as well as hyper-permeability. Vitamins, trace elements and minerals essential for the metabolism of various organic functions, including the synthesis of serotonin, are no longer assimilated correctly. A team of researchers from INRA Bordeaux has highlighted that the lack of serotonin would be due to brain inflammation caused not only by chronic stressbut also by the presence of molecules from a hyperpermeable gut.
In this context of intestinal hyper-permeability, let us note the poor assimilation of Omega 3, natural anti-inflammatories.
Inflammation of the gut mucosa is closely related to intestinal hyperpermeability and vice versa. It is the principle of the snake that bites its own tail.
A clear correlation between microbiota and mental health?
In February 2019, in the journal Nature Microbiology, a study was published in which a team of Belgian researchers presented their work on the correlation between microbiota and mental health.
By analyzing stool samples from more than 1,000 volunteers, they were able to observe that in people with depression, two families of bacteria were less numerous. This was also the case for subjects on antidepressant treatment. These two families of bacteria are the coprococcus and dialister already known for their anti-inflammatory properties.
Despite the fact that this study does not demonstrate the link between bacterial quantity and depression, it does question researchers about possible links between the presence (or absence) of certain families of bacteria in our microbiota and the brain.
Psychobiotics or the administration of specific probiotic strains adapted to this or that type of depression.
Mice fed Lactobacillus rhamnosus had reduced anxiety and depressive symptoms (Mind-Body Institute in Canada).
Anxiety-depressed rats have extremely low GABA levels, whereas certain strains of lactobacilli and bifidobacteria (beneficial gut bacteria) in probiotics produce GABA.
These results are encouraging even though they are rodents and not humans because, indeed, the functioning of the relationship between the nervous and digestive systems is close in the two species.
Currently, many studies are underway to understand the relationship betweenmicrobiota and depressive states. The results obtained through rodent research are extremely encouraging and provide insight into the internal and physiological mechanisms of depression. At this time, there is no question of isolating specific strains of probiotics to ameliorate or prevent depression because it, as we have discussed, is multifactorial. However, this holistic approach to disease including the importance of a quality microbiota is already commonplace in naturopathy for a whole host of conditions including depression.