An article was published in the world's leading scientific journal Nature Communications, the authors of which (once again) tested the connection between depression and the composition of the intestinal microbiome. And once again, such a connection was found: people with symptoms of a depressive disorder in the intestines are significantly more likely to have fewer or more bacteria of certain groups. Before that, in other work, other scientists transplanted bacteria from people with depression into rats grown under sterile conditions, and the rats also developed depression! That is, it can be assumed that at least in some cases there is a causal relationship: certain bacteria provoke depression in their hosts.
Microbiologist and science journalist Irina Yakutenko comments on this discovery in her blog:
“This idea is not new at all, and yet it still causes skepticism among many: how microbes living in the intestines can influence the processes occurring in the head? For a long time it was believed that intestinal bacteria are such microscopic parasites that the body tolerates only because they do all sorts of useful little things like the synthesis of vitamin K. But gradually, as new data accumulated and research methods improved, it became more and more obvious that the role of bacteria has been grossly underestimated, and they are able to influence many different processes from the regulation of weight gain to depression. At a minimum, due to the fact that bacterial cells synthesize all kinds of metabolically active substances, and there are a huge number of these cells inside us.
For example, intestinal bacteria synthesize such an important substance for the work of various systems of our body as serotonin. Actually, 90% of all serotonin in the human body is synthesized by intestinal bacteria. In addition, microbes living inside us produce glutamate, butyrate, GABA - that is, all those substances, violations in the work of which, as we know today, most directly affect, for example, the development of depression.
“But how does serotonin and other substances from the gut get into the brain? the savvy reader will ask. “After all, the general blood flow from the brain is separated by the blood-brain barrier, which is not so easy to overcome.” That's right, it is not easy for substances "from the outside", even from their own intestines, to get into the brain (although the same butyrate, for example, can). But substances produced by intestinal microbes can change its permeability, as a result, something can get there under their influence.
In addition, bacteria and the substances they secrete can affect the production of serotonin and other neurotransmitters in the brain not directly, but through the mediation of the vagus nerve: a very important and super-long, real nerve pathway, the processes of which collect information from almost all organs of our body including the intestines. Feeling certain signals from bacteria and their metabolic products, the vagus nerve can transmit corrective instructions to the very top - in experiments on mice this is shown directly. But such an “alien” correction will do good or harm to the body - it is not known.
What we do know for sure is that mice deprived of gut bacteria do not produce neurotransmitters in their brains in the same way as normal, healthy mice with intact gut flora. But, as numerous studies on animals and humans show, not all yogurts are equally useful, in the sense that not all bacteria have the same positive effect on the synthesis of neurotransmitters.
In general, in this topic there is still much more incomprehensible than what has been studied. It appears that gut bacteria do affect the risk of developing depression as one of the factors. But why some people have one of their composition, others have another, to what extent it depends on the diet, and to what extent - on the genetic characteristics of a person - remains to be seen. But it's funny that we are trying to treat depression by talking to the patient (we are also trying with pills, but alas, the sad truth is that the vast majority of antidepressants are ineffective, more precisely, some are effective, some are not, and to predict what will be outcome in a particular patient is not possible.
However, with psychotherapy, success is also not guaranteed, to put it mildly), and the reason may be that he has an unfortunate composition of microbes, and, you know, they are not very talkative.
I exaggerate, of course, depression is a complex multifactorial phenomenon that depends on both external and internal factors, and it can develop due to various reasons. But it cannot be ruled out that in the future, for the most effective treatment, the doctor will first of all refer a patient with depression to take an analysis of the intestinal microflora, and maybe transplant it from non-depressed donors..."