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What role does the gut play in mood and stress?

New studies show a strong link between the bacteria that populate our intestines and our mental health. In summary, no healthy mind without healthy microbiota.

If you are told: it is “the conductor of our gestures and behaviours”, “the guardian of our emotions”, “the seat of our mental health”… Immediately, you think: the brain! Not that easy. “There is a permanent bilateral dialogue between our intestines and our brain,” says neurobiologist Pierre-Marie Lledo, from the Institut Pasteur. These exchanges use different channels. Blood circulation first: during digestion, the walls of the intestine draw the nutrients the body needs. They pass through the blood and inform the brain about our digestive health. Then through the nervous system. The intestine is innervated by hundreds of millions of neurons – hence its nickname “second” brain – whose primary function is to ensure intestinal motility.

The intestine produces 80% of serotonin, which regulates mood

This “enteric nervous system” does not only act locally: it also communicates with the “central nervous system”, our brain, via the vagus nerve. This is where serotonin circulates, 80% of which is produced in the digestive tract and known to play a role in regulating mood, stress and anxiety. “It is released at the synapses where it plays its role as a neurotransmitter by binding to specific receptors on neurons”, specifies Professor Gabriel Perlemuter, head of the hepato-gastroenterology and nutrition department at Antoine-Béclère hospital, in Clamart (Hauts-de-Seine). What happens in our intestines could therefore influence our emotions!

In this dialogue, our head is much less talkative than our stomach: only 20% of the signals pass in the brain-gut direction! In addition, new studies highlight the link between dysbiosis – an imbalance – of the intestinal microbiota and a disturbance in brain function. “To depress a mouse, just feed it junk food!” likes to repeat Professor Perlemuter, citing a study conducted in 2019 at the University of Graz, Austria. Researchers divided 156 mice into two groups: some were given a diet high in sugars and bad fats, while the others were given a balanced menu. After eight weeks, the mice stuffed with carbohydrates and lipids had not only gained weight but were less swift, less active, less sociable, more fearful… In other words: depressed.

By observing their microbiota, the researchers discovered that it was less diversified. Some bacteria had proliferated at the expense of others. More surprising: “if we transplant the microbiota of mice made depressed by junk food to others with a normal diet, the latter in turn plunge into depression, specifies the specialist. This disease thus ceases to be purely psychic since it is perhaps transmitted by contagion, with the stools. What to see mental pathologies in a new light, as well as certain cognitive disorders.

A team from Inrae (the research institute for food and the environment) transferred intestinal bacteria from stressed adult quail to newborn birds, still lacking in microbiota. Result: they behaved as if they themselves had been under stress, also showing a disturbance of memory! “A stress-fueled loop connects memory and the gut microbiota: stress disrupts the microbiota, which itself recreates a state of stress that disrupts memory,” the researchers concluded.

If the causal link is therefore no longer in doubt, it remains to understand precisely the mechanisms at work. Here again, research is progressing: in 2020, a team from the Institut Pasteur, Inserm and CNRS discovered that a modification of the intestinal microbiota caused by chronic stress in mice had caused a collapse of certain molecules ( endocannabinoids) in the blood and brain. In several subjects, the absence of these substances in the hippocampus – an area involved in the formation of memories and emotions – had led to depressive behavior. “Another study showed that resistance to a well-known antidepressant, Prozac, can be overcome by the administration of probiotics [des bactéries aux effets positifs sur l’organisme] says Pierre-Marie Lledo. But this dialogue between body and mind still conceals many mysteries.

We live in symbiosis with a bacterial population

In the meantime, one thing is certain: our mental health depends at least in part on the bacterial population with which we live in symbiosis and therefore, indirectly, on what we eat. “Before any psychiatric consultation, patients suffering from sleep disorders or anxiety should begin by asking themselves about the contents of their plate, confirms psychiatrist Guillaume Fond. Their diet is often unbalanced, which leads to deficiencies. A varied, nutrient-rich diet can help restore a healthy microbiota and thus alleviate depressive symptoms. »

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