The composition of the gut microbiota can influence decision-making

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Commensal bacteria (red) between the mucus (green) and epithelial cells (blue) in the small intestine of a mouse. Credit: University of Chicago.

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Commensal bacteria (red) between the mucus (green) and epithelial cells (blue) in the small intestine of a mouse. Credit: University of Chicago.

The way we make decisions in a social context can be explained by psychological, social and political factors. But what if there were other forces at work? Hilke Plassmann and her colleagues from the Paris Brain Institute and the University of Bonn show that changes in the gut microbiota can influence our sensitivity to fairness and how we treat others. Their findings are published in the journal PNAS nexus.

The gut microbiota – that is, all the bacteria, viruses and fungi that inhabit our digestive tract – play a crucial role in our body, well beyond digestive function. Recent research underlines its impact on cognition, stress, anxiety, depressive symptoms and behavior; for example, mice raised in a sterile environment have difficulty interacting with other individuals.

While these findings are promising, most of this research is conducted on animals and cannot be extrapolated to humans. Nor can we understand which neuronal, immune or hormonal mechanisms are at work in this fascinating dialogue between brain and gut: researchers observe a link between microbiota composition and social skills, but do not know exactly how one controls the other . .

“The available data suggest that the intestinal ecosystem communicates with the central nervous system through several routes, including the vagus nerve,” explains Plassmann (Sorbonne University), head of the Control-Interoception-Attention Team at the Paris Brain Institute, and professor at the University from Paris. Insead. “It could also use biochemical signals that trigger the release of neurotransmitters, such as dopamine and serotonin, which are essential for proper brain function.”

Studying altruistic punishment

To determine whether the composition of the human gut microbiota could influence decision-making in a social environment, the researcher and her colleagues used behavioral tests, including the famous “ultimatum game” in which one player is given an amount of money that he must divide (fairly or unfairly) with a second player, who is free to reject the offer if she considers it insufficient. In that case, neither player receives any money.

Refusing the sum of money is equivalent to what we call ‘altruistic punishment’, that is, the impulse to punish others when a situation is perceived as unfair: for the second player, restoring equality (no one gets money) sometimes feels more important then obtaining a reward. The ultimatum game is then used as an experimental way to measure sensitivity to fairness.

To take full advantage of this effect, the researchers recruited 101 participants. For seven weeks, 51 people took nutritional supplements with probiotics (beneficial bacteria) and prebiotics (nutrients that promote the colonization of bacteria in the intestines), while 50 others received a placebo. They all participated in an ultimatum game during two sessions at the beginning and end of the supplementation period.

Are bacteria pulling the strings?

The results of the study indicate that the group that received the supplements was much more likely to reject unequal offers at the end of the seven weeks, even if the distribution of the money was slightly unbalanced. Conversely, the placebo group behaved similarly during the first and second testing sessions.

Furthermore, the behavioral change in the supplemented group was accompanied by biological changes: the participants who at the start of the study had the greatest imbalance between the two types of bacteria that dominate the intestinal flora (Firmicutes and Bacteroidetes) experienced the most significant change in the composition of their intestinal microbiota by taking supplements. Moreover, they also showed the greatest sensitivity to honesty during the tests.

The researchers also observed a sharp drop in their levels of tyrosine, a precursor to dopamine, after the seven-week intervention. For the first time, a causal mechanism emerges: the composition of the gut microbiota could influence social behavior via the precursors of dopamine, a neurotransmitter involved in brain reward mechanisms.

“It is still too early to say that intestinal bacteria can make us less rational and more receptive to social considerations,” Plassmann concludes. “However, these new results clarify which biological pathways we should be looking at. The prospect of modulating the gut microbiota through diet to positively influence decision-making is fascinating. We should explore this pathway very carefully.”

More information:
Marie Falkenstein et al, Impact of gut microbiome composition on social decision making, PNAS nexus (2024). DOI: 10.1093/pnasnexus/pgae166

Magazine information:
PNAS nexus

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