The composition of the intestinal microbiota could influence decision-making

The way we make decisions in a social context can be explained by psychological, social and political factors. What if other forces were at work? Hilke Plassmann and 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 Nexus PNAS.

The intestinal microbiota, that is to say all the bacteria, viruses and fungi that populate our digestive tract, plays a central role in our body, well beyond digestive function. Recent research highlights its impact on cognition, stress, anxiety, depressive symptoms and behavior; mice raised in a sterile environment, for example, have difficulty interacting with other individuals.

Although these results are promising, most of this research is conducted on animals and cannot be extrapolated to humans. It also does not allow us to understand which neuronal, immune or hormonal mechanisms are at work in this fascinating dialogue between brain and intestine: researchers observe a link between the composition of the microbiota and social skills but do not know precisely how it ‘one controls the other. .

“The available data suggest that the intestinal ecosystem communicates with the central nervous system via different pathways, including the vagus nerve,” explains Plassmann (Sorbonne University), head of the Control-Interoception-Attention team at the Brain Institute of Paris and professor at On the contrary. “It could also use biochemical signals that trigger the release of neurotransmitters, such as dopamine and serotonin, essential for healthy brain function.”

Studying altruistic punishment

To determine whether the composition of the human intestinal microbiota could influence decision-making in a social context, the researcher and her colleagues used behavioral tests, including the famous “ultimatum game” in which a player receives a sum of money that he must share (fairly or unfairly) with a second player, free to decline the offer if he considers it insufficient. In this case, neither player receives any money.

Refusing the sum of money is equivalent to what we call “altruistic punishment”, that is to say the desire to punish others when a situation is perceived as unfair: for the second player, restore the Equality (no one gets any money) sometimes seems more important than getting a reward. The ultimatum game is then used as an experimental means of measuring fairness sensitivity.

To fully exploit this effect, the researchers recruited 101 participants. For seven weeks, 51 people took food supplements containing probiotics (beneficial bacteria) and prebiotics (nutrients that encourage the colonization of bacteria in the intestine), 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 study results indicate that the group that received the supplements was much more likely to reject unequal offers at the end of the seven weeks, even when the distribution of money was slightly unbalanced. Conversely, the placebo group behaved similarly during the first and second testing sessions.

Furthermore, the change in behavior in the supplemented group was accompanied by biological changes: the participants who, at the start of the study, presented the greatest imbalance between the two types of bacteria that dominate the intestinal flora (Firmicutes and Bacteroidetes ) experienced the greatest imbalance. modification of the composition of their intestinal microbiota with the taking of supplements. Additionally, they also showed the highest sensitivity to fairness during testing.

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 appears: the composition of the intestinal microbiota could influence social behavior via precursors of dopamine, a neurotransmitter involved in brain reward mechanisms.

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