The composition of the gut microbiota can influence decision-making

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

Intestinal microbiota, ie. all the bacteria, viruses and fungi that inhabit our digestive tract play a key role in our body, far beyond the digestive function. Recent studies highlight its effects on cognition, stress, anxiety, depressive symptoms, and behavior; mice raised in sterile conditions, for example, have difficulty interacting with other individuals.

Although these findings are promising, most of this research is done in animals and cannot be extrapolated to humans. It also prevents us from understanding which neuronal, immune or hormonal mechanisms are at work in this fascinating dialogue between the brain and the gut: researchers observe a connection between the composition of the microbiota and social skills, but do not know exactly how one controls the other.

“Available evidence suggests that the gut ecosystem interacts with the central nervous system through various pathways, including the vagus nerve,” explains Plassmann (Sorbonne University), head of the Control-Interoception-Attention Group at the Paris Brain Institute and Professor at Insead. “It can also use biochemical signals that trigger the release of neurotransmitters like dopamine and serotonin, which are essential for the brain to function properly.”

A study of altruistic punishment

To determine whether the composition of a person’s gut microbiota can influence decision-making in social networks, the researcher and her colleagues used behavioral tests, including the famous “ultimatum game”, in which one player is given an amount of money to share (fairly or unfair) with the other player who can reject the offer if he deems it insufficient. In this case, neither player gets any money.

Giving up a sum of money is equivalent to what we call “altruistic punishment,” ie. the desire to punish others when the situation is perceived as unfair: for the other player, restoring equality (no one gets money) sometimes seems more important than receiving a reward. The ultimatum game is then used as an experimental way to measure fairness sensitivity.

To fully exploit this effect, the researchers recruited 101 participants. For seven weeks, 51 people took dietary supplements containing probiotics (good bacteria) and prebiotics (nutrients that promote the colonization of bacteria in the gut), while 50 others received a placebo. They all participated in the ultimate game for two sessions at the beginning and at the end of the feeding period.

Bacteria pulling the strings?

The results of the study show that the supplement group was significantly more likely to reject the unequal offers at the end of seven weeks, even though the distribution of money was slightly unbalanced. In contrast, the placebo group behaved similarly during the first and second test sessions.

Moreover, behavioral change in the supplement group was accompanied by biological changes: participants who had the greatest imbalance between the two types of bacteria that dominate the intestinal flora (Firmicutes and Bacteroidetes) at the beginning of the study experienced the most significant imbalance and change in the composition of the microbiota. intestine with the intake of food supplements. In addition, they also showed the greatest sensitivity to fairness during the tests.

The researchers also observed a dramatic drop in levels of tyrosine, a precursor to dopamine, after the seven-week intervention. A causal mechanism is revealed for the first time: the composition of the gut microbiota can influence social behavior through the precursors of dopamine, a neurotransmitter involved in the brain’s reward mechanisms.

“It’s too early to say that gut bacteria can make us less rational and more susceptible to social judgment,” Plassman 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 exciting. We need to study this pathway very carefully.”