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Increasing evidence suggests that the gut microbiome influences socio-affective behavior, particularly in animals. However, such studies are more challenging to conduct in humans.

A team of researchers from the business school INSEAD, Fontainebleau, France; the Brain Institute; and the University of Bonn, Bonn, Germany, sought to understand how the composition of the microbiome influences social skills.

To do this, they recruited 101 healthy men. "We chose only men for our study, particularly due to the hormonal factors in women that are significant in decision-making," explained Hilke Plassmann, a professor at INSEAD and coauthor of the study published in PNAS Nexus.

Half of the participants received a mixture of prebiotics and probiotics, including Lactobacillus and Bifidobacterium, while the other half received a placebo for 7 weeks. Their microbiome composition was analyzed at the beginning and end of the experiment.

"The participants continued their usual diet. The prebiotics allowed the bacteria to colonize the gut more easily," the researcher explained.

Measuring Sensitivity to Injustice

Participants were invited to take part in a behavioral test called the "ultimatum game," before and after receiving the supplementation. The principle is to allocate a sum of money to a player, who must then share it, fairly or not, with a second player.

The second player can accept or reject the offer, especially if deemed unfair. In this case, neither player receives any money, which is called "altruistic punishment." Indeed, the second player sacrifices their share of money to "punish" the first player for their lack of generosity. This game experimentally measures sensitivity to injustice.

The study results showed that participants who received the supplements were more likely to reject unequal offers at the end of the 7 weeks, even when the imbalance was slight. The placebo group behaved identically during the first and second test sessions.

"This suggests that modifying the gut microbiome made the participants less rational and more human, more sensitive to social considerations," summarized Plassmann.

Role of Tyrosine

The scientists also observed biologic changes. Participants who, at the beginning of the study, had the greatest imbalance between the two types of bacteria dominating the gut flora (Firmicutes and Bacteroidetes) saw their microbiome composition evolve more with the supplements. These individuals showed the greatest sensitivity to injustice during the ultimatum game.

Additionally, blood tests before and after supplementation revealed a decrease in tyrosine levels, a dopamine precursor, after 7 weeks of supplementation.

"The composition of the gut microbiota could influence social behavior through dopamine precursors, which are involved in the brain's reward mechanisms," hypothesized the neuroscientist.

This study needs to be complemented by further research, particularly on other social groups.

"One of the weaknesses of this study is that the sample consists only of men. Additionally, we focused on what happens at the behavioral and biological levels, but we don't know why this occurs at the psychological level," noted Plassmann.

However, the uniqueness of this study lies in its being the first to measure the influence of the microbiome on decision-making in humans. "For future studies, we can try to understand why this psychological reaction is observed," she said.

The importance of the microbiome's role is increasingly studied, including in psychiatric disorders. A meta-analysis published in 2021 in JAMA Psychiatry showed that imbalances in the microbiota can be found in patients experiencing depression, anxiety, bipolar disorder, and schizophrenia. "The difficulty at the moment is that we don't know if it's the change in the microbiota that influences the disease or vice versa. It's the chicken or the egg problem," commented Plassmann.

This story was translated from the Medscape French Edition using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.