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Pre- and Probiotic Supplements Increase Sensitivity to Social Considerations

3D illustration of the microbiota-gut-brain-axis showing signals from the brain to the gastrointestinal tract
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How the gut microbiome impacts behavior and brain function is an area of increasing research interest. Studies have shown that not only do social interactions shape the composition of the microbiome but that the microbiome may also modulate social behavior.

To better understand the modulatory role the gut microbiome may play in social cognition and behavior, researchers investigated whether taking pro- and pre-biotics could affect levels of altruistic punishment. The findings, published in the journal PNAS Nexus, suggest that social decision-making is influenced by not just our brain but also the microorganisms that inhabit our gut.

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The gut microbiomes impact on decision making

Human and animal models suggest that gut microbiota can communicate with the central nervous system through several different pathways including signaling via the vagus nerve. Studies have also shown the microbiota–gut–brain axis is involved in the release and metabolism of dopamine, serotonin and their precursors.

The researchers set out to investigate the effects of human gut microbiome composition on social decision-making and shed light on the underlying mechanisms along the microbiota–gut–brain axis. To do this, the scientists tested whether taking pro- and pre-biotics could affect levels of altruistic punishment.

What is the microbiota–gut–brain axis?

The gut-brain axis is a two-way communication network that links the enteric and central nervous systems. The gut-brain axis allows the brain to influence intestinal activities and the gut to influence mood, cognition, and mental health. The microbiota­–gut–brain axis includes the role of gut microbiota in the biochemical signaling events that take place between the gastrointestinal tract and the central nervous system

Over seven weeks, 51 participants took symbiotic supplements that contained the beneficial bacteria Lactobacillus and Bifidobacterium. Another 50 participants acted as controls and took placebos.

Before and after the seven-week intake of the dietary supplement, participants were asked to play the ultimatum game, a task from behavioral economics historically used to assess bargaining and altruistic behavior.

In the game, one player controls a pot of money and can offer a share or split to a second player. The second player can accept the offer and take the money or can reject the offer, in which case neither player receives any money. Rejecting an unfair offer is interpreted as altruistic punishment, as the rejecter sacrifices whatever small share is on offer to punish the first player for being ungenerous.

Players who took the dietary supplements were more likely to reject offers. In particular, players who took the supplements were more likely to reject 30%–40% splits; all players tended to reject very unequal splits.

Players who had a high ratio of Firmicutes to Bacteroidetes at the beginning of the study saw the greatest changes in both gut makeup and rates of altruistic punishment. The supplements reduced plasma levels of the dopamine precursor tyrosine in some participants. Players who experienced this reduction who saw the most significant increase in altruistic punishment.

Controlling altruistic behavior with dietary interventions

These findings demonstrated a causal effect of changing the gut microbiome on altruistic punishment behavior. According to the authors, people who changed their gut microbiome to a state regarded as healthier became less rational and more sensitive to social considerations.

The findings show that changes in microbiome composition might also affect dopamine-precursor availability independent of dietary intake of these precursors. This suggests that the microbiome is both a potential mediator of previously demonstrated dietary effects on behavior and a diet-independent contributor to dopamine-precursor availability.

The authors said, “Our findings challenge the classical view in cognitive sciences that complex behaviors such as social decision-making are only a function of higher cognitive processes located in cortical brain areas.”

The study used only male participants with a body mass index (BMI) between 20 and 34 and who didn’t adhere to a special diet, such as a vegan, gluten-free or allergy-related one, as such a lifestyle would limit the findings' generalizability. Further studies could test the effects in more diverse samples and across different diets and existing health conditions.

The effects of interventions targeted at the gut microbiome may strongly depend on the gut microbiome composition and on other individual factors. The researchers call for future studies to further dissect these kinds of interactions and to link individual malleability of the gut microbiome and social behavior to more stable personality factors and genetic dispositions.

Other studies could also test whether the gut microbiome could serve as a target for interventions to improve social decision-making in health and disease.

Reference: Falkenstein M, Simon MC, Mantri A, Weber B, Koban L, Plassmann H. Impact of the gut microbiome composition on social decision-making. PNAS Nexus. 2024;3(5):pgae166. doi: 10.1093/pnasnexus/pgae166

This article is a rework of a press release. Material has been edited for length and content.