Does gut flora affect social relationships?

Social phobia, a condition inducing anxiety and intense fear in unfamiliar group interactions, affects many individuals. Onset can be as early as childhood or adolescence, persisting for years and leading to avoidance behaviors that impair quality of life. Addressing this challenge has seen attempts with pharmacological interventions, yet their efficacy remains limited and inconsistent.

However, a new discovery is shedding light on a potential solution for social phobia. Research shows that individuals with this disorder have a distinct gut microbiota compared to those without it. Surprisingly, these microorganisms may influence how the brain processes social interactions.

What is the intestinal microbiota? Many bacteria live in the human body, mainly in the gut, forming a complex system known as intestinal flora, microbiota or microbiome. It consists of over 1,000 types of bacteria that change over time due to factors like diet, antibiotic use, childbirth method and modern lifestyles. The structure and changes in the gut microbiota are dynamic. While the body provides a nutrient-rich environment, the gut flora offers metabolic, protective, and structural functions independent of a person’s genes. This microbiota helps remove toxins, prevents harmful bacteria from colonizing the intestines, supports the immune system, and controls inflammation in the body.

Fecal microbiota transplants are now being used to study how they influence changes in the recipient’s characteristics. For instance, when feces from an obese mouse are given to a thin, germ-free mouse, it becomes more prone to overeating and gains body fat. However, the exact mechanism by which the microbiota causes this effect is still unknown. Research has also revealed that these transplants can transfer psychological and physiological traits related to conditions like depression, anxiety, irritable bowel syndrome and schizophrenia. This suggests that gut flora can indeed impact brain function.

This led a team of researchers from the Karolinska Institute in Stockholm to study whether a specific intestinal microbiota can cause social fear. They transferred fecal matter from humans with social phobia to male mice to observe changes in social fear, sociability, cognition, stress coping behaviors, and gut functions in the mice. The mice that received fecal matter from humans with social phobia showed increased sensitivity to social fear, as confirmed by Pavlovian tests. These findings highlight the significant impact of gut microbiota on the brain function of mice.

Although organs or biological matter have been transplanted from animals to humans on several occasions, the reverse transfer — human biological material transplanted to animals — is less common. This is due to ethical and practical concerns, such as transplanting from a person with social phobia to one without it. However, transferring the microbiota linked to this phobia to a smaller animal like a mouse consistently shows an influence that could extend along the mammalian evolutionary chain, potentially affecting our human brain. This suggests that altering the microbiota of individuals with social phobia might offer a promising way to treat or alleviate their condition.

Sign up for our weekly newsletter to get more English-language news coverage from EL PAÍS USA Edition