Researchers working with animals have shown that altering their gut bacteria can affect their stress levels and make them less anxious. In humans, the link between gut microbes and mental health is just starting to become clear. A new study published in Psychopharmacology suggests that fostering the growth of helpful bacteria helps modulate how we process information and could perhaps help ease anxiety and depression.

There are probiotics (“good” bacteria), and then there are “prebiotics,” ingestible fibers that help augment the growth of our beneficial indigenous microbiotia. And researchers are increasingly interested in knowing if (and how) these could be used to improve a patient’s response to psychiatric drugs. So, to study the effects of prebiotics on the secretion of the stress hormone cortisol, Oxford’s Philip Burnet and colleagues recruited 45 healthy people ages 18 to 45 to ingest either one of two types of prebiotics or a placebo every day.

At the end the three-week treatment period, the volunteers completed several computerized tasks that allowed the team to assess how they process emotionally salient information -- and whether they were biased towards the positive or the negative. The researchers also sampled cortisol levels in the participants’ saliva after waking up in the morning -- called “salivary cortisol awakening response” -- before and after the prebiotic (or placebo) was administered.

Compared with the control group, people who took prebiotics tended to pay more attention to positive information, and their salivary cortisol awakening response was significantly lower. The findings suggest that those in the prebiotic group have "less anxiety about negative or threatening stimuli," Burnet tells Live Science. However, they didn’t find any change in the recruits’ self-rated levels of stress and anxiety. This could have been due to the short period of time they were on the prebiotics, he adds.

This suppression of the stress response is consistent with previous findings of microbiota changes, though researchers remain unsure of how these changes might be affecting the brain. According to Burnet, gut bacteria may be affecting the immune system, which, in turn, could be influencing the brain.