The plethora of microbes living in the human gut not only affect people's physical health, they may also influence mental health, according to a growing body of research.

Recent studies in animals show that changes in the gut bacteria community appear to make mice less anxious, and also affect levels of the stress hormone cortisol.

In humans, there is some very early evidence of a link between gut bacteria and mental health. A new study from England found that supplements that boost "good" bacteria in the gut (called "prebiotics") may alter the way people process emotional information, suggesting that changes in gut bacteria may have anti-anxiety effects.

Scientists are now interested in studying whether probiotics (strains of good bacteria) or prebiotics (carbohydrates that serve as food for those bacteria) could be used to treat anxiety or depression, or if the substances improve patients' response to psychiatric drugs, said study author Philip Burnet, a researcher in the University of Oxford's department of psychiatry. [5 Ways Gut Bacteria Affect Your Health]

But experts caution that the idea that taking a probiotic or a prebiotic could improve mental health in humans is still an unproven hypothesis that needs to be investigated with further research.

"It's becoming a very interesting question in the field," said Dr. Roger McIntyre, a professor of psychiatry and pharmacology at the University of Toronto. "The animal data looks really interesting, and looks very suggestive, [but] we're still waiting for that convincing human study," McIntyre said.

Could bacteria reduce anxiety?

In the new study from England, 45 healthy people ages 18 to 45 took either a prebiotic or a placebo, every day for three weeks. At the end of the study, they completed several computer tests to assess how they processed emotional information, such as negative and positive words.

During one computer test, people who took the prebiotic paid less attention to negative information, and more attention to positive information, compared with people who took a placebo. A similar effect has been seen in people who take drugs for depression or anxiety, and the finding suggests that the people in the prebiotic group had "less anxiety about negative or threatening stimuli," Burnet said.

The study also found that people who took the prebiotics had lower levels of cortisol in their saliva when they woke up in the morning, compared with people who took a placebo. High cortisol levels have been linked with stress, anxiety and depression, Burnet said. The study was published in the Dec. 3 issue of the journal Psychopharmacology, and was funded in part by Clasado Research Services, which makes prebiotics.

The researchers said they did not find any change in the study participants' self-rated levels of stress and anxiety. This may have been because the participants did not take the prebiotic long enough to have an effect, or because they already had fairly low levels of stress and anxiety to begin with, Burnet said.

However, a 2011 study from France found that people who took probiotics for 30 days did have reduced levels of psychological distress. Because this is just one study, its findings need to be confirmed in future research.

In a 2013 study, UCLA researchers gave women milk with or without probiotics, and then scanned their brains while they viewed photos of people with emotional facial expressions. Those who took the probiotics had less activity in their brains in areas involved in processing emotions, compared with those who did not take the probiotic.

Behind the link

Researchers aren't sure exactly how changes in gut bacteria might affect the brain. Some researchers suspect that the vagus nerve — which conveys sensory information from the gut to the brain — plays a role. Gut bacteria may also affect the immune system, which could, in turn, influence the brain, Burnet said.

A recent study by McIntyre and colleagues found that giving people the antibiotic minocycline reduced symptoms of depression. However, because the study did not analyze participants' gut microbes, it's not known whether the effect on depression was due to changes in gut bacteria, or a different mechanism, McIntyre said. The study has not yet been published.

McIntyre said future studies are needed to better understand whether gut microbes play a role in psychiatric disorders, like depression and anxiety, and which bacteria species are important.

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