Evidence that suggests our gut bacteria play complex roles in maintaining and impacting overall health is piling up. One new review of the scientific literature has now found that regulating intestinal microbiota may help relieve the symptoms of anxiety.

Recent research suggests that the bacteria that naturally populate the human gut may play an extensive role not just in a person's physical health, but also in their mental well-being. One study drew a link between certain groups of bacteria and a higher risk of mental health conditions, including depression. Another suggested that our gut bacteria may influence certain brain mechanisms and contribute to anxiety.

Now, researchers from the Shanghai Mental Health Center at Shanghai Jiao Tong University School of Medicine in China have evaluated the evidence of 21 studies—involving 1,503 participants in all—that looked at different interventions set to regulate the intestinal microbiota and whether they had any effect on symptoms of anxiety.

Among the researchers were Beibei Yang, Jinbao Wei, Peijun Ju, and Jinghong Chen. The findings, which appeared yesterday in the journal General Psychiatry, emphasize the idea that scientists should not ignore the possible role of the intestinal flora when providing solutions for mental health.

In the introduction to their study paper, the researchers explain that although studies in mice have suggested that interventions that regulate the balance of intestinal bacterial populations can be helpful in reducing behaviors consistent with a state of anxiety, there is currently no scientific consensus regarding the effectiveness of these interventions.

Through their review and meta-analysis, the scientists hope to get closer to some of the answers sought by those interested in the relationship between the gut and the brain.

Over 50% of studies found positive effects

The studies that the team evaluated chose different types of intervention. Of the 21 studies, 14 used probiotics—or "good" bacteria—as the main agent in their interventions regulating intestinal flora. The remaining seven opted for interventions that did not use probiotics, such as simply adjusting a person's typical diet.

Of the 14 that used probiotic-centered interventions, seven used a single probiotic, two used two kinds of probiotics, and five used three or more different types of probiotics.

The team found that 11 out of the 21 studies (52%) concluded that interventions regulating intestinal flora helped reduce anxiety symptoms. More specifically, among the studies that used probiotics in their interventions, 36% concluded that the strategy was effective. Among the studies that did not use probiotics, 6 out of 7 suggested that the interventions helped alleviate anxiety. "It is worth mentioning that the efficiency of supplementation of nonprobiotic preparations is as high as 86%," the authors write in their study paper.

Five of the studies used interventions regulating intestinal flora to supplement traditional anxiety treatments. Among these, only the studies that did not use probiotics led to an improvement of anxiety symptoms.

Why are some interventions more effective?

The researchers also found that nonprobiotic interventions on their own seemed to have a greater positive effect than interventions using probiotics, at a rate of 80% effectiveness in the former vs 45% effectiveness in the latter.

This, they say, may be because interventions such as adjusting one's daily diet might contribute more to regulating the microbiome by offering different sources of energy to the bacteria that populate the intestines.

"The energy source of gut microbiota growth is mainly food," explain the study authors. "Adjusting the gut microbiota through modulating dietary structure can directly change the energy-supplying structure of gut microbiota and this plays a decisive role in the growth of gut microbiota, so the effect is obvious."

Because the recent research was observational in nature, the scientists warn that the results do not speak conclusively of cause and effect. However, more than half of the studies they looked at offered high-quality data that suggested that regulating the intestinal microbiota could be helpful in reducing anxiety symptoms. The authors conclude that their findings, if additional research supports them, may have important clinical implications. They say:

"In the clinical treatment of anxiety symptoms, in addition to the use of psychiatric drugs for treatment, we can also consider regulating intestinal flora to alleviate anxiety symptoms."

"Especially for patients with somatic diseases who are not suitable for the application of psychiatric drugs for anxiety treatment, probiotic methods and/or nonprobiotic ways [...] can be applied flexibly according to clinical conditions," the researchers conclude.