The human intestine is home to trillions of bacteria, called the gut microbiome, that play a crucial role in digestion. An increasing body of evidence suggests these microbes also influence our brain and behavior. Now, researchers of a new systematic review think that changing our gut flora could help ease anxiety symptoms. 

Researchers at Shanghai Jiao Tong University School of Medicine analyzed 21 studies that focused on gut bacteria interventions to treat anxiety. The review, which involved 1,503 subjects, found that certain approaches may help alleviate the condition. The study is published in the journal General Psychiatry.

Two main approaches were used to manipulate gut bacteria in the participants, which the team called "interventions to regulate intestinal flora" (IRIF). In seven of the studies, the patients' diet was altered, while in the other 14, they were given probiotic supplements. Probiotics are also known as “good” or “friendly” bacteria, they are found in certain foods, and can help with depleted gut flora. Of the probiotic studies, seven used only one type of bacteria, two used two types of bacteria, and the remaining five used three or more kinds of bacteria.

For all the studies in the systematic review, the researchers found a modest positive effect. They state that 11 of the studies (52 percent) showed a marked improvement in anxiety symptoms following IRIF – five from the probiotic approach and six from the diet approach. The review also provided a glimpse into the combination of traditional anxiety medications with gut bacteria regulation. The researchers found that only the studies that used the diet approach (non-probiotic) paired with anxiety medications showed improvements.

Taken by themselves, the diet interventions had a rate of efficacy of 86 percent. This could possibly be related to the more effective growth of diverse bacteria types following a change in diet. Alternatively, the issue may be with the probiotics themselves, as the bacteria in the supplements could be competing against each other and not delivering the necessary changes to the gut microbiome. It's also possible that only certain types of bacteria make a difference, which the researchers did not control for in this review. Another limitation is the length of the studies – they all happened over only a couple of months. The authors are cautious about making claims about the effects of diet or probiotics in managing anxiety. They state that more studies are necessary as this research doesn't pinpoint a causal link between them.

"There are two kinds of interventions (probiotic and non-probiotic interventions) to regulate intestinal microbiota, and it should be highlighted that the non-probiotic interventions were more effective than the probiotic interventions. More studies are needed to clarify this conclusion since we still cannot run meta-analysis so far," conclude the authors.