Numerous psychiatric and neurological conditions have been linked to disturbances in our gut microbiome. However, clinical trials have shown promise but lack conclusive results.

Andrew Moseson and two patients with bipolar disorder participated in a small trial led by Valerie Taylor, a psychiatrist at the University of Calgary in Canada. Taylor became interested in the gut microbiota after two people with bipolar disorder approached her, and a faecal microbiota transplant proved to be transformative for Moseson, but less so for the others.

John Cryan, a neurobiologist at University College Cork in Ireland studied a small sample of people with depression and found that they harboured microbial communities that differed from those of healthy people. In December 2023, Cryan and his colleagues showed that symptoms experienced by people with social anxiety disorder could similarly be transferred to rodents through faecal transplants, according to an August 18 2025 report in Nature.

More targeted studies are needed to determine which species of gut microbiota affect these mental health disorders. So far, studies found that probiotics known as psychobiotics when used are effective as an add-on, but not when used alone. 

Wolfgang Marx, a nutritional psychiatrist at Deakin University in Geelong, Australia, found a Mediterranean diet reduced symptoms in people with depression, as prebiotics to support overall health.

Researchers like Cryan concludes that looking only at changes in microbiota composition will not be enough to get to the bottom of how psychobiotics affect mood. 

https://www.nature.com/articles/d41586-025-02633-4

https://www.sciencedirect.com/science/article/abs/pii/S0166432822003497