Probiotics alone or combined with prebiotics may help ease depression but their role in lessening anxiety is less clear, according a research review.

A review of the available evidence, published in BMJ Nutrition Prevention & Health, searched for relevant studies published in English between 2003 and 2019, which looked at the potential therapeutic contribution of pre-and probiotics in adults with depression and/or anxiety disorders.

It found that probiotic supplements either alone or in combination with prebiotics may be linked to measurable reductions in depression.

Out of 71 studies, the researchers found only seven studies that met all the criteria for inclusion. All seven investigated at least one probiotic strain; four looked at the effect of combinations of multiple strains.

Every study showed a significant fall or improvement in anxiety symptoms and/or clinically relevant changes in biochemical measures of anxiety and/or depression with probiotic or combined pre-probiotic use. Of the 12 different probiotics investigated, 11 were potentially useful, the findings showed.

Two-way relationship between the brain and digestive tract

In the UK in 2016-17, 1.4 million people were referred with mental health issues, over half of them (53%) had anxiety or stress related disorders, while a third (33%) had depression.

A two-way relationship exists between the brain and digestive tract, known as the gut-brain axis. And the possibility that the microbiome - the range and number of bacteria resident in the gut - might help treat mental ill health has become a focus of interest in recent years.

The researchers highlight several caveats to their review: none of the included studies lasted very long; and the number of participants in each was small. But on the basis of the preliminary evidence to date, pre- and probiotic therapy warrant further investigation, they suggest.

The researchers added: “Purely from the information gathered for this review, it is valid to suggest that, for patients with clinically recognised depression: isolate, or adjuvant prebiotic therapy is unlikely to affect an individual’s experience of their condition in a quantitatively evident way; and that isolate or adjuvant, probiotic/combined prebiotic-probiotic therapy may offer a quantitatively measurable improvement in parameters relating to depression.

“However, there are inadequate data to suggest anything meaningful to support or refute the use of either pre/probiotic agents (or a combination of both) in patients with clinically recognised anxiety disorders; this would be a useful area to investigate further.”

For more news and articles on depression and anxiety go to our psychiatry section