A new review suggests that probiotics, either taken by themselves or when combined with prebiotics, may help ease depression.

While the underlying mechanism remains unclear, probiotics may help reduce the production of inflammatory chemicals, such as cytokines, as is the case in inflammatory bowel disease, say the researchers. Or they may help direct the action of tryptophan, a chemical thought to be important in the gut-brain axis in psychiatric disorders.

The study is published in the journal BMJ Nutrition Prevention & Health.

Foods that broaden the profile of helpful bacteria in the gut are collectively known as probiotics, while prebiotics are compounds that help these bacteria to flourish.

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.

To investigate this further, the research team looked 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.

Out of an initial group of 71 studies, only 7 met all the criteria for inclusion in the review. All 7 studies investigated at least 1 probiotic strain; 4 looked at the effect of combinations of multiple strains.

Overall, 12 probiotic strains were featured in the selected studies, primarily Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidium. One study looked at combined pre-probiotic treatment, while one looked at prebiotic therapy by itself.

Although the studies varied considerably in their design, methods used, and clinical considerations, all of the studies concluded that probiotic supplements either alone or in combination with prebiotics may be associated with measurable reductions in depression.

In addition, each study showed a significant 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 studied, 11 were potentially useful, according to the findings.

The researchers note some limitations to their review: None of the included studies lasted very long; and the number of participants in each was small. This makes it difficult to draw any strong conclusions about the overall effects, whether they are long-lasting, and whether there might be any unwanted side effects linked to prolonged use, they say.

Still, on the basis of the preliminary evidence to date, pre- and probiotic therapy warrant further investigation, they say.

As anxiety disorders and depression affect people in different ways, they require treatment approaches that take account of these complexities, they say. “In this way, with a better understanding of the mechanisms, probiotics may prove to be a useful tool across a wide range of conditions,” they write.

In addition, individuals with depression and/or anxiety disorders often have other underlying conditions, such as impaired insulin production and irritable bowel syndrome, they point out.

“As such, the effect that probiotics have on patients with [common mental disorders] may be twofold: they may directly improve depression in line with the observed findings of this review, and/or they might beneficially impact a patient’s experience of their [common mental disorder] by alleviating additional comorbidities,” they write.

Source: BMJ