Probiotics either taken by themselves or when combined with prebiotics, may help to ease depression, suggests a new review of the available evidence.
But as to whether they might help to lessen anxiety isn't yet clear, say the researchers.
Foods that increase the community of beneficial bacteria in the gut are collectively known as probiotics, while prebiotics are compounds that help these bacteria flourish.
A two-way relationship exists between the brain and digestive tract, known as the gut-brain axis. And the possibility that the microbiota -- the range and number of microbes residing in the gut -- might help treat mental health problems has become a focus of interest in recent years.
For the analysis, researchers searched for relevant studies published between 2003 and 2019, which looked at the therapeutic potential of prebiotics and probiotics in adults with depression and/or anxiety disorders.
Just seven studies met all the criteria for inclusion. All seven investigated at least one probiotic strain; four looked at the effect of combinations of multiple strains.
In all, 12 probiotic strains featured in the selected studies, primarily Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidium.
The studies varied considerably in their design, methods used, and clinical considerations, but all of them concluded that probiotic supplements either alone or in combination with prebiotics may be linked to measurable reductions in depression.
And every study showed a significant decline 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.
None of the studies lasted very long and the number of participants in each was small. This makes it difficult to draw any firm conclusions about the overall effects, whether they are long-lasting, and whether there might be any unwanted side effects associated with prolonged use.
The preliminary evidence to date, though, does suggest that pre- and probiotic therapy warrant further investigation.
How probiotics might help treat depression
Probiotics may help reduce the production of inflammatory chemicals in the brain, such as cytokines.
Or they may help direct the action of tryptophan, an amino acid thought to be important in the gut-brain axis in psychiatric disorders.
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