When we hear of probiotics, we typically think of benefits related to better digestion and absorption.
The British Medical Journal published a review of existing evidence from 2003 to 2019 showing there's more to the picture, though. It appears that probiotics, taken either with or without prebiotics, may help ease depression.
Is it really possible that our gut microbiome can help treat mental health disorders? Well, the gut-brain axis has definitely been established as a two-way line of communication between the gastrointestinal tract and the central nervous system. Because of this two-way road, it is believed that both areas - the gut and the brain - can influence each other's function and condition.
While all the studies examined in the review varied in how they were carried out, some were short in duration, and some had relatively few participants, they all concluded that probiotic supplementation (alone or in combination with prebiotics) helped produce a reduction in depressive symptoms that could be measured.
The journal Psychiatry Research conducted a review of 19 double-blind, randomized, placebo-controlled trials with a total of 1901 participants. They found that probiotics were most effective in reducing depressive symptoms in those with major depression. Those in the general population with no depression diagnosis did not experience any significant anti-depressive effects.
How could probiotics possibly affect the brain?
Some researchers believe it has something to do with a reduction in inflammatory chemicals, such as cytokines.
For others, it may be less of a direct improvement in depression and simply an improvement in other gastrointestinal disorders that aggravate depressive symptoms, such as inflammatory bowel disease.
Finally, some of the researchers believe it has to do with tryptophan. Tryptophan can be used by the body in two main ways, only one of which will lead to its conversion to serotonin, the "feel good" hormone. Probiotics may influence whether or not tryptophan will end up being metabolized and converted to serotonin.
At best, this review and these studies provide the needed evidence to warrant further investigation into this area of interest. As the Annals of general psychiatry puts it, the evidence is very exciting but more double-blind randomized control trials with larger sample sizes (the gold standard of scientific studies) are needed before we can make a direct claim.
Until then, however, we can still benefit from taking probiotics appropriately and making them a daily part of our diet.
Sanjay Noonan, Meena Zaveri, Elaine Macaninch, Kathy Martyn. Food & mood: a review of supplementary prebiotic and probiotic interventions in the treatment of anxiety and depression in adults. BMJ Nutrition, Prevention & Health, 2020; bmjnph-2019-000053 DOI: 10.1136/bmjnph-2019-000053
Wallace, C., & Milev, R. (2017). The effects of probiotics on depressive symptoms in humans: a systematic review. Annals of general psychiatry, 16, 14. https://doi.org/10.1186/s12991-017-0138-2
Psychiatry Research. Volume 282, December 2019, 112568