Background: There is a lot of evidence for a bidirectional communication between the gut and brain. Dysbiosis and increase intestinal permeability may lead to a systemic low-grade inflammatory response or various neuroactive bacterial metabolite may cross gut barrier. Pro-inflammatory cytokines or bacterial metabolites such as short-chain fatty acid (SCFA) are known to pass through blood brain barrier and altered neurotransmitter metabolism or increase production of neurotoxic pathways. In this review we hypothesized that restoring the gut microbiota ecosystem could improve mental disorders. We reviewed literature for human evidence proving clinical relevance of probiotics intake in mental disorders.
Subjects and methods: We searched literature with keywords "depression" or "major depressive disorder" and "probiotic". We selected randomized control trial and we considered having both outcomes concerning impact on depressive symptoms but also on inflammation biomarkers, microbiota composition, cerebral nervous system or cognition.
Results: Seven out of fourteen randomized control trial reported significant improvement on depressive symptoms in patients taking probiotics. Besides improvement in depressive symptoms, we found decrease in inflammatory markers such as IL-6, decrease in serum kynurenine level, changes in microbiota diversity and abundance of species correlated to depressive disorder and higher cognitive performance.
Conclusions: Probiotic seems to be secure and more effective on depression when used in supplement to usual antidepressant and in mild to moderate depression. We highlighted positive impact on vulnerability factors prevent further worsening. Probiotics could have anti-inflammatory effect acting on inflammatory markers well known to have a role on pathogenesis of depression. A strong correlation between neuroactive metabolites and a relative abundance of microbiota bacterial species underlined importance to consider the gut-brain axis in mental disorders.
Keywords: major depressive disorder – probiotic – inflammation – cognition - microbiota.