Accumulating data show that probiotics may be beneficial in reducing depressive symptoms. We conducted an updated meta-analysis and evaluated the effects of probiotics on depressive symptoms. A systematic search of six databases was performed, and the results were reported according to Preferred Reporting Items for Systematic Reviews and Meta-analyses, with the priori-defined protocol registered at PROSPERO (CRD42018107426). In total, 19 double-blind, randomized, placebo-controlled trials with a total of 1901 participants were included in the qualitative synthesis. Participants treated with probiotics showed significantly greater improvement in depressive symptoms than those receiving placebo. The clinical population was stratified by clinical diagnosis into those with major depressive disorder (MDD) and those with other clinical conditions. The beneficial effect of probiotics on depressive symptoms was significant in patients with MDD, but not in those with other clinical conditions and in the general population. In addition, multiple strains of probiotics were more effective in reducing depressive symptoms. In conclusion, altering the gut–brain axis with probiotics may be an approach to improve depression severity. It is essential to verify the efficacy of specific combinations or strains of probiotics for depressive symptoms by conducting studies with a larger sample size in the future.
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