Ahmad Esmaillzadeh - Department of Community Nutrition, School of Nutritional Sciences and Dietetics, Tehran University of Medical sciences, Tehran, Iran

Introduction: Interest in the gut-brain axis and emerging evidence that the intestinal microbiota can influence the function of central nervous system has led to the hypothesis that probiotic supplementation might have a positive effect on psychological symptoms, such as depression. Despite several studies about the effect of probiotic supplementation on depressive symptoms, there are some controversies in this area. This study aimed to summarize findings from earlier studies on te effects of probiotics on depression. Methods: Relevant studies published up to March 2018 were searched through PubMed, MEDLINE, SCOPUS, EMBASE, and Google Scholar, using following suitable MESH and non-MESH key words: ( Lactobacillus [All Fields] OR Streptococcus [All Fields] OR Saccharomyces [All Fields] OR Enterococcus [All Fields] OR Bifidobacterium [All Fields] OR Probiotic [All Fields] OR Lactococcus [All Fields]) AND ( mood [All Fields] OR depression [All Fields] OR MDD [All Fields] OR suicide [All Fields]). No restrictions of language or time of publication were used. To avoid missing any publication, we also examined reference lists of all included studies as well as review articles.Results: Overall, 10 clinical trials enrolling a total of 1349 patients were included. Polling effect sizes, no significant effect of probiotic supplementation on depression was found (SMD: -0.128, 95% CI: -0.261 to 0.00463, P=0.059). Subgroup analysis showed a significant reduction in depression scores among patients with mild to moderate depressive disorders following probiotic supplementation (SMD: -0.684, 95% CI: -1.296 to -0.0712, P=0.029), while the effect of probiotic was non-significant in healthy individuals (SMD: -0.0999, 95% CI: -0.235 to 0.0348, P=0.146). Conclusion: Probiotic supplementation did not affect depression symptoms in adults. However, it might reduce depression scores among patients with mild to moderate depressive disorders. Between-study heterogeneity was high among included studies. Moreover, the majority of available RCTs were conducted in healthy individuals. Therefore, further RCTs are required to reach a firm conclusion.