recent meta-analysis of 34 placebo-controlled clinical trials evaluated the effects of probiotics and prebiotics on depression and anxiety. The supplementation with probiotics showed small but significant effects on depression (23 trials, P<0.01) and anxiety (22 trials, P=0.03). The duration of probiotic intervention ranged from 8 days to 45 weeks. A significant effect was observed in the studies with probiotic use longer than a month. There were no significant differences between prebiotics (GOS, FOS, or short-chain FOS) and placebo supplementation for depression or anxiety.
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Liu RT et al. Prebiotics and probiotics for depression and anxiety: A systematic review and meta-analysis of controlled clinical trials. Neurosci Biobehav Rev 2019;102: 13-23