Effectiveness of probiotic/prebiotic/synbiotic treatments on anxiety: A systematic review and meta-analysis of randomized controlled trials.

BACKGROUND: Anxiety can adversely affect human well-being. This meta-analysis aimed to evaluate the effects of interventions that alter the gut microbes (including probiotics, prebiotics, and synbiotics) on anxiety. METHODS: A systematic meta-analysis of the effects of probiotics, prebiotics, and synbiotics on anxiety was conducted by searching randomized controlled trials (RCTs) in 13 databases. The primary outcomes were the pre- and post-intervention anxiety scores in the intervention and placebo groups. Anxiety scores were extracted as standard mean differences (SMDs) and pooled based on a random effects model. Subgroup analyses of anxiety scales, health status, gastrointestinal symptoms, flora strains, treatment type, probiotic dose, region, and treatment duration were also performed. RESULTS: 29 RCTs (2035 participants) were included, revealing that both probiotics and synbiotics significantly reduced anxiety scores. Additionally, anxiety scores did not significantly reduce when comparing prebiotics and placebos. LIMITATIONS: Owing to the small combined effect size of probiotic/prebiotic/synbiotic treatments and the relatively few studies on prebiotics and synbiotics included in the analysis, the findings of probiotic/prebiotic/synbiotic treatments are preliminary. CONCLUSIONS: Our study indicated that probiotics and synbiotics can reduce anxiety scores; however, it might be premature to conclude their clinical efficacy in alleviating anxiety due to the small effect size. There is no consensus regarding the optimal dose, treatment duration, treatment type, or probiotic strain to improve anxiety. Moreover, the mechanisms by which probiotics and synbiotics improve anxiety remain unclear. More RCTs are needed to determine the mechanisms of action and to identify appropriate markers to clarify their effects.