Background & aims: Clinical trials have reported controversial results regarding the effectiveness of probiotics in alleviating functional constipation in adults. We reviewed relevant randomized controlled trials to elucidate the effectiveness of probiotics on constipation symptoms in adults with functional constipation.

Methods: We searched Medline, the Cochrane Library, Web of Science, and Google Scholar for relevant articles published up to April 2019. The primary outcomes of interest were stool frequency, gut transit time (GTT), stool consistency, and bloating. Two authors independently performed the study selection, risk-of-bias assessment, and data extraction. The outcome data were extracted from each included study and synthesized using weighted mean differences (WMDs) or standardized mean differences (SMDs). Pooled data synthesis was performed using a random-effects model.

Results: In total, 2327 relevant studies were identified, 15 of which were found to be eligible randomized controlled trials and were included in the meta-analysis. Pooling of the extracted data demonstrated that probiotic consumption significantly reduced the whole GTT by 13.75 h [95% confidence interval (CI): -21.93 to -5.56 h] and increased the stool frequency by 0.98 (95% CI: 0.36 to 1.60) bowel movements per week. This increase was significant with the consumption of multispecies probiotics [at least two bacteria; WMD: 1.22 (95% CI: 0.50 to 1.94) bowel movements per week] but not with the consumption of Bifidobacterium lactis [WMD: 1.34 (95% CI: -0.27 to 2.94) bowel movements per week] or B. longum [WMD: -0.02 (95% CI: -0.56 to 0.53) bowel movements per week] alone. Multispecies probiotics (WMD: 1.37; 95% CI: 0.72 to 2.01), but not single-species probiotics (WMD: 1.18; 95% CI: -0.59 to 2.96), improved stool consistency (WMD: 1.30; 95% CI: 0.22 to 2.38). Similarly, multispecies probiotics (at least two bacteria; WMD: -0.49; 95% CI: -0.85 to -0.13), but not single-species probiotics (WMD: -0.24; 95% CI: -0.55 to 0.07), significantly decreased bloating. Performance bias were high, whereas detection bias was unclear because of inadequate reporting.

Conclusion: Consumption of probiotics, in particular, multispecies probiotics, may substantially reduce the GTT, increase the stool frequency, and improve the stool consistency. Thus, probiotics can be regarded as safe and natural agents for alleviation of functional constipation in adults.