AIM: To determine the efficacy of probiotic supplementation on intestinal transit time(ITT) in adults and to identify factors that influence these outcomes. METHODS: We conducted a systematic review of randomized controlled trials of probiotic supplementation that measured ITT in adults. Study quality was assessed using the Jadad scale. A random effects meta-analysis was performed with standardized mean difference(SMD) of ITT between probiotic and control groups as the primary outcome. Meta-regression and subgroup analyses examined the impact of moderator variables on SMD of ITT.RESULTS: A total of 15 clinical trials with 17 treatment effects representing 675 subjects were included in this analysis. Probiotic supplementation was moderately efficacious in decreasing ITT compared to control, with an SMD of 0.38(95%CI: 0.23-0.53, P < 0.001). Subgroup analyses demonstrated statistically greater reductions in ITT with probiotics in subjects with vs without constipation(SMD: 0.57 vs 0.22, P < 0.01) and in studies with high vs low study quality(SMD: 0.45 vs 0.00, P = 0.01). Constipation(R~2 = 38%, P < 0.01), higher study quality(R~2 = 31%, P = 0.01), older age(R~2 = 27%, P = 0.02), higher percentage of female subjects(R~2 = 26%, P = 0.02), and fewer probiotic strains(R~2 = 20%, P < 0.05) were predictive of decreased ITT with probiotics in meta-regression. Medium to large treatment effects were identified with B. lactis HN019(SMD: 0.67, P < 0.001) and B. lactis DN-173 010(SMD: 0.54, P < 0.01) while other probiotic strains yielded negligible reductions in ITT relative to control.CONCLUSION: Probiotic supplementation is moderately efficacious for reducing ITT in adults. Probiotics were most efficacious in constipated subjects, when evaluated in high-quality studies, and with certain probiotic strains. AIM: To determine the efficacy of probiotic supplementation on intestinal transit time (ITT) in adults and to identify factors that affect these outcomes. METHODS: We conducted a systematic review of randomized controlled trials of probiotic supplementation that measured ITT in adults. Study quality was assessed using the Jadad scale. A random effects meta-analysis was performed with standardized mean difference (SMD) of ITT between probiotic and control groups as the primary outcome. Meta-regression and subgroup analyzes examined the impact of moderator variables on SMD of ITT. RESULTS: A total of 15 clinical trials with 17 treatment effects representing 675 subjects were included in this analysis. Probiotic supplementation was moderately efficacious in decreasing ITT compared to control, with an SMD of 0.38 (95% CI: 0.23-0.53, P <0.001 ). Subgroup analyzes of relatively greater reductions in ITT with probiotics in subjects with vs without constipation (SMD: 0.57 vs 0.22, P (P <0.01), and in studies with high versus low study quality (R 2 = 31%, P = 0.01) .Constipation = 0.01), older age (R ~ 2 = 27%, P = 0.02), higher percentage of female subjects (R ~ 2 = 26%, P = 0.02) <0.05) were predictive of decreased ITT with probiotics in meta-regression. Medium to large treatment effects were identified with B. lactis HN019 (SMD: 0.67, P <0.001) and B. lactis DN- <0.01) while other probiotic strains yielded negligible reductions in ITT relative to control. CONCLUSION: Probiotics were mostly for augment ITT in adults. Probiotics were most efficacious in constipated subjects, when evaluated in high-quality studies, and with certain probiotic strains .