Non-Digestible Oligosaccharides and Constipation: A Systematic Review and Meta-Analysis of Randomized Trials on Stool Frequency, Stool Consistency, and Fermentation Biomarkers

Background: Chronic constipation lacks effective long-term treatments. Non-digestible oligosaccharides (NDOs) are short-chain carbohydrates that resist digestion and may improve bowel function. This systematic review and meta-analysis examines the effect of NDOs on constipation-related outcomes in humans. Methods: We searched Ovid MEDLINE, Embase, and Web of Science (2010–May 2025) for randomized controlled trials (RCTs) comparing NDOs with placebo, reporting stool frequency, stool consistency, fecal pH, or short-chain fatty acids (SCFAs). Data were pooled using random-effects meta-analysis. All effect estimates are reported as standardized mean differences (SMDs) with 95% confidence intervals (CIs). Subgroups were analyzed based on baseline constipation status and treatment duration. Results: We included 20 RCTs (1786 participants) evaluating seven NDO types. NDO supplementation significantly increased stool frequency overall, with larger effects in constipated individuals (SMD 0.99, 95% CI 0.58–1.28) than in non-constipated population (SMD 0.30, 95% CI 0.10–0.51). By duration, shorter interventions (≤3 weeks) yielded greater frequency gains (SMD 0.89, 95% CI 0.40–1.38) than longer ones (SMD 0.24, 95% CI 0.09–0.38). While the overall effect on stool consistency was non-significant, constipated patients (SMD 0.46, 95% CI 0.19–0.74) and short-term trials (SMD 0.20, 95% CI 0.03–0.37) showed modest improvements. NDOs also lowered fecal pH (SMD −1.02, 95% CI −1.25–−0.79). Data on SCFAs were inconclusive and based on very limited studies. Conclusions: NDOs modestly increase stool frequency and lower fecal pH, with greater effects in constipated individuals and short-term interventions. However, evidence certainty remains low due to heterogeneity and study limitations. Further studies are needed to establish clinical utility.