Abstract
Background & Aims:
Dietary fiber is first-line for functional constipation (FC), but its mechanism remains unclear. Emerging evidence suggests gut–brain axis modulation. We conducted a meta-analysis of randomized controlled trials (RCTs) to evaluate fiber's clinical efficacy and its effects on gut–brain axis biomarkers in FC.
Methods:
We searched PubMed, Embase, Web of Science, and Cochrane CENTRAL from inception to January 2026 for RCTs assessing fiber supplementation in Rome-diagnosed FC patients. Primary outcomes: spontaneous bowel movements (SBMs), complete SBMs (CSBMs), stool consistency. Secondary outcomes: short-chain fatty acids (SCFAs), serotonin (5-HT), inflammatory markers. Random-effects models calculated mean differences (MD) or standardized mean differences (SMD) with 95% CIs.
Results:
Fourteen RCTs (1,284 participants) were included. Fiber significantly increased SBMs (MD 1.42/week; 95% CI 0.98–1.86) and CSBMs (MD 0.96/week; 95% CI 0.52–1.40), and improved stool consistency (SMD 0.64; 95% CI 0.39–0.89). Fiber increased total SCFAs (SMD 0.78; 95% CI 0.45–1.11) and 5-HT (SMD 0.52; 95% CI 0.18–0.86), and reduced inflammatory markers (SMD −0.34; 95% CI −0.62 to −0.06). Soluble/fermentable fibers at >10 g/day for ≥4 weeks showed greater efficacy. Evidence certainty: moderate to high.
Conclusions:
Dietary fiber improves clinical outcomes in FC and is associated with changes in gut–brain axis–related biomarkers, including microbial metabolites, serotonergic signalling, and inflammatory markers. These associations are consistent with effects that extend beyond stool bulking; however, the clinical significance of the biomarker changes is not yet established, and the findings should be interpreted as hypothesis-generating rather than as evidence that fermentable fiber restores neuroenteric homeostasis.