The Impact of Dietary Fiber Supplementation on Gut-Brain Axis Biomarkers and Symptoms in Functional Constipation: A Meta-Analysis of Randomized Controlled Trials

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.