Fiber intake and laxation in people with normal bowel function: a systematic review

Optimal evidence-based dietary recommendations for dietary fiber have not been established.

This study aims to conduct a systematic review of fiber and gut motility (laxation) in noninfants with normal bowel function.

We searched PubMed, Embase, and CINAHL from inception through 4 March, 2025, and existing systematic review reference lists. We included randomized controlled trials (RCTs) in the general population without bowel dysfunction that compared fiber amounts or types and assessed stool consistency, fecal weight, fecal frequency, or gut transit time. We categorized fibers according to their solubility, viscosity, and fermentability and performed hierarchical Bayesian regression models and spline analyses across reported total and added fiber intake doses, with subanalyses by fiber type. We assessed study risk of bias and strength of evidence (SoE).

We identified 113 eligible trials, with low or moderate risk of bias. Models provided moderate SoE that increasing fiber intake (per g/d total fiber) yields softening stool consistency [0.013 units, 95% credible interval (CrI): 0.009, 0.016 on the Bristol Stool Scale], increasing total fecal weight (1.76 g/d, 95% CrI: 1.58, 1.94) and dry fecal weight (0.47 g/d, 95% CrI: 0.42, 0.51), increasing fecal frequency (0.053 additional bowel movements/wk, 95% CrI: 0.042, 0.065), and shortening gut transit times (-0.24 h, 95% CrI: -0.36, -0.12). Associations between fiber intake and outcomes varied across fiber doses and type. Low-solubility and low-fermentability fibers had the strongest associations with laxation outcomes, with low to medium SoE. However, findings are indirect, based on models, and there is insufficient evidence in specific subpopulations.

RCTs support small to modest effects of dietary fiber on laxation in generally healthy people with normal bowel function. Low-solubility and low-fermentability fibers yielded the strongest effects. Future studies should better characterize fiber properties, assess wider ranges of fiber doses, and comprehensively report participants' total and background fiber intake. This study was registered at PROSPERO as CRD42024522380.