Effects of Dietary Fiber Supplementation on Gut Microbiota and Bowel Function in Healthy Adults: A Randomized Controlled Trial

This randomized, double-blind, controlled trial evaluated the effects of 4-week dietary fiber supplementation on gut microbiota, bowel-related quality of life, and secondary outcomes, including sleep and skin condition. A total of 105 healthy adults received either low-fiber foods (2.2 g/day total fiber, 1.2 g/day fermentable fiber) or high-fiber foods (8.2 g/day total fiber, including 6.4 g/day fermentable fiber). Gut microbiota was analyzed by 16S rRNA sequencing. Outcomes included stool diary, JPAC-QOL (Japanese version of the Patient Assessment of Constipation Quality of Life), OSA-MA (Oguri-Shirakawa-Azumi sleep inventory MA version), skin questionnaires, and fecal organic acids. The high-fiber group showed significant improvements in JPAC-QOL and increases in SCFA-associated genera such as <i>Anaerostipes</i>, <i>Bifidobacterium</i>, and <i>Fusicatenibacter</i>. These taxa positively correlated with other beneficial bacteria, including <i>Faecalibacterium</i>, suggesting ecological cooperation. The effects on sleep and skin were limited but correlated with beneficial bacteria, implying possible gut-brain and gut-skin axes involvement. This study demonstrated that short-term fiber supplementation meaningfully improved the bowel-related quality of life and beneficially modulated the gut microbiota in healthy adults. Although the systemic effects were modest, microbial shifts suggest that higher fiber intake may provide broader health benefits with longer interventions. This study was registered in the UMIN Clinical Trial Registry (UMIN000054712).