The effect of dietary fiber based on fermentability and viscosity on the gut microbiome composition in chronic kidney disease: a systematic review of experimental and clinical trials.

BACKGROUND: Chronic kidney disease (CKD) is associated with alterations in gut microbial composition and diversity. Accordingly, dietary fiber has emerged as a promising microbiome-targeted intervention. However, the influence of fiber fermentability and viscosity remains largely understudied. OBJECTIVE: To systematically evaluate the effects of dietary fiber supplementation interventions, classified by viscosity and fermentability, on gut microbial composition in CKD across human and animal studies, and to determine whether these fiber properties influence gut microbiota composition. METHODS: A systematic search of PubMed, Embase, CINAHL, and Cochrane Library (through February 2026) identified randomized controlled trials and controlled animal studies assessing isolated dietary fiber in CKD. Eligible studies reported gut microbiota composition. Risk of bias was assessed with ROB-2 and SYRCLE, and an overall level of evidence (strong, moderate, weak, or inconclusive) was assigned based on study quality and consistency of findings. Alpha and beta diversity, as well as bacterial taxa changes, were then reported according to these evidence rankings. RESULTS: Twenty-two studies (9 human, 13 animal) were included, comprising 236 participants and 274 animals with CKD. Dietary fiber supplementation demonstrated limited and inconsistent effects on gut microbiota composition in human studies. Inulin and resistant starch were associated with weak evidence of changes in beta diversity, but most taxa across phylum and genus levels remained unchanged, often supported by strong or weak evidence depending on the taxonomic level. Modest increases were observed in a small number of genera, including Verrucomicrobia, Akkermansia, Clostridium by inulin, Oscillospiraceae UCG-002, and Subdoligranulum by resistant starch type 2 in human studies, which were not consistent across animal studies. Beta-glucan showed no significant effects on alpha or beta diversity or microbial composition. In contrast, weak evidence from animal studies suggested that beta-glucan may increase alpha diversity. CONCLUSION: Isolated dietary fiber interventions were associated with variable changes in gut microbiota composition in CKD, with limited consistency across human and animal studies, and no clear pattern based solely on fermentability or viscosity. Future studies should use standardized sequencing approaches, harmonized taxonomic reporting, and detailed characterization of fiber properties to clarify whether specific fibers reproducibly modulate microbial community structure.This trial was registered at PROSPERO as CRD42023483468.