Over-the-counter fiber supplementation associates with metabolic and microbial shifts in rectal mucosa

Fiber supplements are the first line of treatment for all patients with benign anorectal disease. In addition, fiber has been found to support gut microbiota and reduce inflammation; however, their integrated effect on the colonic mucosal environment is incompletely defined. We evaluated how mixed-fiber formulations may collectively shape the rectal mucosal environment through coordinated effects on microbiota composition, short-chain fatty acid (SCFA) handling, and host transcription. We evaluated multi-compartment responses to a 28-day mixed-fiber intervention in 37 participants with benign anorectal disease (~15 g/day; 7.5g psyllium husk, 8g wheat dextrin) and assessed the microbiota (16S rRNA), the metabolome (GC-MS and LC-MS), and transcriptome (subset n=10) before (PRE) and after (POST) supplementation. Changes in KEGG pathways were tested with gene set enrichment analysis (GSEA). Associations were tested by Spearman correlation, and microbiota-based prediction of SCFA responses by random-forest regression. Microbiota diversity was stable after the fiber intervention, however, 10 mucosal ASVs were differentially abundant including Parabacteroides and Lachnospiraceae NK4A136 group while SCFAs, propionate and isobutyrate, both decreased in rectal tissue and butyrate decreased in serum. To understand these shifts in the metabolome, we proceeded with the transcriptome where we identified 130 differentially expressed genes and enrichment of metabolic and SCFA-related pathways. Random forest regression captured a modest signal for serum butyrate. Taken together, short-term mixed-fiber supplementation produced selective rectal mucosal taxonomic shifts and enriched mucosal metabolic and SCFA-metabolism program despite reduced SCFA pools, suggesting that even brief over-the-counter fiber supplementation may reshape the mucosal metabolic environment.