Gastrointestinal Transit Time Shapes Mouse Gut Microbiome Responses to Dietary Fiber Content and Regulates Satiety Hormones

Intestinal transit time (TT) is strongly associated with gut microbiota composition and metabolism, with longer TT linked to proteolytic fermentation and shorter TT to saccharolytic fermentation in humans. However, the impact of diet on these associations remains poorly understood. We investigated how fiber and TT shape microbial metabolism and host responses in mice (n=48). Mice were assigned to low-, medium-, or high-fiber diets, and each dietary group was subsequently split into saline controls or loperamide-treated subgroups to prolong TT. Dietary fiber increased cecal alpha diversity, while loperamide treatment reduced it, particularly for mice on medium/high-fiber diets. Beta diversity analysis revealed clustering by both diet and loperamide treatment. With higher fiber intake, intestinal acetate and butyrate levels increased, while intestinal branched-chain fatty acids (BCFAs), urinary/serum p-cresol sulfate (pCS), and urinary phenylacetyl glycine (PAGly) decreased dose-dependently. Loperamide treatment induced fiber-concentration-dependent shifts in microbial metabolites, reducing cecal acetate in mice on low/medium-fiber diets, decreasing acetate and butyrate in the proximal colon of mice fed a high-fiber diet, elevating circulating BCFAs in medium/high-fiber mice, and increasing circulating indoxyl sulfate (IS) in medium-fiber mice. Portal vein levels of the satiety hormones glucagon-like peptide 1 (GLP-1) and peptide tyrosine-tyrosine (PYY) increased with loperamide treatment in low/medium-fiber groups. These findings demonstrate that TT modulates the gut microbiome and metabolome in a dietary fiber-concentration-dependent manner and exerts strong effects on gut satiety hormone levels, largely independently of dietary fiber intake.