Psyllium and Glucomannan as Viscous Fiber Modulators of the Gut–Microbiome–Incretin Axis: Molecular Links to Metabolic Inflammation, Barrier Function, and GLP-1 Receptor Agonist Therapy

Dietary fiber is an under-consumed nutritional substrate that supports gut microbial metabolism, intestinal barrier integrity, enteroendocrine signaling, and cardiometabolic regulation. Among dietary fibers, psyllium and glucomannan are clinically accessible viscous, gel-forming soluble fibers with distinct but complementary physicochemical profiles. This narrative review examines how these fibers act through luminal viscosity, nutrient diffusion, bile acid and cholesterol handling, short-chain fatty acid production, epithelial barrier support, enteroendocrine L-cell signaling, and low-grade metabolic inflammation. Psyllium has the strongest clinical support for stool normalization, glycemic modulation, and LDL cholesterol reduction, whereas glucomannan provides marked viscosity and water-holding capacity that may support satiety, lipid modulation, and weight-management strategies when appropriately hydrated and tolerated. We also discuss the emerging nutritional context of glucagon-like peptide-1 receptor agonist therapy, in which appetite suppression, reduced meal volume, delayed gastrointestinal transit, and constipation may reduce dietary fiber intake and fermentable substrate delivery to the colon. The pain- and mood-related implications are framed as hypothesis-generating extensions, because direct clinical evidence that psyllium or glucomannan improves these outcomes remains limited. A psyllium-centered, selectively glucomannan-supported strategy may help close the fiber gap and support bowel function, microbial metabolite signaling, and cardiometabolic stability during modern metabolic and weight-loss therapies.