Role of fermentation-resistant and non-fermentable fibers in gastrointestinal health: mechanisms, benefits, and challenges.

Fermentability is largely dictated by fiber structure, which shapes physiochemical properties and microbial accessibility. While many fibers are rapidly fermented, others are only partially degraded or remain non-fermentable yet still provide benefits such as bulking, improved motility, and altered colonic environment. Interindividual variation in gut physiology and microbiota composition adds further complexity, making it challenging to predict fiber-specific outcomes. Mechanistic studies of these resistant fibers and their interactions with other dietary polysaccharides remain relatively limited. Dietary fibers are indigestible carbohydrate polymers that are structurally and functionally diverse, encompassing natural, extracted, and synthetic forms. Their glycosidic linkages are selectively degraded by colonic bacteria that express specialized enzymes generating gases and metabolites, including short-chain fatty acids. Delaying rather than inhibiting fermentation is desirable as it delivers these beneficial metabolites to distal regions of the colon, where they exert protective and anti-carcinogenic effects. This review focuses on the physiochemical properties of fermentation-resistant and non-fermentable fibers, highlighting their role in gastrointestinal health. We emphasize magnetic resonance imaging to noninvasively monitor gut motility and regional fermentation dynamics in vivo. We discuss how structural attributes can prolong fermentation and target distal colonic metabolism, ultimately helping to guide tailored dietary interventions to maximize fiber's therapeutic potential.