The Structure–Decoding–Conversion–Effect Paradigm of Natural Polysaccharides for Gut Microbiota Remodeling in Ulcerative Colitis

Ulcerative colitis (UC), a chronic inflammatory bowel disease, is closely associated with disturbances in the gut microbiota. Natural polysaccharides, owing to their unique “indigestibility” and prebiotic properties, represent a potential strategy for intervening in UC by remodeling the gut microecology. This review summarizes the mechanisms by which natural polysaccharides alleviate UC through modulation of the gut microbiota, with a particular focus on the structure–activity relationship between the structural features of natural polysaccharides and their microbiota-regulating functions. Analytical studies indicate that polysaccharides with distinct structures can be recognized and degraded by specific carbohydrate-active enzymes (CAZymes) in the gut microorganisms, leading to the targeted enrichment of beneficial genera such as Roseburia, Lactobacillus, and Akkermansia, while simultaneously suppressing pro-inflammatory genera such as Escherichia–Shigella and Helicobacter. This structure-dependent microbial remodeling ultimately enhances the production of key metabolites and exerts comprehensive therapeutic effects, including repair of the intestinal barrier, suppression of excessive inflammation, and alleviation of oxidative stress, via activation of signaling pathways such as AMP-activated protein kinase (AMPK) and nuclear factor erythroid 2-related factor 2 (Nrf2) and inhibition of pathways such as nuclear factor kappa-B (NF-κB). By exploring the paradigm of “Structure–Decoding–Conversion–Effect” based on precise microecological regulation of polysaccharide structures, this paper provides a crucial theoretical foundation and design strategy for developing targeted microecological interventions.