INFOGEST Digestion and In Vitro Rat Fecal Fermentation of Bletilla striata Polysaccharides: Digestive Stability and Dose-Specific Microbiota–Metabolite Responses

Natural polysaccharides have considerable potential to modulate the gut microbial ecosystem. This study integrated INFOGEST-based simulated gastrointestinal digestion, in vitro fermentation using rat feces, untargeted metabolomics, and 16S rRNA sequencing to characterize the digestive stability of Bletilla striata polysaccharides (BSP) and the dose-specific microbial and metabolic responses to BSP. During digestion, BSP exhibited changes in total and reducing sugar content while retaining appreciable antioxidant activity. During fermentation, BSP promoted microbial growth and reduced pH. The medium dose selectively enriched Bifidobacterium, whereas the high dose favored Lactobacillus and Ligilactobacillus, accompanied by lower relative abundances of potentially unfavorable taxa, including Escherichia–Shigella and Proteus. Metabolomic analysis further revealed that BSP predominantly altered amino acid, aromatic amino acid, and carbohydrate metabolism, generating distinct dose-specific metabolic signatures. Overall, BSP remained available for microbial utilization after digestion and reshaped both microbial composition and metabolic profiles in a dose-specific manner, indicating its promise as a functional food ingredient.