In Vitro Gastrointestinal Digestion of Grifola frondosa Polysaccharides and Their Enhancement of GABA Production via Gut Microbiota Modulation

<b>Background</b>: The water-soluble <i>Grifola frondosa</i> polysaccharides (GFPs) are the primary bioactive component of the edible and medicinal fungus <i>Grifola frondosa</i>. However, the digestive behavior of GFPs in the human gastrointestinal (GI) tract and their subsequent interaction with gut microbiota (GM) to exert health effects remain unclear. <b>Methods</b>: In this study, GFPs were extracted based on a traditional hot water decoction. An in vitro simulated GI digestion model and a human fecal microbiota fermentation model were established to systematically investigate the digestive stability of GFPs, GM modulation, and metabolite changes. <b>Results:</b> Results showed that GFPs remained structurally stable during in vitro oral, gastric, and small intestinal digestion, allowing them to reach the colon intact for microbial fermentation. During colonic fermentation, GFPs were efficiently degraded by GM, and significantly increased the relative abundance of beneficial bacteria such as <i>Akkermansia</i>, <i>Bacteroides</i>, <i>Parabacteroides</i>, and <i>Lactobacillus</i> while reducing the abundance of pathogenic <i>Escherichia-Shigella</i>. Meanwhile, GFPs enriched metabolites beneficial for intestinal health, among which γ-aminobutyric acid (GABA) was the most significantly upregulated. Single-strain fermentation confirmed that <i>Lactobacillus</i> (<i>L. plantarum</i>) was the core GABA-producing genus. <b>Conclusions:</b> This study highlights the potential of GFPs as prebiotics for GM modulation, expands the understanding of the health-promoting effects of fungal polysaccharides, and provides a theoretical basis for the development of GFP-based functional foods.