Bioactivity and prebiotic potential of insoluble dietary fiber from discarded Chinese cabbage outer leaves and the metabolomic dynamics of bound polyphenols during alkaline extraction, in vitro gastrointestinal digestion and colonic fermentation

Insoluble dietary fiber from Chinese cabbage outer leaves (CIF) is abundant in bound polyphenols (BPs), while the release of BPs and their influence on the function of CIF remain unclear. This study compared the changes in CIF before and after the removal of BPs and characterized the BPs profiles during gastrointestinal and colonic fermentation. CIF exhibited a markedly denser microstructure with fewer pores and markedly higher antioxidant and α-glucosidase inhibitory activities than BPs-removed CIF (BPR-CIF). UHPLC-OE-MS analysis identified 342 phenolic compounds in CIF, predominantly flavonoids, phenolic acids, and coumarins. The release of BPs during in vitro gastrointestinal digestion was 2.59% (mainly flavonoids, coumarins, and lignans) with kaempferol 3-sophoroside-7-glucoside as the representative compound, and 37.25% (mainly phenolic acids, phenylpropanoids, and stilbenoids) with salicylic acid, mandelic acid, and trans-cinnamic acid as the representative compounds during colonic fermentation, which was more effectively released. The phase dependence of the BPs was further confirmed using quantitative HPLC analysis. Microbiomic data revealed that CIF-BPs reduced the Firmicutes / Bacteroidota ratio and enriched beneficial microbiota (e.g., Bifidobacterium , Lachnoclostridium, and Blautia ) while inhibiting pro-inflammatory microbiota (e.g., Bilophila , Eggerthella, and Fusobacterium ). And the decrease in pH and increase in short-chain fatty acid concentrations further demonstrated these prebiotic effects. Furthermore, the observed microbial changes showed a strong correlation with elevated levels of specific phenolic compounds. These findings enhance our understanding of the interactions between BPs and CIF as natural complexes and provide valuable insights into the high-value utilization of outer leaf waste of Chinese cabbage.