Bioavailability of Coix Seed Polyphenols in a MKN28/Caco-2 Continuous Transport Model and Their Lipid-Lowering Effects via Modulating Adipocyte Differentiation of 3T3-L1 Cells

At present, most of the research on coix seed polyphenols (CSPs) focuses on the separation, purification, structure analysis, and biological functions of specific components, and few studies have considered the overall bioavailability and the metabolites that play a role after digestion and absorption and their biological functional activities. In this study, we constructed a MKN28 and Caco-2 cell monolayer continuous transport model (MCTM) to study the bioavailability of CSPs in the digestion and absorption stages of the stomach and small intestine. Using this model, we innovatively divided CSPs into easy-to-digest and hard-to-digest polyphenols and studied their intracellular lipid-lowering function and their influence on human intestinal flora. Transwell experiments showed that ferulic acid, rutin, naringin, arbutin, and syringetin had high transmembrane transport efficiency, especially syringetin. The methylation reaction in the monolayer membrane of Caco-2 cells may be the reason for the higher transport rate of syringetin. Further experiments showed that CPL reduced TG accumulation by more than 50% during 3T3-L1 differentiation and promoted the transformation of adipocytes into brown cells (p < 0.05). Finally, in vitro fermentation experiments showed that CSP_AP can increase the abundance of Lactobacillus and Bifidobacterium of human gut microbiota at the genus level (p < 0.05).