Exploring the Hypoglycemic Activity and Mechanism of Polyphenols From Highland Barley Through <i>Lactobacillus acidophilus</i> Fermentation

The polyphenols in grains are highly active, but some polyphenols in highland barley are in a bound form and have extremely low bioavailability. Fermentation by lactic acid bacteria (LAB) is capable of altering the functionality of foods. This research investigated the effects of fermentation with different LAB, such as Lactobacillus acidophilus (LAC), Lactobacillus casei (LCA), Lactobacillus rhamnosus (LRH), Lactobacillus plantarum (LPL), and Lactobacillus bulgaricus (LBU), on the hypoglycemic activity and mechanism of polyphenols in highland barley. The hypoglycemic activity of the fermentation products was measured by in vitro antioxidant, enzyme activity, and glucose consumption experiments. Untargeted metabolomic analysis used UHPLC-Q Exactive HF-X/MS to reveal distinct metabolic profiles among the fermented groups. Molecular docking and western blot experiments were conducted to elucidate the mechanism underlying the hypoglycemic effect of fermentation products. Polyphenolic antioxidant activity in highland barley and its inhibitory activities against α-glucosidase and α-amylase were increased after LAC fermentation. Furthermore, the fermented extracts improved glucose consumption in HepG2 cells. The content determination and metabolomic analysis showed that fermented highland barley polyphenols were increased, and 113 differential phenolic metabolites were identified and annotated, among which 44 exhibited a significant upregulation compared with raw highland barley polyphenols. At the molecular level, the polyphenol extract upregulated PI3K and phosphorylated Akt expression in HepG2 cells. Overall, the results indicate that fermentation by LAC biotransformed highland barley polyphenols into smaller molecules with improved hypoglycemic activities, thereby enhancing their bioavailability.