Bound Polyphenols from Red Quinoa Prevailed over Free Polyphenols in Reducing Postprandial Blood Glucose Rises by Inhibiting α-Glucosidase Activity and Starch Digestion

Inhibiting α-glucosidase activity is important in controlling postprandial hyperglycemia and, thus, helping to manage type-2 diabetes mellitus (T2DM). In the present study, free polyphenols (FPE) and bound polyphenols (BPE) were extracted from red quinoa and their inhibitory effects on α-glucosidase and postprandial glucose, as well as related mechanisms, were investigated. HPLC-MS analysis showed that the components of FPE and BPE were different. FPE was mainly composed of hydroxybenzoic acid and its derivatives, while BPE was mainly composed of ferulic acid and its derivatives. BPE exhibited stronger DPPH and ABTS antioxidant activities, and had a lower IC50 (10.295 mg/mL) value in inhibiting α-glucosidase activity. The inhibition kinetic mode analysis revealed that FPE and BPE inhibited α-glucosidase in a non-competitive mode and an uncompetitive mode, respectively. Furthermore, compared to FPE, BPE delayed starch digestion more effectively. BPE at 50 mg/kg reduced postprandial glucose increases comparably to acarbose at 20 mg/kg in ICR mice. These results could provide perspectives on the potential of BPE from red quinoa, as a functional food, to inhibit α-glucosidase activity, delay postprandial glucose increases and manage T2DM.