The combination effects of quercetin on starch and digestive enzymes reduce postprandial blood glucose in rats

Quercetin is the most abundant flavonoid found in human diet. In the present study, the effects of quercetin on postprandial hyperglycemia (PPHG) were investigated in two ways. First, the complexes of quercetin and starch were prepared and characterized. Quercetin can occupy the helix of starch and interact with it through secondary bonds. The digestibility of starch decreased with the increasing proportion of quercetin in the complexes. Second, the inhibitory activities of quercetin on α-glucosidase and α-amylase were investigated in both in vitro and in vivo. Quercetin showed very strong inhibitory activity on α-glucosidase, while its inhibitory activity on α-amylase was far weaker than acarbose. Quercetin is the non-competitive inhibitor of α-glucosidase. Fluorescence titration showed that quercetin and α-glucosidase had one binding site and the binding constant lgKa was 3.70. After oral administration of quercetin–starch complexes in rats, the maximum level of postprandial blood glucose was reduced and delayed. Besides, quercetin also improved the PPHG after administration of sucrose and maltose in rats. Hence, it is deduced that the regulation of quercetin on PPHG is the result of its combination effects on starch and digestive enzymes. The complexes of quercetin and starch may be useful for the development of low-glycemic food.