Jeju Citrus (Citrus unshiu) Leaf Extract and Hesperidin Inhibit Small Intestinal α-Glucosidase Activities In Vitro and Postprandial Hyperglycemia in Animal Model

Citrus fruits are widely distributed in East Asia, and tea made from citrus peels has demonstrated health benefits, such as a reduction in fever, inflammation, and high blood pressure. However, citrus leaves have not been evaluated extensively for their possible health benefits. In this study, the α-glucosidase-inhibitory activity of Jeju citrus hot-water (CW) and ethyl alcohol (CE) extracts, along with hesperidin (HP) (a bioactive compound in citrus leaf extracts), was investigated, and furthermore, their effect on postprandial blood glucose reduction in an animal model was determined. The hesperidin contents of CW and CE were 15.80 ± 0.18 and 39.17 ± 0.07 mg/g-extract, respectively. Hesperidin inhibited α-glucosidase (IC₅₀, 4.39), sucrase (0.50), and CE (2.62) and demonstrated higher α-glucosidase inhibitory activity when compared to CW (4.99 mg/mL). When using an SD rat model, during sucrose and starch loading tests with CE (<i>p</i> < 0.01) and HP (<i>p</i> < 0.01), a significant postprandial blood glucose reduction effect was observed when compared to the control. The maximum blood glucose levels (C<i>max</i>) of the CE administration group decreased by about 15% (from 229.3 ± 14.5 to 194.0 ± 7.4, <i>p</i> < 0.01) and 11% (from 225.1 ± 13.8 to 201.1 ± 7.2 hr·mg/dL, <i>p</i> < 0.05) in the sucrose and starch loading tests, respectively. Our findings suggest that citrus leaf extracts standardized to hesperidin may reduce postprandial blood glucose levels through the observed inhibitory effect against sucrase, which results in delayed carbohydrate absorption. Our findings provide a biochemical rationale for further evaluating the benefits of citrus leaves.