(P26-008-25) Effect of Onion Peel on Carbohydrate Enzymes and Glucose Transport in Vitro

Objectives: Hyperglycemia is a significant risk factor for developing type 2 diabetes and related metabolic diseases. Postprandial hyperglycemia is a rapid and significant increase in blood glucose levels after eating a meal. This phenomenon arises from the hydrolysis of carbohydrates into glucose, followed by its absorption into the blood using sodium-glucose co-transporter 1 (SGLT1) and glucose transporter 2 (GLUT2) in the intestine. Therefore, regulating intestinal carbohydrate-hydrolyzing enzymes and glucose transporters may help mitigate sudden spikes in blood glucose levels. This study investigated the glucose-lowering effect of onion peel extract (OPE) and its major bioactive compound, quercetin (Q) in vitro.

Methods: OPE was prepared using 70% ethanol (v/v) by liquid-liquid extraction, and then OPE and Q were digested using gastrointestinal digestive enzymes in vitro. The effects of OPE, digested OPE (DOPE), Q, and digested Q (DQ) on carbohydrate digestion were examined by measuring intestinal carbohydrate-hydrolyzing enzyme activities with acarbose, anti-diabetic medicine as a positive control. Glucose uptake was determined using a fluorescent glucose in Caco-2 cells.  The expression of genes and proteins related to glucose uptake was measured using a qPCR and a western blot.

Results: Both OPE and Q, with or without digestion, demonstrated inhibitory effects on α-glucosidase and α-amylase. However, DOPE was found to be more effective than OPE, Q, and DQ at concentrations above 2.5 µg/mL. Among them, DOPE and Q exhibited the strongest inhibition, showing IC₅₀ values of 5.83 and 5.506 µg/mL for α-glucosidase and 4.879 and 5.847 µg/mL for α-amylase, respectively, compared to OPE, DQ, and acarbose. Moreover, maltase, sucrase, and glucoamylase were inhibited at 20 µg/mL of these samples, with similar effects to 1 mg/ml acarbose. OPE, DOPE, Q, and DQ also reduced glucose uptake by decreasing the expression of SGLT1 and GLUT2 genes and proteins by activating AMP-activated protein kinase.

Conclusions: The results indicated that the extract was more effective than a single compound, and that gastrointestinal digestion enhanced its effect. Onion peel and quercetin may potentially prevent postprandial hyperglycemia by inhibiting carbohydrate-hydrolyzing enzymes and glucose transport in the intestine.

Funding Sources: USDA