Diets high in calories and sweetened foods with
disaccharides frequently lead to exaggerated postprandial spikes in
blood glucose. This state induces immediate
oxidant stress and
free radicals which trigger oxidative stress-linked
diabetic complications. One of the therapeutic approaches for decreasing
postprandial hyperglycemia is to retard absorption of
glucose by the inhibition of
carbohydrate hydrolyzing
enzymes, α-
amylase and α-
glucosidases, in the digestive organs. Therefore, the inhibitory activity of Korean onion (Allium cepa L.) extract against rat intestinal α-
glucosidases, such as
sucrase,
maltase, and porcine pancreatic α-
amylase were investigated in vitro and in vivo. The content of
quercetin in
ethyl alcohol extract of onion skin (EOS) was 6.04 g/100 g dried weight of onion skin. The in vitro half-maximal inhibitory concentrations (IC(50)) of EOS and
quercetin, a major phenolic in onion, on rat intestinal
sucrase were 0.40 and 0.11 mg/mL, respectively. The postprandial
blood glucose lowering effects of EOS and
quercetin were compared to a known
type 2 diabetes drug (
Acarbose), a strong α-
glucosidase inhibitor in the Sprague-Dawley (SD) rat model. In rats fed on
sucrose, EOS significantly reduced the
blood glucose spike after
sucrose loading. The area under the
blood glucose-time curve (AUC(last)) in EOS-treated SD rats (0.5 g-EOS/kg) was significantly lower than in untreated SD rats (259.6 ± 5.1 vs. 283.1 ± 19.2 h·mg/dL). The AUC(last) in
quercetin-treated SD rats (0.5 g-
quercetin/kg) was similar to in EOS-treated group (256.1 ± 3.2 vs. 259.6 ± 5.1 h·mg/dL). Results from this study indicates that although
quercetin does have
blood glucose lowering potential via α-
glucosidase inhibition, there are other bioactive compounds present in onion skin. Furthermore, the effects of two weeks administration of EOS in a high
carbohydrate-dietary mixture (Pico 5053) on
sucrase and
maltase activities in intestine were evaluated in SD rat model. Compared to the upper and middle parts of intestine, the activities of
sucrase in the lower parts of intestine remained significantly higher after two weeks of EOS treatment. These results indicate that EOS may improve exaggerated postprandial spikes in
blood glucose and
glucose homeostasis since it inhibits intestinal
sucrase and thus delays
carbohydrate absorption, although clinical trials are needed.