Onion peel extracts ameliorate hyperglycemia and insulin resistance in high fat diet/streptozotocin-induced diabetic rats

Background: Quercetin derivatives in onions have been regarded as the most important flavonoids to improve diabetic status in cells and animal models. The present study was aimed to examine the hypoglycemic and insulin-sensitizing capacity of onion peel extract (OPE) containing high quercetin in high fat diet/streptozotocin-induced diabetic rats and to elucidate the mechanism of its insulin-sensitizing effect. Methods. Male Sprague-Dawley rats were fed the AIN-93G diet modified to contain 41.2% fat and intraperitoneally injected with a single dose of streptozotocin (40 mg/kg body weight). One week after injection, the rats with fasting blood glucose levels above 126 mg/dL were randomly divided into 4 groups to treat with high fat diet containing 0 (diabetic control), 0.5, or 1% of OPE or 0.1% quercetin (quercetin equivalent to 1% of OPE) for 8 weeks. To investigate the mechanism for the effects of OPE, we examined biochemical parameters (insulin sensitivity and oxidative stresses) and protein and gene expressions (pro-inflammatory cytokines and receptors). Results: Compared to the diabetic control, hypoglycemic and insulin-sensitizing capability of 1% OPE were demonstrated by significant improvement of glucose tolerance as expressed in incremental area under the curve (P = 0.0148). The insulin-sensitizing effect of OPE was further supported by increased glycogen levels in liver and skeletal muscle (P < 0.0001 and P = 0.0089, respectively). Quantitative RT-PCR analysis showed increased expression of insulin receptor (P = 0.0408) and GLUT4 (P = 0.0346) in muscle tissues. The oxidative stress, as assessed by superoxide dismutase activity and malondialdehyde formation, plasma free fatty acids, and hepatic protein expressions of IL-6 were significantly reduced by 1% OPE administration (P = 0.0393, 0.0237, 0.0148 and 0.0025, respectively). Conclusion: OPE might improve glucose response and insulin resistance associated with type 2 diabetes by alleviating metabolic dysregulation of free fatty acids, suppressing oxidative stress, up-regulating glucose uptake at peripheral tissues, and/or down-regulating inflammatory gene expression in liver. Moreover, in most cases, OPE showed greater potency than pure quercetin equivalent. These findings provide a basis for the use of onion peel to improve insulin insensitivity in type 2 diabetes.