As quercetin has been shown to inhibit aldose reductase, the first enzyme in the polyol pathway, a theoretical basis exists for its use in the prevention of long-term diabetic complications such as cataracts, nephropathy, retinopathy and neuropathy (Chaudhry et al 1983). Quercetin may also provide beneficial effects in people with diabetes by decreasing oxidative stress and preserving pancreatic beta-cell integrity (Coskun et al 2005).
Preliminary evidence suggests a possible antinociceptive activity of quercetin, probably through modulation of opioidergic mechanism, suggesting a potential for the treatment of diabetic neuropathic pain (Anjaneyulu & Chopra 2003). Topical application of quercetin in combination with ascorbyl palmitate and vitamin D3 has been tested in a randomised, placebo-controlled, double-blind trial of 34 men and women (age 21-71 years) with diabetic neuropathy. The QR-333 preparation or placebo was applied three times daily for 4 weeks to each foot experiencing symptoms. QR-333 was well tolerated and reduced the severity of numbness, jolting pain, and irritation from baseline values and improved QOL scores (Valensi et al 2005).
The diabetic status of rats fed high-dose quercetin (1 g/kg) was found to be ameliorated by approximately 25%; however, the amounts used were considerably higher than those commonly used in humans (Shetty et al 2004). Intraperitoneal injection of quercetin has also demonstrated an ability to improve glucose tolerance, and cholesterol and triglyceride levels in diabetic, but not normoglycaemic, rats and increase the number of pancreatic islets in both groups (Vessal et al 2003). However, these results cannot necessarily be applied to oral doses in humans and further research is required to confirm any potential benefits.
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