Quercetin is used in the treatment of acute and chronic allergic symptoms, such as hayfever and chronic rhinitis. The anti-inflammatory activity of quercetin and its ability to stabilise mast cells, neutrophils and basophils and inhibit histamine release provides a rationale for its use in these indications.
In a study of 123 patients sensitised to house dust mite and displaying nasal symptoms of mild to severe perennial allergic rhinitis, nasal scrapings were taken and histamine release measured as a percentage of the total content in the specimen. Antigen exposure resulted in an increase in mast cells of the epithelial layer of the nasal mucosa resulting in nasal hypersensitivity. Quercetin inhibited histamine release by 46-96% in a dose-dependent manner.
Large-scale human trials are required to fully elucidate the potential for quercetin to inhibit allergic symptoms caused by the release of histamine.
Quercetin has also been used as an adjunct in the management of asthma, often in combination with vitamin C because of its anti-allergic activity and ability to inhibit leukotriene synthesis. Controlled studies are still required to determine its effectiveness.
PREVENTING DIABETIC COMPLICATIONS
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. Quercetin may also provide beneficial effects in people with diabetes by decreasing oxidative stress and preserving pancreatic beta-cell integrity.
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. 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.
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. 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. However, these results cannot necessarily be applied to oral doses in humans and further research is required to confirm any potential benefits.
In addition to the potential reduction in diabetic cataract formation afforded by the inhibition of aldose reductase, quercetin may also reduce oxidative stress associated with the initiation of maturity onset cataracts.
Cataracts may result from oxidative damage to the lens, which causes a disruption of the redox system, membrane damage, proteolysis, protein aggregation and a loss of lens transparency. Quercetin has been shown to inhibit oxidative damage to the lens and maintain lens transparency in vitro. Further trials are warranted to confirm the effects of oral doses in humans.
PREVENTING CARDIOVASCULAR DISEASE
The cardioprotective properties of quercetin, demonstrated in animal and in vitro studies, provide a theoretical basis for the use of quercetin in the prevention of cardiovascular disease; however, current human data is less encouraging.
A double-blind, placebo-controlled study investigating the effects of a quercetin-containing supplement on plasma quercetin status, risk factors for heart disease and serum/platelet fatty acid levels was conducted on 27 healthy men and women with cholesterol levels of 4.0-7.2 mmol/L. The subjects consumed a quercetin-containing supplement (1 g quercetin/day) or rice flour placebo for 28 days. Quercetin intakes were approximately 50-fold greater than dietary intakes previously associated with lower coronary heart disease mortality in epidemiologic studies. Plasma quercetin concentrations were approximately 23-fold greater in subjects consuming the quercetin capsules than in the placebo group. Quercetin supplementation did not alter serum total, LDL- or HDL-cholesterol or triglyceride levels, or other cardiovascular disease or thrombogenic risk factors such as platelet thromboxane B2 production, blood pressure or resting heart rate. This is in contrast to a previous trial, which demonstrated inhibition of platelet aggregation and signalling and thrombus formation at doses of 150 mg or 300 mg quercetin-4′-0-beta-D-glucoside, were not reproduced using the dose and form in this study. There was also no effect on the levels of omega-3 or omega-6 polyunsaturated fatty acids in serum or platelet phospholipids. Further investigation with larger and longer term trials is required to determine the effects and safety of quercetin in the prevention of cardiovascular disease in humans.
Thirty men with category Ilia or lllb chronic pelvic pain syndrome received either placebo or quercetin 500 mg twice daily for 1 month. Sixty seven percent of the treated subjects had at least a 25% improvement in symptoms, compared to 20% of the placebo group. In a follow-up, unblinded, open-label study, 17 additional men received the same dose of quercetin (combined with bromelain and papain to enhance absorption) for 1 month. The combination increased the response rate from 67% to 82%. The anti-inflammatory, antioxidant and immunomodulating activities of quercetin may help to explain these results.
Early concerns that quercetin may be carcinogenic have not been supported by recent research. Quercetin is primarily found in fruit and vegetables, which have been shown to decrease the risk of certain human cancers when consumed regularly, thereby casting doubt on this proposition. In fact, the anticarcinogenic effects of quercetin seen in laboratory animals suggest a possible preventative role, and in vitro and epidemiological studies have suggested potential benefits in the prevention of colon, lung, prostate and breast cancer.
Human studies using extremely low dose supplementation of quercetin (30 mg/day) have so far provided more questions than answers about the exact mechanism/s by which quercetin may exert its effects but much remains unknown.
Quercetin: Other Uses
Although the potential benefits of quercetin in allergic conditions have yet to be elucidated by human trials, quercetin is often used to treat conditions such as hayfeverand other respiratory allergies and histamine-related conditions. In practice, quercetin is also commonly used to stabilise the integrity of blood vessel walls and address conditions resulting from capillary fragility and used in conjunction with vitamin C for the treatment of viral infections.
Quercetin: Dosage Range
GENERAL DOSE RANGE
• 200-1 500 mg daily taken in divided doses
• Chronic prostatitis: 500 mg (combined with bromelain and papain to enhance absorption) twice daily
• Acute allergies: 2 g every 2 hours for 2 days (often used with vitamin C)
• Chronic allergies: 2 g daily
• Asthma: as an adjunct to standard treatment 2 g daily