Quercetin

, a flavonoid with anti-inflammatory and antioxidant effects, has recently been reported to improve symptoms of NBP and PD. An uncontrolled study reported that 500 mg of quercetin twice daily for at least two weeks significantly improved symptoms in 59% of men with chronic prostatitis. These results were confirmed in a double-blind study, in which similar treatment with quercetin for one month improved symptoms in 67% of men with NBP or PD. Another uncontrolled study combined 1,000 mg per day of quercetin with the enzymes bromelain and papain, resulting in significant improvement of symptoms. Bromelain and papain promote absorption of quercetin and have anti-inflammatory effects as well.

In a double-blind study of trained athletes, the incidence of upper respiratory tract infections following a three-day period of intensive exercise was significantly lower in people who took quercetin than in those who received a placebo (5% versus 45%). The amount of quercetin used was 500 mg twice a day, beginning three weeks before, and continuing for two weeks after, the intensive exercise.

Many of the effects of allergic reactions are caused by the release of histamine, which is the reason antihistamine medication is often used by allergy sufferers. Some natural substances, such as vitamin C and flavonoids, including , have demonstrated antihistamine effects in test tube, animal, and other preliminary studies. However, no research has investigated whether these substances can specifically reduce allergic reactions in humans.

, a flavonoid found in most plants, has an inhibiting action on lipoxygenase, an enzyme that contributes to problems with asthma. No clinical trials in humans have confirmed whether quercetin decreases asthma symptoms. Some doctors are currently experimenting with 400 to 1,000 mg of quercetin three times per day.

, a flavonoid, protects LDL cholesterol from damage. While several preliminary studies have found that eating foods high in quercetin lowers the risk of heart disease, the research on this subject is not always consistent, and some research finds no protective link. Quercetin is found in apples, onions, black tea, and as a supplement. In some studies, dietary amounts linked to protection from heart disease are as low as 35 mg per day.

The flavonoid may also help by blocking sorbitol accumulation in the eye. This may be especially helpful for people with diabetes, though no clinical trials have yet explored whether quercetin actually prevents diabetic cataracts.

Flavonoids are a group of compounds found in some plant foods and medicinal herbs. An antiviral action of some flavonoids has been observed in a number of test tube experiments., one of the flavonoids, has shown particularly strong antiviral properties in the test tube; however, one study did not find quercetin to be of benefit to mice with a viral infection. It is not known whether flavonoids can be absorbed in amounts sufficient to exert an antiviral effect in humans, and therefore their possible role in the treatment of childhood exanthems remains unknown.

Because coumarin, hydroxyethylrutosides, and diosmin are not widely available in the United States, other flavonoids, such as quercetin, rutin, or anthocyanosides (from bilberry), have been substituted by doctors in an attempt to obtain similar benefits. The effect of these other flavonoids against edema has not been well studied. Also, optimal amounts are not known. However, in one study, quercetin in amounts of 30-50 mg per day corrected abnormal capillary permeability (leakiness), an effect that might improve edema. A similar effect has been reported with rutin at 20 mg three times per day. Doctors often recommend 80-160 mg of a standardized extract of bilberry, three times per day.

In test tube studies, , a flavonoid, has inhibited an enzyme involved in the production of uric acid in the body. In a double-blind trial, supplementation with 500 mg of quercetin once a day for 4 weeks significantly decreased blood levels of uric acid by an average of 8% in men with uric acid levels in the high-normal range. Decreasing uric acid levels may help prevent gout attacks.

is an increasingly popular treatment for hay fever even though only limited preliminary clinical research has suggested that it is beneficial to hay fever sufferers.

Quercetin has demonstrated multiple actions that may benefit people with type 1 diabetes, including reducing glucose absorption, increasing insulin release, and promoting glucose uptake by cells. Despite a wealth of animal studies indicating its potential in prevention and treatment of diabetes and its complications, clinical trials have not yet been performed to explore whether quercetin actually benefits people with diabetes.

Quercetin, a flavonoid widely found in the plant world, has well established antioxidant and anti-inflammatory properties. Higher dietary intake of quercetin has been correlated with a lower risk of type 2 diabetes in one study. Animal and laboratory research suggests quercetin may have a role to play in improving metabolism in type 2 diabetes and protecting against diabetes complications. A topical quercetin preparation, which also contains vitamins C and D, was found in a human clinical trial to be effective for relieving symptoms of neuropathy related to type 2 diabetes.