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Quercetin quer·ce·tin (ˈkwər-sə-tən) is a plant-derived flavonoid, specifically a flavonol, used as a nutritional supplement. Laboratory studies show it may have anti-inflammatory and antioxidant properties , and it is being investigated for a wide range of potential health benefits.


Quercetin has been shown to increase energy expenditure in rats, but only for short periods (fewer than 8 weeks) . Effects of quercetin on exercise tolerance in mice have been associated with increased mitochondrial biogenesis.

The American Cancer Society says that while quercetin "has been promoted as being effective against a wide variety of diseases, including cancer," and "some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans." In the amounts consumed in a healthy diet, quercetin "is unlikely to cause any major problems."

High dietary intake of fruits and vegetables is associated with reduction in cancer, and some scientists suspect quercetin may be partly responsible. Research shows that quercetin influences cellular mechanisms in vitro and in animal studies, and there is limited evidence from human population studies that quercetin may reduce the risk of lung cancer.

Some researchers believe quercetin should not be used by healthy people (for prevention) until it can be shown that quercetin doesn't itself cause cancer. In laboratory studies of cells (in vitro), quercetin produces changes that are also produced by compounds that cause cancer (carcinogens), but these studies don't report increased cancer in animals or humans. The U.S. Food and Drug Administration has not approved any health claims for quercetin. There is current early-stage clinical research on quercetin addressing safety and efficacy against sarcoidosis, asthma and glucose absorption in obesity and diabetes (February 2009).

Quercetin is the aglycone form of a number of other flavonoid glycosides, such as rutin and quercitrin, found in citrus fruit, buckwheat and onions. Quercetin forms the glycosides quercitrin and rutin together with rhamnose and rutinose, respectively. Quercetin is classified as IARC group 3 (no evidence of carcinogenicity in humans).

Occurrence

Quercetin is a naturally-occurring polar auxin transport inhibitor.

Foods rich in quercetin include capers (1800 mg/kg), lovage (1700 mg/kg), apples (440 mg/kg), tea (Camellia sinensis), onion, especially red onion (higher concentrations of quercetin occur in the outermost rings), red grapes, citrus fruit, tomato, broccoli and other leafy green vegetables, and a number of berries including cherry, raspberry, bog whortleberry (158 mg/kg, fresh weight), lingonberry (cultivated 74 mg/kg, wild 146 mg/kg), cranberry (cultivated 83 mg/kg, wild 121 mg/kg), chokeberry (89 mg/kg), sweet rowan (85 mg/kg), rowanberry (63 mg/kg), sea buckthorn berry (62 mg/kg), crowberry (cultivated 53 mg/kg, wild 56 mg/kg), and the fruit of the prickly pear cactus. A recent study found that organically grown tomatoes had 79% more quercetin than "conventionally grown".

A study by the University of Queenslandmarker, Australia, has also indicated the presence of quercetin in varieties of honey, including honey derived from eucalyptus and tea tree flowers.

Possible medicinal properties

From in vitro studies, quercetin has demonstrated significant anti-inflammatory activity by inhibiting both manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it exerts potent antioxidant activity and vitamin C-sparing action .

In vitro, quercetin shows some antitumor activity. Cultured skin and prostate cancer cells showed significant mortality (compared to nonmalignant cells) when treated with a combination of quercetin and ultrasound Note that ultrasound also promotes topical absorption by up to 1,000 times, making the use of topical quercetin and ultrasound wands an interesting proposition.

Recent studies have supported that quercetin may help men with chronic prostatitis, and both men and women with interstitial cystitis, possibly because of its action as a mast cell inhibitor.

Quercetin may have positive effects in combating or helping to prevent cancer, prostatitis, heart disease, cataracts, allergies/inflammations, and respiratory diseases such as bronchitis and asthma . It also has been claimed to have antidepressant properties, however any claim of quercetin action against neurological diseases should be treated with skepticism due to the fact that quercetin is a neurotoxin in vitro.

It has also been claimed that quercetin reduces blood pressure in hypertensive subjects.

It also may be found in dietary supplements.

An 8-year study found that the presence of three flavonolskaempferol, quercetin, and myricetin — in the person's normal diet were associated with a reduced risk of pancreatic cancer, a rare but frequently fatal disease, of 23 percent in current tobacco smokers. There was no benefit to people that have never smoked or that had previously quit smoking.

In mice, an oral quercetin dose of 12.5 to 25 mg/kg increased gene expression of mitochondrial biomarkers and improved exercise endurance.

An in vitro study showed that quercetin and resveratrol combined inhibited production of fat cells.

Despite these preliminary indications of possible health benefits, quercetin has neither been confirmed as a specific therapeutic for any condition nor has it been approved by any regulatory agency. A bioavailability study done on rats showed that ingested quercetin is extensively metabolized into non-active phenolic acids, with more than 96% of the ingested amount excreted within 72 hours, indicating actual physiological roles, if they exist, involve quercetin in only minute amounts.

Drug interactions

Quercetin is contraindicated with some antibiotics; it may interact with fluoroquinolones (a type of medicinal antibiotic), as quercetin competitively binds to bacterial DNA gyrase. Whether this inhibits or enhances the effect of fluoroquinolones is not entirely clear.

Quercetin is also a potent inhibitor of CYP3A4 and CYP2C9, which are enzymes that break down most drugs in the body. As such, quercetin would be expected to increase serum levels, and therefore effects, of drugs metabolized by this enzyme.

In cattle, there is a synergystic interaction between bovine papillomavirus-2 infection and exposure to quercetin, promoting bladder neoplasia, clinically presenting as enzootic haematuria. A similar effect is seen on exposure to the bracken fern Pteridium aqualinum, and the chemical ptaquiloside found within it.

Glycosides



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