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Quercetin

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Quercetin /ˈkwɜrsɨtɨn/ is a flavonol, plant-derived flavonoid, 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.

Informed medical opinion

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 or benefits."

Adequate dietary intake of fruits and vegetables may reduce the risk of cancer. Research shows that quercetin influences cellular mechanisms in vitro and, in animal studies, there is evidence from human population studies that quercetin may, in a very limited fashion, reduce the risk of certain cancers.

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 black & green tea (Camellia sinensis; 2000-2500mg/kg), capers (1800 mg/kg), lovage (1700 mg/kg), apples (44 mg/kg), onion, especially red onion (1910 mg/kg) (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 Queensland, Australia, has also indicated the presence of quercetin in varieties of honey, including honey derived from eucalyptus and tea tree flowers.

Biosynthesis

The biosynthesis of quercetin is summarized in the above figure. Phenylalanine(1) is converted to 4-coumaroyl-CoA(2) in a series of steps known as the general phenylpropanoid pathway using phenyl ammonia-lyase, cinnamate-4-hydroxylase, and 4-coumaroylCoA-ligase. 4-coumaroyl-CoA(2) is added to three molecules of malonyl-CoA(3) to form tetrahydroxychalcone(4) using 7,2’-dihydroxy, 4’-methoxyisoflavanol synthase. Tetrahydroxychalcone(4) is then converted into naringenin(5) using chalcone isomerase. Naringenin(5) is then converted into eriodictyol(6) using flavanoid 3’ hydroxylase. Eriodictyol(6) is then converted into dihydroquercetin(7) with flavanone 3-hydroxylase which is then converted into quercetin using flavanol synthase.

Possible medicinal properties

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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, cultured skin and prostate cancer cells were suppressed (compared to nonmalignant cells) when treated with a combination of quercetin and ultrasound.

Studies indicate that quercetin may inhibit chronic prostatitis, and interstitial cystitis, possibly resulting from its action as a mast cell inhibitor.

Quercetin may have inhibitory properties against cancer, prostatitis, heart disease, cataracts, allergies/inflammations, and respiratory diseases, such as bronchitis and asthma.

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

An 8-year study found that the presence of three flavonols — kaempferol, quercetin, and myricetin — in the person's normal diet was associated with 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.

A possible antagonistic interaction occurred in rats between quercetin and niacin (vitamin B3) which, in sustained, high doses, produces a cutaneous reddening or flush.

Despite these preliminary indications of possible medicinal effects, 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 certain.

Quercetin is described as an inhibitor of CYP2C9. Sources are inconsistent as to whether quercetin is an inhibitor or inducer of CYP3A4. CYP2C9 and CPY3A4 are members of the cytochrome P₄₅₀ mixed-function oxidase system, and as such are enzymes involved in the metabolism of xenobiotics in the body. In either case, quercetin may alter serum levels, and potentially, effects of drugs metabolized by these enzymes.

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

• Guaijaverin is the 3-O-arabinoside of quercetin.
• Hyperoside is the 3-O-galactoside of quercetin.
• Isoquercitin is the 3-O-glucoside of quercetin.
• Spiraeoside is the 4'-O-glucoside of quercetin.

See also

References

External links

• UMM Quercetin Info Page (University of Maryland Medical Center Website)