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Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox referencesQuercetin is a flavonoid and, to be more specific, a flavonol. It 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.
• 1 Occurrence
• 2 Medicinal properties
• 3 Drug interactions
• 4 See also
• 5 References
• 6 External links  Occurrence
Quercetin is a naturally-occurring polar auxin transport inhibitor.
Foods rich in quercetin include capers (1800mg/kg), lovage (1700mg/kg), apples (440mg/kg), tea (Camellia sinensis), onions, especially in red onions (higher concentrations of quercetin occur in the outermost rings), red grapes, citrus fruits, broccoli and other leafy green vegetables, cherries, and a number of berries including raspberry, bog whortleberry (158 mg/kg, fresh weight), lingonberry (cultivated 74mg/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 53mg/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.
 Medicinal properties
Quercetin is found to be the most active of the flavonoids in studies, and many medicinal plants owe much of their activity to their high quercetin content. Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation. For example, it inhibits both the manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it exerts potent antioxidant activity and vitamin C-sparing action.
Quercetin also shows anti-tumour properties. A study in the British Journal of Cancer showed that, when treated with a combination of quercetin and ultrasound at 20 kHz for 1 minute duration, skin and prostate cancers show a 90% mortality within 48 hours with no visible mortality of normal cells. 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 can 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 quercitin is a neurotoxin in vitro.
It also may be found in dietary supplements.
An 8-year study found that three flavonols — kaempferol, quercetin, and myricetin — were associated with a reduced risk of pancreatic cancer of 23 percent.
 Drug interactions
Quercetin is contraindicated with 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, an enzyme that breaks 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.
Properties: Quercetin has many health promoting effects, including improvement of cardiovascular health, reducing risk for cancer. Quercetin has anti-inflammatory and anti-allergic effects. All these activities are caused by the strong antioxidant action of quercetin. It will help to combat free radicals molecules, which can damage cells.
As many other flavonoids, quercetin prevents the oxidation of LDL (bad) cholesterol.
The anti-inflammatory action of quercetin is caused by the inhibition of enzymes, such as lipoxygenase, and the inhibition of inflammatory mediators. Quercetin also inhibits the release of histamine, which causes congestion, by basophils and mast cells.
Studies have shown that quercetin reduces the cancer risk of prostate, ovary, breast, gastric and colon cells.
Quercetin also seems to reduce the production of uric acid, by inhibiting the xanthine oxidase, thereby easing gout symptoms.
Studies have shown an improved lung function and lower risk of certain respiratory diseases (asthma and bronchitis) for people with high apple (rich in quercetin) intake.
Description: Quercetin is the most abundant of the flavonoids. Quercetin belongs to the flavonoids family and consist of 3 rings and 5 hydroxyl groups. Querctin is also a building block for other flavonoids. Quercetin occurs in food as a aglycone (attached to a sugar molecule). Only a small percentage of the ingested quercetin will get absorbed in the blood.
Distribution: Quercetin is found in many common foods including apple, tea, onion, nuts, berries, cauliflower and cabbage.
Research Reviews: Dietary Intakes of Flavonols, Flavones and Isoflavones by Japanese Women and the Inverse Correlation between Quercetin Intake and Plasma LDL Cholesterol Concentration
Tissue Distribution of Quercetin in Rats and Pigs
The Effect of Quercetin on SW480 Human Colon Carcinoma Cells: a Proteomic Study
Quercetin inhibits eNOS, microtubule polymerization, and mitotic progression in bovine aortic endothelial cells.
Rat Gastrointestinal Tissues Metabolize Quercetin
Journal of Nutrition. 2001;131:1938-1941.)
© 2001 The American Society for Nutritional Sciences
Quercetin Glucuronides but Not Glucosides Are Present in Human Plasma after Consumption of Quercetin-3-Glucoside or Quercetin-4'-Glucoside1
Aloys L. A. Sesink*2, Karen A. O'Leary and Peter C. H. Hollman*
* State Institute for Quality Control of Agricultural Products (RIKILT), P.O. Box 230, 6700 AE Wageningen, The Netherlands and the Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA, UK
2To whom correspondence should be addressed. E-mail: firstname.lastname@example.org
The nature of quercetin conjugates present in blood after consumption of quercetin glucosides is still unclear. In this study, we analyzed plasma of volunteers that had consumed 325 µmol of either quercetin-3-glucoside or quercetin-4'-glucoside as an oral solution. Quercetin metabolites were extracted with acetonitrile/phosphoric acid and these extracts were analyzed using a high performance liquid chromatography with Coularray detection that distinguishes between the glucuronidated and the glucosylated forms of quercetin. No intact quercetin glucosides and only trace amounts of aglycone were found in human plasma, irrespective of the glucoside ingested. This was confirmed by spiking the plasma with glucoside standards. The major components in plasma had the same retention time as quercetin glucuronide standards. These plasma components disappeared after treatment of the plasma with bovine liver ß-glucuronidase, under reformation of quercetin, and showed the same oxidation pattern as the glucuronides. These results suggest that after consumption of quercetin glucosides, quercetin glucuronides are major metabolites in plasma.
KEY WORDS: • flavonoids • quercetin glucuronides • intestinal absorption • human • bioavailability