In red onions, higher concentrations of quercetin occur in the outermost rings and in the part closest to the root, the latter being the part of the plant with the highest concentration. One study found that organically grown tomatoes had 79% more quercetin than chemically grown fruit. Quercetin is present in various kinds of honey from different plant sources.
Effects of consumption by humans and other animals
Quercetin itself (aglycone quercetin), as opposed to quercetin glycosides, is not a normal dietary component. In a bioavailability study in rats, radiolabelled quercetin-4'-glucoside was converted to phenolic acids as it passed through the gastrointestinal tract, producing compounds not monitored in previous animal studies of aglycone quercetin. All but 4% was recovered within 72 hours (69% in urine), indicating low retention and high excretion, a characteristic of ingested polyphenols. Quercetin may also induce insulin secretion by activation of L-type calcium channels in the pancreatic β-cells. Quercetin has not been confirmed scientifically as a specific therapeutic for any condition nor approved by any regulatory agency. The U.S. Food and Drug Administration has not approved any health claims for quercetin.
Hyperoside (which is the 3-O-galactoside of quercetin) is a strong inhibitor of HBsAg and HBeAg secretion in 2.2.15 cells.
It has been shown that quercetin has greater inhibitory effect on HCV replication than other flavonoids. 
Quercetin is an effective bronchodilator and helps reduce the release of histamine and other allergic or inflammatory chemicals in the body.
Quercetin has demonstrated significant anti-inflammatory activity in mouse models of asthma because of direct inhibition of several initial processes of inflammation.
Laboratory studies have investigated quercetin's potential for use in anti-cancer applications. The American Cancer Society says 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."
Serum IgE levels are highly elevated in eczema patients, and virtually all eczema patients are positive for allergy testing. Excessive histamine release can be minimized by the use of antioxidants. Quercetin has been shown to be effective in reducing IgE levels in rodent models.
Several laboratory studies show quercetin may have anti-inflammatory properties, and it is being investigated for a wide range of potential health benefits.
Quercetin has been reported to be of use in alleviating symptoms of pollinosis. An enzymatically modified derivative was found to alleviate ocular but not nasal symptoms of pollinosis.
Studies done in test tubes have shown quercetin may prevent immune cells from releasing histamines which might influence symptoms of allergies.
A study with rats showed that quercetin effectively reduced immediate-release niacin (vitamin B3) flush, in part by means of reducing prostaglandin D2 production. A pilot clinical study of four humans gave preliminary data supporting this.
Quercetin may have properties of a calcineurin inhibitor, similar to cyclosporin A and tacrolimus, according to one laboratory study.
Quercetin may be effective in the treatment of fibromyalgia because of its potential anti-inflammatory or mast cell inhibitory properties shown in laboratory studies.
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. In mice, an oral quercetin dose of 12.5 to 25 mg/kg increased gene expression of mitochondrialbiomarkers and improved exercise endurance. Quercetin was also demonstrated to act as agonist of PPARgamma, nuclear receptor that is current pharmacological target for the treatment of metabolic disorders such as diabetes type 2.
It has also been claimed that quercetin reduces blood pressure in hypertensive and obese subjects in whom LDL cholesterol levels were also reduced.
In vitro studies showed quercetin and resveratrol combined inhibited production of fat cells and vascular smooth muscle cell proliferation.
Supplements of quercetin with vitamin C and niacin does not cause any significant difference in body mass or composition and has no significant effect on inflammatory markers, diagnostic blood chemistries, blood pressure, and blood lipid profiles.
Quercetin is contraindicated with some antibiotics; it may interact with fluoroquinolones (an antibiotic), as quercetin competitively binds to bacterial DNA gyrase. Whether this inhibits or enhances the effect of fluoroquinolones is not certain.
AHFS Drug Information (2010) identifies quercetin as an inhibitor of CYP2C8, and specifically names it as a drug with potential to have harmful interactions with taxol/paclitaxel. As paclitaxel is metabolized primarily by CYP2C8, its bioavailability may be increased unpredictably, potentially leading to harmful side-effects.
Quercetin is described as an inhibitor of CYP2C9. Quercetin is an inhibitor and inducer of CYP3A4 (in other words, it reduces the enzyme's activity in the short term, but the body responds by producing more of it). CYP2C9 and CYP3A4 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, therefore, effects of drugs metabolized by these enzymes.
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