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 growntomatoes had 79% more quercetin than chemically grown fruit. Quercetin is present in various kinds of honey from different plant sources.
Following dietary ingestion, quercetin undergoes rapid and extensive metabolism that makes the biological effects presumed from in vitro studies unlikely to apply in vivo.
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 European Food Safety Authority evaluated possible health claims associated with consumption of quercetin, finding that no cause-and-effect relationship has been established for any physiological effect.
Although quercetin is under basic and early-stage clinical research for a variety of disease conditions, there exists no sufficient evidence that it has any beneficial effect in the human body. The US FDA has issued warning letters, e.g., to emphasize that quercetin is not a defined nutrient, cannot be assigned a dietary content level and is not regulated as a drug to treat any human disease.
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.
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