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 non-organically grown fruit. Quercetin is present in various kinds of honey from different plant sources.
The bioavailability of quercetin in humans is low and highly variable (0–50%), and it is rapidly cleared with an elimination half-life of 1–2 hours after ingesting quercetin foods or supplements. Following dietary ingestion, quercetin undergoes rapid and extensive metabolism that makes the biological effects presumed from in vitro studies unlikely to apply in vivo.
Quercetin has been studied in basic research and small clinical trials. While supplements have been promoted for the treatment of cancer and various other diseases, there is no high-quality evidence that quercetin (via supplements or in food) is useful to treat cancer or any other disease.
The US Food and Drug Administration has issued warning letters to several manufacturers advertising on their product labels and websites that quercetin product(s) can be used to treat diseases. The FDA regards such quercetin advertising and products as unapproved – with unauthorized health claims concerning the anti-disease products – as defined by "sections 201(g)(1)(B) and/or 201 (g)(1)(C) of the Act [21 U.S.C. § 321(g)(1)(B) and/or 21 U.S.C. § 321(g)(1)(C)] because they are intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease", conditions not met by the manufacturers.
There has been little research into the safety of quercetin supplementation in humans, and the results are insufficient to give confidence that the practice is safe. In particular, there is a lack of safety information on the effect of quercetin supplementation for pregnant women, breastfeeding women, children, and adolescents. The hormonal effects of quercetin found in animal studies raise the suspicion of a parallel effect in humans, particularly in respect of estrogen-dependent tumors.
Quercetin supplementation can interfere with the effects of medications. The precise nature of this interaction is known for some common medicines, but for many, it is not.
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