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 which were not met by the manufacturers.
In preliminary human studies, oral intake of quercetin in doses up to one gram per day over three months did not cause adverse effects. The safety of using quercetin in dietary supplements during pregnancy and lactation has not been established.
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^ abJanice L King (2 March 2017). "Warning Letter to Cape Fear Naturals". Inspections, Compliance, Enforcement, and Criminal Investigations, US Food and Drug Administration. Retrieved 29 November 2018.
^ abRonald Pace (17 April 2017). "Warning Letter to DoctorVicks.com". Inspections, Compliance, Enforcement, and Criminal Investigations, US Food and Drug Administration. Retrieved 29 November 2018.