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 reported to inhibit the oxidation of other molecules and hence is classified as an antioxidant. Quercetin contains a polyphenolic chemical substructure that stops oxidation by acting as a scavenger of free radicals that are responsible for oxidative chain reactions.
Quercetin has been studied in basic research and small clinical trials. While quercetin supplements have been promoted for the treatment of cancer and various other diseases, there is no evidence that quercetin (via supplements or in food) is useful to treat cancer or any disease. The US Food and Drug Administration has issued warning letters to several manufacturers advertising unauthorized health claims for treating diseases on their product labels and websites for quercetin product(s) deemed by the FDA as unapproved "drugs under 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 and/or articles intended to affect the structure or any function of the human body."
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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