Antioxidantes y quercetina

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Antioxidants and Cancer III: Quercetin
Davis W. Lamson, MS, ND, and Matthew S. Brignall, ND
Abstract Quercetin is a flavonoid molecule ubiquitous in nature. A number of its actions make it a potential anti-cancer agent, including cell cycle regulation, interaction with type II estrogen binding sites, and tyrosine kinase inhibition. Quercetin appears to be associated with little toxicity whenadministered orally or intravenously. Much in vitro and some preliminary animal and human data indicate quercetin inhibits tumor growth. More research is needed to elucidate the absorption of oral doses and the magnitude of the anti-cancer effect. (Altern Med Rev 2000;5(3):196-208)

Quercetin (3,3’,4’,5,7-pentahydroxyflavone; Figure 1, R= OH) belongs to an extensive class ofpolyphenolic flavonoid compounds almost ubiquitous in plants and plant food sources. Frequently quercetin occurs as glycosides (sugar derivatives); e.g., rutin (Figure 1) in which the hydrogen of the R-4 hydroxyl group is replaced by a disaccharide. Quercetin is termed the aglycone, or sugarless form of rutin. Two extensive volumes, the proceedings of major meetings on plant flavonoids, presented much ofthe biological and medical data about quercetin in 1985 and 1987.1,2 Quercetin is the major bioflavonoid in the human diet. The estimated average daily dietary intake of quercetin by an individual in the United States is 25 mg.3 Its reputation as an antioxidant stems from the reactivity of phenolic compounds with free radical species to form phenoxy radicals which are considerably less reactive.Additionally, one can envision a polyphenolic compound easily oxidizable to a quinoid form (similar to vitamin K) and participating in the redox chemistry of nature. In recent years, research about quercetin has ranged from considering it potentially carcinogenic to examination of its promise as an anti-cancer agent. Four pressing questions arise. Is additional dietary supplementation safe? Isquercetin absorbed and bioavailable when given orally? Is it active against malignant human cells and could its use be developed? Are additional routes such as intravenous or transdermal safe or more advantageous? It is the object of this review to present evidence about these concerns and outline gaps in the available data which need to be filled in order to determine whether quercetin has anappreciable role in future cancer therapy.
Davis W. Lamson, MS, ND Private practice, Tahoma Clinic- Kent, WA. Coordinator of Oncology, Bastyr University- Kenmore, WA. Correspondence Address: 9803 17th Ave. NE, Seattle, WA. 98115. email: Matthew S. Brignall, ND 1999 graduate, Bastyr University. Currently doing a research fellowship at Tahoma Clinic in Kent, WA.

Page 196 Alternative Medicine Review ◆ Volume 5 Number 3 ◆ 2000 Copyright©2000 Thorne Research, Inc. All Rights Reserved. No Reprint Without Written Permission

Antioxidants & Cancer: Quercetin

Figure 1. Structure of Naturally Occurring Flavonoids Showing Numbering of Ring Atoms
R1 HO O R2 R4 R3 O

In quercetin R1, R2, and R3 are all OH: rutin is a glycoside of quercetin inwhich R4 is the disaccharide, rutinose (β-1-L-rhamnosido-6-o-glucose). From Rahman, A., et al., Carcinogenesis 1989;10:1833-1839.

Absorption of Quercetin
Most animal and human trials of oral dosages of quercetin aglycone show absorption in the vicinity of 20 percent. An early trial in rabbits showed 25 percent of a 2-2.5 g oral dose was accountable for in the urine.4 In light of more recentfindings of urinary excretion, this is a questionable result.5 Rats eating a diet supplemented with 0.2-percent quercetin for three weeks attained a serum concentration of 133 microM, mainly in sulfated and glucuronidated forms.6 Humans fed fried onions containing quercetin glucosides equivalent to 64 mg of the aglycone form reached a maximum serum concentration of 196 ng/ml (0.6 microM) 2.9 hours...