Quercetin is a phytochemical that is naturally found in many foods, such as red wine, green tea, onions, apples and leafy vegetables. Most of what we know about quercetin has been derived from preclinical studies, as not many studies on quercetin as a monotherapy have been conducted. Quercetin exhibits strong antioxidant, antiinflammatory, and possibly some antihistamine and antitumor activity. Quercetin also inhibits the activity of an enzyme breaks down norepinephine. These activities are theorized to be at least partially responsible for some of the healthy benefits of foods such as red wine and green tea. Quercetin is often included in dietary supplement formulations for its antioxidant, cellular protective and cardiovascular effects. As quercetin exhibits antioxidant benefits, it is believed to also be beneficial for prevention of oxidative stress and macular degeneration.
Quercetin is rarely used as a monotherapy, but as a component of our foods, and possibly a component of dietary supplement formulations, is thought to promote a strong antioxidant effect. We still know little about quercetin’s clinical effectiveness or even its pharmacokinetics, but there are promising preclinical and early clinical studies which indicate that it is one of the synergistic components of our foods that bring multiple health benefits (Pignatelli et al., 2000).
The effect of quercetin was studied in two clinical studies in combination with other substances (sodium nucleinate in one study, and tocopherol acetate in the other) for treatment of Flexner’s Dysentary. In combination with sodium nucleinate it was found to reduce symptoms of dyspepsia, promote the arrest of bacterial isolation, and to normalize immune status. In combination with tocopherol acetate, it promoted healthy clinical indices and immune status (Iushchuk et al., 1991; Frolov et al., 1993).
Another clinical study investigated the effect of two of the flavonoids from red wine, quercetin (10-20 micromol/l) and catechin (50-100 micromol/l) on platelet aggregation. The study was designed to test whether or not flavonoids had a synergistic effect through their antioxidant function in producing collagen-induced platelet aggregation, which in turn stimulates the phospholipase C pathway. Neither substances on their own have been found to effect platelet function. The study found that the combination of flavonoids caused a blunting of hydrogen peroxide production, that in turn inhibited platelet function and activated phospholipase C. The authors suggested that the results of this study indicate a possible explanation in the decreased risk of cardiovascular disease that comes with moderate red wine consumption (Pignatelli et al., 2000).
Inflammatory Diseases of the Parotid Glands
Timofeev et al. (1990) found high effectiveness in the quercetin treatment of inflammatory diseases of the parotid glands. The study involved 26 patients that were administered a 0.1% quercetin solution, and the effectiveness of treatment was assessed through clinical examination and cytologic study of the saliva.
Graefe et al. (1999) performed two clinical studies to determine the pharmacokinetics of quercetin. The elimination half-life of quercetin was found to be 2.4 h and 0.7 h, the volume of distribution at steady-state was 92.61 and 6.21, and total body clearance was 34.6/lxh and 28.1/lxh. The absorption of quercetin was found to be from 0 to 50% of the dose. The authors admitted the data so far was inconsistent and inconclusive, and it could be attributed to highly sensitive assay methodology.
Ferry et al. (1996) performed a Phase I dosing clinical trial with quercetin. The recommendation for Phase II clinical trial was a 1400 mg/m2 bolus dose in three week or one week intervals. The authors concluded the quercetin to be safe at those levels, and that there was lyphocyte tyrosine kinase activity and evidence of antitumor activity observed in the study.
Safety / Dosage
No widely accepted dosage has been determined for quercetin. As an antioxidant, vasodilator and blood thinner, quercetin has been used with intakes ranging from 100-1000 mg (in divided doses) with no apparent side effects, but it is though to be active at the lower levels of usage. As it has been shown to have vasodilatory and blood thinning properites, quercetin is contraindicated for those with risk of bleeding.
1.Ferry DR, Smith A, Malkhandi J, Fyfe DW, deTakats PG, Anderson D, Baker J, Kerr DJ. Phase I clinical trial of the flavonoid quercetin: pharmacokinetics and evidence for in vivo tyrosine kinase inhibition. Clin Cancer Res. 1996 Apr;2(4):659-68.
2.Frolov VM, Peresadin NA, Khomutianskaia NI, Pshenichnyi IIa. The efficacy of quercetin and tocopherol acetate in treating patients with Flexner's dysentery. Lik Sprava. 1993 Apr;(4):84-6.
3.Graefe EU, Derendorf H, Veit M. Pharmacokinetics and bioavailability of the flavonol quercetin in humans. Int J Clin Pharmacol Ther. 1999 May;37(5):219-33.
4.Iushchuk ND, Frolov VM, Peresadin NA. The treatment of protracted forms of Flexner dysentery. Ter Arkh. 1991;63(11):19-22.
5.Pignatelli P, Pulcinelli FM, Celestini A, Lenti L, Ghiselli A, Gazzaniga PP, Violi F. The flavonoids quercetin and catechin synergistically inhibit platelet function by antagonizing the intracellular production of hydrogen peroxide. Am J Clin Nutr. 2000 Nov;72(5):1150-5.
6.Timofeev AA, Maksiutina NP, Topchii DV, Voitenko GN, Balanda PP. The use of a solution of quercetin for the treatment of inflammatory diseases of the parotid glands. Klin Khir. 1990;(12):20-2.
EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.