Like many other substances in natural medicine, quercetin is a type of plant-based chemical or phytochemical. In fact, quercetin is a common chemical pigment found in the rinds and barks of a wide variety of plants. Quercetin, a flavonol or potent antioxidant, is found in red wine, onions, green tea, apples, berries and cruciferous vegetables.1
Its value in treating and preventing many illnesses has been recognized for decades and has made it a very popular component in the herbal and supplement marketplace.
Background
Quercetin, derived from the Latin term quercetum, meaning “oak forest,” was named in 1857.2 Since then, the substance has been isolated and studied as an adjunctive therapy for many conditions that are known to be either caused or exacerbated by oxidative damage.
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Science
As an antioxidant, quercetin is thought to stabilize cell membrane permeability and reduce the transport of oxidative agents such as prostaglandins and other cytokines.3 Quercetin also has vascular effects, such as the inhibition of platelet aggregation, and it promotes endothelial function by allowing rapid transport of nitric oxide and subsequent vasodilation.3
Other benefits that come from membrane stabilization include antihistaminic effects due to mast cell stabilization.3 Studies evaluating the potential value of quercetin on cancer cells due to immunomodulatory up-regulation and inhibition of cellular proliferation are also under way.4
As an antihistamine, quercetin has been studied for its action in subjects with atopic asthma or reactive airway disease. In one study, researchers developed various extracts of quercetin and applied them to cultured cell lines of human airway smooth muscle that had been stimulated by antigenic compounds.5 Since human muscle that is asthmatic for long periods of time eventually undergoes cellular hypertrophic proliferation, the quercetin extract was used to determine its inhibitory effect on that process. The extract was found to totally abolish the proliferation of smooth muscle cells in the test culture.5
Quercetin also has an effect on allergic rhinitis. Researchers have determined the histamine-blocking effect of quercetin compared to an active control using cromolyn (Nasalcrom).6 Data showed a mast-cell stabilization blocking histamine release that was 46% lower in untreated cells, compared with 58% stabilization in cells treated with cromolyn.6
In cardiovascular diseases, quercetin has multiple potential benefits. Scientists have noted that populations whose diets were particularly high in flavonol-rich foods have a lower incidence of atherosclerosis and cardiac diseases.7
A group of British scientists studied the use of a quercetin extract in six healthy human subjects.8 The participants’ blood was sampled at designated time frames after ingestion of the extract. The samples were monitored to determine the level of platelet-aggregating activity. Platelet aggregation was virtually eliminated at all testing times and serum concentrations of quercetin.8
In a larger randomized, placebo-controlled trial, 42 patients with prehypertension or stage 1 hypertension were given either placebo or a quercetin supplement for 28 days.9 Subjects with prehypertension showed no change after treatment in either arm of the trial, but those with stage 1 hypertension showed a nearly 10-mm Hg drop in systolic pressure and a 5-mm Hg drop in diastolic readings.9
Safety, interactions
Quercetin has not been studied in pregnant or lactating women, or in infants and children. Consequently, patients in those groups should avoid use of quercetin unless approved by their healthcare provider. The most common side effect of quercetin is headache and tingling of the arms and legs. Isolated cases of kidney damage have been reported after prolonged, high-dose usage.10 Concomitant use with fluoroquinolones may reduce the effectiveness of the antibiotic.10
Dosage and cost
Recommended daily oral intake of quercetin as a supplement varies, but the average dose is 500 mg twice daily. At this dosage, a month’s supply costs about $20. Quercetin can be found in powder-filled capsules, liquids or teas.10
Summary
Quercetin is a potent, well-tolerated antioxidant that is a safe supplement for persons who want to prevent illness or improve chronic disease management. Quercetin is recommended as adjunctive alternative medication in most patient populations.
References
1. ACS cancer treatment information page. American Cancer Society website. Available at www.cancer.org /Treatment.
2. Boots A, Haenen G, Bast A. Health effects of quercetin: from antioxidant to nutraceutical. Eur J Pharmacol. 2008;585:325-337.
3. Formica JV, Regelson W. Review of the biology of Quercetin and related bioflavonoids. Food Chem Toxicol. 1995;33:1061-1080.
4. Bobe G, Weinstein SJ, Albanes D, et al. Flavonoid intake and risk of pancreatic cancer in male smokers. Cancer Epidemiol Biomarkers Prev. 2008;17:553-562. Available at cebp.aacrjournals.org/content/17/3/553.full.
5. Chaabi M, Freund-Michel V, Frossard N, et al. Anti-proliferative effect of Euphorbia stenoclada in human airway smooth muscle cells in culture. Journal Ethnopharmacol. 2007;109:134-139.
6. Otsuka H, Inaba M, Fujikura T, Kunitomo M. Histochemical and functional characteristics of metachromatic cells in the nasal epithelium in allergic rhinitis: studies of nasal scrapings and their dispersed cells. J Allergy Clin Immunol. 1995;96:528-536. Available at www.jacionline.org/article/S0091-6749%2895%2970297-0 /fulltext.
7. Bischoff SC. Quercetin: potentials in the prevention and therapy of disease. Curr Opin Clin Nutr Metab Care. 2008;11:733-740.
8. Hubbard GP, Wolfram S, Lovegrove JA, Gibbins JM. Ingestion of quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in humans. J Thromb Haemost. 2004;2:2138-2145. Available at onlinelibrary.wiley.com /doi/10.1111/j.1538-7836.2004.01067.x/full.
9. Edwards R, Lyon T, Litwin SE, et al. Quercetin reduces blood pressure in hypertensive subjects. J Nutr. 2007;137:2405-2411. Available at jn.nutrition.org /content/137/11/2405.full.
10. UMMC complementary medicine page. University of Maryland Medical Center website. Available at www.umm .edu/altmed/articles/quercetin-000322.htm.
All electronic documents accessed on October 15, 2011.
