Quercetin is a major flavonol, one of the almost 4,000 flavonoids (antioxidants) that occur in foods of plant origin, such as red wine, onions, green tea, apples, berries, and Brassica vegetables (cabbage, broccoli, cauliflower, turnips). Quercetin is also found in Gingko biloba, St. John's wort, and American elder.
Quercetin and rutin (another flavonol) are used in many countries as vasoprotectants (protects blood vessels) and are ingredients of numerous multivitamin preparations and herbal remedies. They occur mainly as glycosides, which means they are linked with various sugars. However, the ability of the body to absorb these compounds is questionable.
Quercetin and other flavonols have a wide variety of biological effects, but the scientific evidence for use in the prevention or treatment of disease is weak. Quercetin has been considered as a therapy for cardiovascular diseases, high cholesterol, diabetic cataracts, inflammation, ischemic injury, chronic prostatitis, chronic venous insufficiency, gastrointestinal ulceration, hepatitis, allergies, asthma, viral infections, and hay fever.
Review of the literature shows that there have been several studies on the association of quercetin and with the risk reduction for coronary heart disease and stroke, high blood pressure, cancers, and a few studies on other medical conditions. However, there is a lack of strong evidence to support any of these conditions.
Allium cepa, American elder, apples, Artemisia abrotanum L, AS195 Folia vitis viniferae (red vine leaf extract), biflavonoids, bilberries, black currants, black tea, brassica vegetables (cabbage, broccoli, cauliflower, turnips, kale), buckwheat tea, citrus bioflavonoid, endive, flavones, flavonoids, flavonols, flavon(ol)-glycosides, Ginkgo biloba extract, ginkgo flavone glycosides, grapefruit, green tea, Hypericum perforatum L., isoflavones, isorhamnetin, isoquercitrin, kaemferol, meletin, Myrtaceae, naphthodianthrones, onion, parsley (Petroselinum crispum (Mill.) Nym.), phytodrug (QG-5), phytoestrogens, pine bark extract, polyphenol, Psidium guajava L. (Fam.), quercetin aglycone, quercetin chalcone, quercetin dimethyl-ethers, quercetin glucoside, quercetin glucuronides, quercetin rutin, quercetin rutinoside, red vine leaf extract, red wine, red wine phenolics, rhamnose molecule, rutin, Sambucas nigra L., sophretin, St. John's wort, STW-3, Tycho, Venenkapseln, Venoruton (O-(beta-hydroxyethyl) rutosides (HR).
These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.
Several of the effects of flavonoids that have been observed in laboratory and animal studies suggest that they might be effective in reducing cardiovascular disease risk. Studies in humans using polyphenolic compounds from red grapes showed improvement in endothelial function in patients with coronary heart disease. Antioxidant and cholesterol-lowering effects are proposed.
Population-based studies report that quercetin is associated with a reduced risk of coronary heart disease and stroke. Quercetin supplementation reduced blood pressure in hypertensive rodents and hypertensive humans.
Immune function (after intense exercise)
Quercetin does not appear to affect changes in the immune system caused by intense exercise. However, it reduced the number of respiratory tract infections in people who participated in intense cycling. More research is needed.
Pancreatic cancer prevention
Some research suggests that quercetin may help prevent pancreatic cancer in smokers. However, quercetin did not have this effect in non-smokers or former smokers. More research is needed to determine if quercetin is beneficial.
Prostatitis/chronic pelvic pain syndrome
There is some evidence that quercetin may be useful for the treatment of chronic prostatitis and chronic pelvic pain syndrome. Further research is needed to confirm these results.