Quercetin
Quercetin /ˈkwɜrsɨtɨn/, a flavonol, is a flavonoid, in other words, a plant pigment with a molecular structure like or derived from flavone.[2] It is found in fruits, vegetables, leaves and grains. It can be used as an ingredient in supplements, beverages, or foods.
Quercetin itself (aglycone quercetin), as opposed to quercetin glycosides, is not a normal dietary component. In a bioavailability study in rats, radiolabelled quercetin-4'-glucoside was converted to phenolic acids as it passed through the gastrointestinal tract, producing compounds not monitored in previous animal studies of aglycone quercetin.[20] All but 4% was recovered within 72 hours (69% in urine), indicating low retention and high excretion, a characteristic of ingested polyphenols. Quercetin may also induce insulin secretion by activation of L-type calcium channels in the pancreatic β-cells.[21] Quercetin has not been confirmed scientifically as a specific therapeutic for any condition nor approved by any regulatory agency. The U.S. Food and Drug Administration has not approved any health claims for quercetin.[22]
Preliminary research[edit source | editbeta]
Antiviral[edit source | editbeta]
Hyperoside (which is the 3-O-galactoside of quercetin) is a strong inhibitor of HBsAg and HBeAg secretion in 2.2.15 cells.[23]
Quercitrin and myricetin 3-O-beta-D-galactopyranoside inhibit HIV-1 reverse transcriptase, all with IC50 values of 60 μM.[24]
Asthma[edit source | editbeta]
Quercetin is an effective bronchodilator and helps reduce the release of histamine and other allergic or inflammatory chemicals in the body.[25]
Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation.[26]
Cancer[edit source | editbeta]
The American Cancer Society says while quercetin "has been promoted as being effective against a wide variety of diseases, including cancer," and "some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans." In the amounts consumed in a healthy diet, quercetin "is unlikely to cause any major problems or benefits."[27]
In laboratory studies of cells in vitro, quercetin produces changes that are also produced by compounds that cause cancer (carcinogens), but these studies do not report increased cancer in animals or humans.[28][29][30]
From laboratory studies it is conjectured that quercetin may affect certain mechanisms of cancer.[31][32] An 8-year study found the presence of three flavonols — kaempferol, quercetin, and myricetin — in a normal diet was associated with 23% reduced risk of pancreatic cancer, a rare but frequently fatal disease, in tobacco smokers.[33] There was no benefit in subjects who had never smoked or had quit smoking.
In vitro, cultured skin and prostate cancer cells were suppressed (compared to nonmalignant cells) when treated with a combination of quercetin and ultrasound.[34]
In laboratory culture studies, quercetin increased the sensitivity of resistant colorectal tumors with microsatellite instability to the chemotherapy drug, 5-fluorouracil.[35]
Eczema[edit source | editbeta]
Serum IgE levels are highly elevated in eczema patients, and virtually all eczema patients are positive for allergy testing. Excessive histamine release can be minimized by the use of antioxidants. Quercetin is frequently used therapeutically in allergic conditions, including asthma and hayfever, eczema, and hives.[36]
Inflammation[edit source | editbeta]
Several laboratory studies show quercetin may have anti-inflammatory properties,[37][38] and it is being investigated for a wide range of potential health benefits.[38][39]
Quercetin has been reported to be of use in alleviating symptoms of pollinosis.[40] An enzymatically modified derivative was found to alleviate ocular but not nasal symptoms of pollinosis.[41][42]<[43]
Studies done in test tubes have shown quercetin may prevent immune cells from releasing histamines which might influence symptoms of allergies.[44][45]
A study with rats showed that quercetin effectively reduced immediate-release niacin (vitamin B3) flush, in part by means of reducing prostaglandin D2 production.[46] A pilot clinical study of four humans gave preliminary data supporting this.[47]
Quercetin may have properties of a calcineurin inhibitor, similar to cyclosporin A and tacrolimus, according to one laboratory study.[48]
Fibromyalgia[edit source | editbeta]
Quercetin may be effective in the treatment of fibromyalgia because of its potential anti-inflammatory or mast cell inhibitory properties shown in laboratory studies.[49]
Metabolic syndrome[edit source | editbeta]
Quercetin has been shown to increase energy expenditure in rats, but only for short periods (fewer than 8 weeks).[37] Effects of quercetin on exercise tolerance in mice have been associated with increased mitochondrial biogenesis.[38] In mice, an oral quercetin dose of 12.5 to 25 mg/kg increased gene expression of mitochondrial biomarkers and improved exercise endurance.[38]
It has also been claimed that quercetin reduces blood pressure in hypertensive[50] and obese subjects in whom LDL cholesterol levels were also reduced.[51]
In vitro studies showed quercetin and resveratrol combined inhibited production of fat cells[52] and vascular smooth muscle cell proliferation.[53]
Supplements of quercetin with vitamin C and niacin does not cause any significant difference in body mass or composition[54] and has no significant effect on inflammatory markers, diagnostic blood chemistries, blood pressure, and blood lipid profiles.[55]
Quercetin has also been shown to have the ability to selectively inhibit MAO-A which may explain some of its pharmacological properties