Quercetin is a water-soluble plant pigment called a flavonoid typically found in red wine, green tea, onions, apples, and leafy vegetables. In the body, quercetin is a potent antioxidant and anti-inflammatory activity, protecting cellular structures and blood vessels from the damaging effects of free radicals. It improves blood vessel strength (capillary fragility). Quercetin inhibits the activity of an enzyme (catechol-O-methyltransferase) that breaks down the neurotransmitter norepinephrine. This effect may lead to elevated levels of norepinephrine and an increase in energy expenditure (thermogenesis) and fat oxidation. It also means quercetin acts as an antihistamine leading to relief of allergies and asthma. As an antioxidant, it reduces LDL cholesterol and offers protection from heart disease. Quercetin blocks an enzyme that leads to accumulation of sorbitol, which has been linked to nerve, eye, and kidney damage in diabetics. It may prevent cataract formation. Quercetin is considered a phytoestrogen.
Dietary Sources: Quercetin is found in apples, green tea, black tea, and onions, with smaller amounts in green vegetables and beans.
Dosage: Although there are no widely recognized standard dosage recommendations for quercetin, intakes of as high as 100-1000mg per day, consumed in 2-3 divided doses have been used with no apparent adverse side effects.
Safety: No adverse side effects have been identified relating to typical doses of quercetin in dietary supplements.
The scientific literature on flavonoids, including quercetin, shows the following areas of research.
1. Chronic treatment with flavonoids, including quercetin, reverses cognitive deficits in aged and LPS-intoxicated mice.
2. Quercetin successfully prevented the premature senescent phenotype and up-regulation induced by hydrogen peroxide.
3. Dietary antioxidants, including quercetin, could play a significant role in the reduction of inflammatory responses.
4. Quercetin in particular, paired with ascorbic acid, may be of therapeutic benefit in protecting neurovasculature structures in skin from oxidative damage.
5. In human platelet-rich plasma, quercetin prevented the secondary aggregation and blocked ATP release from platelets induced by epinephrine or ADP.
6. Flavonoids inhibit platelet function by blunting hydrogen peroxide production and, in turn, phospholipase C activation.
7. Flavonoids in regularly consumed foods may reduce the risk of death from coronary heart disease in elderly men.
8. The habitual intake of flavonoids and their major source (tea) may protect against stroke.
9. Quercetin shows potential in the prophylaxis of cardiovascular diseases.
10. Quercetin reduces the elevated blood pressure, the cardiac and renal hypertrophy and the functional vascular changes in rats.
11. Oral low dose quercetin is cardioprotective, possibly via a mechanism involving protection of mitochondrial function.
12. Two flavonoids, quercetin and silybin, characterized as free radical scavengers, exert a protective effect preventing the decrease in the dehydrogenase/oxidase ratio observed during ischemia-reperfusion.
13. Quercetin shows therapeutic potential in CsA-induced nephrotoxicity.
14. Dietary quercetin and metabolites are active in inhibiting oxidative damage in the lens and thus could play a role in prevention of cataract formation.
15. Quercetin provided its strong antioxidant property of protecting cells against H2O2-induced oxidative stress and calcium dysregulation.
16. Daily administration of quercetin for 5 days markedly prevented the cardiac hypertrophy in AAC mice.
17. Citrus flavonoids are effective inhibitors of human breast cancer cell proliferation in vitro, especially when paired with quercetin, which is widely distributed in other foods.
18. Quercetin glycosides are capable of inhibiting lipoxygenase-induced LDL oxidation more efficiently than ascorbic acid and alpha-tocopherol.
Quercetin Abstracts (30)