What is Quercetin?

Quercetin, a flavonol, is widely distributed plant derived flavonoid and it can be naturally found in fruits, vegetables, leaves and grains. Nowadays, it also constitutes as an ingredient in food supplements, beverages and foods. Its name was derived from quercetum meaning oak forest after Quercus. It is also known by the names Quercitin, Quercetine, Sophoretin, Meletin, flavin meletin, quertin, Xanthaurine and Quercetol.

Quercetin is a yellow crystalline powder with a melting point of 316 degrees and having its solubility in water. Its IUPAC name is (3, 3’, 4’, 5, 7-pentahydroxyflavone) or (3’, 4’, 5, 7-tetrahydroxyflavonol). It is a compound known to exist as the chemical backbone for the other flavonoids such as rutin, hesperidin, naringin and tangeritin.

Quercetin is produced in plant from phenylalanine and malonyl-CoA through numerous enzymatic processes. In plants, it is mainly present in the conjugated forms with sugars forming aglycone-glycone (sugar) complexes. It is modified in plants by the processes like glycosylation, methylation, acetylation and prenylation.

Absorption: – Currently, two proposed hypotheses of the absorption mechanisms are accepted. First being the active uptake by Na+ dependent glucose transporter and second is the absorption of quercetin by passive diffusion. Studies show that humans can absorb 52% quercetin glycosides, 24% pure quercetin and 17% quercetin rutinoside. 

Metabolism: – Numerous processes of quercetin metabolism in liver (absorbed quercetin) and in the gut of intestine by microorganisms (unabsorbed form) yield different quercetin metabolites with more or less purported beneficial effects.

Health Benefits of Quercetin

Anti-oxidant activity: – This activity is regarded as the most important effect of the quercetins. After being metabloised in the liver, it is found in the plasma as quercetin glucuronide forms and other metabolites which have various hydroxyl groups on them. These groups donate electrons and quench the free oxygen radical species (superoxides, hydroxyl radicals etc) and thus contribute to the anti-oxidant effect.

Foods you can inculde in your diet – apples, onions, tea and wine.

Anti-inflammatory action: – Uncontrolled inflammatory response may contribute to chronic diseases like arthritis, allergy, Atherosclerosis, cancer and aging. Therefore it becomes necessary to keep a check on the inflammatory response. Quercetin has been found to possibly block the production and release of various inflammatory agents such as eicosanoids including leukotrienes, prostaglandins and cytokines which are majorly involved in the allergic inflammation. During acute and chronic inflammation, it also attenuates various pathophysiological conditions and limits the tissue damage associated with inflammation thereby exhibiting anti-inflammatory activity. 

Foods you can inculde- onions, garlic, tea and wine.

Anti-asthmatic agent: – Asthama, a type of inflammatory disease during which basophills, mast cells and other allergens are activated resulting in the release of inflammatory agents like cytokines causing inflammation. Higher Quercetin intake was reported to attenuate the synthesis and release of cytokines, thus inhibting the inflammation response thereby reducing the incidence of asthma.
Foods you should inculde –onions, garlic. Onions were widely used in the folk medicine to treat allergies and asthma.

Anti-cancer effects: – In vitro studies have revealed that quercetin inhibits the growth of breast carcinogens, colorectal cancer cells, squamous cell, acute lymphoid, pancreatic tumor cells, myeloid leukemia cells, ovulatory cells, endometrial, non-small-cell lung, prostate cancer and lymphoblastoid cell lines.Quercetin

These effects are believed to be mediated by processes like cell cycle regulation, binding to cellular type-II oestrogen binding sites, inhibition of tyrosine protein kinase activity, influence on phosphoinositide phosphorylation, inducing programmed cell death (apoptosis), inhibition of overproduction of tumor necrosis factor (TNF)-alpha and tumor angiogenesis. 

Quercetin reportedly induces cell death through the inhibiton of EGFR tyrosine kinase activity and modulates protein phosphorylation in case of pancreatic cancer cells. Laborartories studies found that people who consumed quercetin in their normal diet showed a reduction in pancreatic cancer risk by 23%. In prostate cancer as well, it exhibits the growth inhibitory activity by decreasing the phosphorylation of protein. In MCF-7 mammary cancer cells, Quercetin was shown to exhibit growth inhibition and apoptosis by inhibiting cell-cycle related kinases and other regulators which justifies the anti-cancer effect. While in other studies, Quercetin was reported to detoxify compounds which possessed carcinogenic properties, drugs, pollutants etc which were done so by an increase in the intestinal enzyme activities, thereby preventing gastro-intestinal cancer and also demonstrated up-regulation in the expression of tumor suppressor protein and other genes thus inhibiting the risk of colorectal tumors.  

Men who consumed higher quercetin content food showed a lower incidence of lung cancer.

Prevention of Cardiovascular diseases (CVD): – In vitro studies showed that intake of dietary flavonoids i.e quercetin were inversely associated with stroke incidence majorly in older men. This activity was attributed to properties like prevention of platelet aggregation (thereby showing antithrombotic effects), antioxidant (protection of LDL from oxidation) and anti-inflammatory properties. However, it should be noted that consumption of quercetin is not associated in any way with the reduction of total and LDL cholesterol or increase of HDL cholesterol. Quercetin’s protective effects on CVD also reported to attenuate hypertension by inhibiting ACE activity and suppressing the activity of Angiotensin-II enzyme. Quercetin also elicits Coronary vasorelaxation. However, apart from the above findings, existence of uncertainity in few areas regarding benefits of quercetin on CVD still prevails.   

Inhibition of membrane lipid peroxidation: – In vitro studies revealed that Quercetin acts as chain braking antioxidants thereby terminating the chain reaction of lipid peroxidation occuring in the cell membrane regions.

Cataract: – Studies on rats show that quercetin might have a possible role in inhibiting Hydrogen peroxide-induced opacification (oxidative damage) of the lens, thus causing a reduction in the incidences of cataract.

Miscellaneous benefits of Quercetin:

Studies with Preliminary evidences suggest that Quercetin intake might me helpful in Type-2 diabetes, Schizophrenia, allergic rhinitis and in strengthening fragile capillaries and connective tissues. Studies also report that it can also be useful as antiviral (against herpes simplex, respiratory synctial viruses) and antimicrobial agent. Though more controlled clinical trials on humans are required before any conclusions can be made on the above effects.

Good sources of Quercetin

Among dietary flavonoids, Quercetin is the most abundant flavonol. It is commonly found in apples (440mg/kg), citrus fruits, berries (raspberries, strawberries, cranberry, lingonberry), onion (191mg/kg); particularly the outer rings of red and yellow onions (Greece, USA and former Yugoslavia), broccoli, red wine (main source of quercetin in Italy), cherries, green leafy vegetables, tomatoes, green beans, tea (2-2.5g/kg); black, green and buckwheat tea (Japan and The Netherlands), cocoa and olives. Preliminary studies by University of Queensland, Australia indicated the presence of quercetin in varieties of honey (derived from eucalyptus and tea tree flowers).   

Quercetin: Dosage and Precautions

Recommended dose of Quercetin

About 25-50 mg of Quercetin is obtained from a typical Western dietary intake daily. For therapeutic effects, typical oral dose ranges from 400-500 mg thrice daily. For best absorption, it should be administered about 20 minutes before the meals. The dose shouldn’t be exceeded than mentioned above as it may prove injurious to your health and might have toxic effects.

As demonstrated in in-vitro studies, high doses have proved to be toxic and can show mutagenic and genotoxic activities. When quercetin is administered orally, you might experience headache and tingling of the extremities.

Possible drug interactions:

  • Contra-indicated with some antibiotics (eg fluoroquinolones).
  • Quercetin is described as the inhibitor of CYP2C9.

Contraindications/Precautions:

None have been reported so far. Though, due to insufficient data and findings regarding its safety in the use of quercetin in case of pregnancy and breast feeding, it is advisable you avoid its use or consult your physician.

Recommendations

It is recommended that due to the uncertainities, you should consume Quercetin only from the natural dietary food sources and you should be careful while taking high doses of quercetin supplements as adverse effects are yet to be studied and concluded in humans.