Quercetin
Quercitin is a class of flavonoids, a group of plant pigments, largely responsible for the colors of many fruits and flowers. Recent research suggests that flavonoids may be useful in the treatment and prevention of many health conditions. Many of the medicinal actions of foods, juices, herbs, and bee pollen are now known to be directly related to their flavonoid content. More than 4000 flavonoid compounds have been characterized and classified according to their chemical structure. Flavonoids are a bit of an enigma to scientists, because they are quite reactive compounds. They can enter into almost any type of reaction known to organic chemistry, such as oxidation-reduction reactions, carbonyl reaction, acid-base reactions, free-radical reaction, hydrophobic interactions, tautomery, and isomerizations. As such, characterization of the many diverse physiologic properties of flavonoids is a considerable challenge to biochemists and researchers alike.
Quercetin has demonstrated significant anti-inflammatory activity due to direct inhibition of several of the initial processes of inflammation via interaction with calcium channels or calmodulin (the intracellular calcium-binding protein), or both. It works through other mechanisms as well, such as inhibiting mast cell and basophil degranulation, neutrophil and monocyte lysosomal secretion, prostaglandin (most notably, leukotriene) formation, lipid peroxidation, and the resultant cascade of effects that are often a result of these processes.
Flavonoids as a group possess significant antiviral activity, with quercetin having the greatest antiviral activity against herpes virus type I, parainfluenzae 3, polio virus type I, and respiratory syncytial virus. Quercetin was shown, in vitro, to inhibit both viral replication and infectivity. In vivo studies in animals also showed quercetin to inhibit viral infection. This would suggest that quercetin might be of some benefit in viral infections, including the common cold.
Many flavonoids have also been shown to inhibit tumor formation, with quercetin consistently demonstrated to be the most effective. In experimental models, quercetin demonstrated significant antitumor activity against a wide range of cancers, including squamous cell carcinoma, leukemia, and cancers of the prostate breast, ovaries, colon, rectum, and brain. The cancer preventive effects of quercetin have been attributed to various mechanisms, including antioxidative activity, inhibition of enzymes that activate carcinogens, modification of signal transduction pathways, and interactions with receptors and other proteins, such as the androgen receptor involved in the development and progression of prostate cancer. Clinically, quercetin holds great promise in reversing multidrug resistance (MDR) in cancer cells. Transporter-mediated active efflux of cytotoxic agents is one of the best characterized mechanisms by which cancer cells develop MDR. Quercetin demonstrated multitargeted effects in reversing MDR.
Quercitin has relatively poor bioavailability and high variability of absorption. Improved clinical results may be achieved with newer forms of quercetin, such as enzymatically modified isoquercitrin (EMIQ) to overcome these shortcomings.
Dosage
The recommended dosage range for quercetin is 200 to 400 mg (Quercitin) or 35 to 50 mg of EMIG, 20 minutes before meals (three times per day). If quercetin is being used for its anti-inflammatory properties and bromelain supplementation is also indicated, administration with bromelain may enhance absorption. Combination preparations of proteolytic enzymes and flavonoids have been shown to have significant anti-inflammatory activity in experimental studies.
Safety
Quercetin appears to be well tolerated and the weight of the available evidence supports the safety of quercetin even in large doses.