Quercetin, a bioflavonoid obtained from buckwheat and citrus fruits, appears to stabilize the membranes of the mast cells that release histamine. Since quercetin's action is preventive, it's best taken daily a week or two before pollen season and continued throughout. Quercetin is used widely for sinus problems and aids in capillary fragility. Yellow onions contain high levels of quercetin, an especially potent antioxidant with some anti-inflammatory properties. It should be avoided during pregnancy.
Citrus Bioflavonoids help protect capillaries, prevent bruising, and intensify the effect of vitamin C in the body. In fact, Bioflavonoids are essential for total vitamin C effectiveness. Citrus Bioflavonoids provide natural antiviral, anti-inflammatory and anti-allergy properties. In addition, certain bioflavonoids are beneficial for fighting infections, free radical damage, viruses, and common colds. Bioflavonoids have also been used to help alleviate symptoms related to allergies and respiratory conditions. Citrus Bioflavonoids are sometimes considered vitamins because they possess the properties of a vitamin, and are sometimes referred to as vitamin P. They are found in several forms, including hesperidin, eriodictyl, quercetin, rutin, and hesperitin. They cannot be produced by the body, and therefore have to be taken as dietary supplements. Typical sources for Bioflavonoids also include the natural substances found in the peels of citrus fruits, peppers, and black currants. Other sources include apricots, cherries, grape fruit, grapes, lemons, oranges, prunes, rose hips.
Quercetin is typically found in plants as glycone or carbohydrate conjugates. Quercetin itself is an aglycone or aglucon. That is, quercetin does not possess a carbohydrate moiety in its structure. Quercetin glycone conjugates include rutin and thujin. Rutin is also known as quercetin-3-rutinoside. Thujin is also known as quercitrin, quercetin-3-L-rhamnoside, and 3-rhamnosylquercetin. Onions contain conjugates of quercetin and the carbohydrate isorhamnetin, including quercetin-3,4'-di-O-beta glucoside, isorhamnetin-4'-0-beta-glucoside and quercetin-4'-0-beta-glucoside. Quercetin itself is practically insoluble in water. The quercetin carbohydrate conjugates have much greater water solubility then quercetin.
Quercetin is known chemically as 2-(3, 4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one and 3,3',4'5,7-pentahydroxy flavone. Quercetin is a phenolic antioxidant and has been shown to inhibit lipid peroxidation. The putative anti-ulcer and gastroprotective effects of quercetin may, in part, be accounted for by this activity. In vitro and animal studies have shown that quercetin inhibits degranulation of mast cells, basophils and neutrophils. Such activity could account, in part, for quercetin's putative anti-inflammatory, anti-allergy and immunomodulating activities. Other in vitro and animal studies suggest that quercetin inhibits tyrosine kinase and nitric oxide synthase and that it modulates the activity of the inflammatory mediator, NF-kappaB. The mechanisms of anti-viral (in some cases enhanced with vitamin C) and anti-cancer activity that have been observed, again in in vitro and in animal studies, are unknown.
Aldose reductase, also known as alditol: NADP+ oxidoreductase, is the first enzyme of the polyol pathway. Hyperglycemia enhances the flow rate of the polyol pathway and this has been linked to such diabetic complications as cataracts, retinopathy, neuropathy and nephropathy. Quercetin is known to inhibit aldose reductase.