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Doctor's Best, Quercetin Bromelain, 180 Veggie Caps

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Jarrow Formulas, Quercetin, 500 mg, 100 Capsules

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Quercetin with Vitamin C and Bioflavonoids, 50 Capsules, 250 mg, From Natrol

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Quercetin, 300, 60 Capsules, From NutriCology

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Natural Factors Quercetin Bioflavonoid Complex, 90 Capsules, From Natural Factors

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Now Foods, Quercetin with Bromelain, 120 Veg Capsules

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Quercetin Plus, 60 vCaps, 1 g, From Olympian Labs

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Source Naturals Activated Quercetin 100 Tab

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Activated Quercetin, 200 Capsules, From Source Naturals

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NutraDrops Quercetin, 4 fl oz (118.28 ml), From Source Naturals

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Activated Quercetin, 200 Tablets, From Source Naturals

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Solaray, QBC Plex, 120 VegCaps

Solaray, QBC Plex, 120 VegCaps

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Doctor's Best Acai Extract, 120 Capsules, 500mg, From Doctor's Best

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Enzymatic Therapy, Resveratrol~Forte, 125 mg, 60 Veg Capsules

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Estrogenex, Anti-Estrogen, 90 Capsules, Hi-Tech Pharm

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Cox 2Tame, 90 Softgels, From Jarrow

Cox 2Tame, 90 Softgels, From Jarrow

Cost Per Serving : $0.16

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Jarrow Formulas, Vision Optimizer, 90 Veggie Caps

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Quercetin belongs to a group of polyphenolic substances known as flavonoids. Quercetin is a member of the class of flavonoids called flavonols. It is widely distributed in the plant kingdom in rinds and barks. Especially rich sources of quercetin include onions, red wine, green tea and St. John's wort.

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 a flavonol (a subclass of flavonoids) and is a potent antioxidant, providing cardiovascular protection by reducing oxidation of LDL cholesterol. Quercetin is one half of the rutin molecule, another flavonol, and is the more active antioxidant. Keep out of reach of children. Quercetin is a potent and versatile flavonoid and phytonutrient.

Quercetin is part of the coloring found in the skins of apples and red onions. Quercetin is isolated and sold as a nutritional supplement.

What Does Quercetin Do?: Quercetin is a powerful anti-oxidant. Quercetin is also a natural anti-histamine, and anti-inflammatory. Research shows that quercetin may help to prevent cancer, especially prostate cancer.

Quercetin's anti-histamine action may help to relieve allergic symptoms and asthma symptoms. The anti-inflammatory properties may help to reduce pain from disorders such as arthritis. Man who are concerned about prostate problems would also benefit from quercetin. Quercetin may also help reduce symptoms like fatigue, depression and anxiety.

Quercetin may have antioxidant, anti-inflammatory, antivral, immunomodulatory, anticancer and gastroprotective activities. It may also have anti-allergy activity and activity in preventing secondary complications of diabetes.

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 NO Formula 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.

About 25% of an ingested dose of quercetin is absorbed from the small intestine and is transported to the liver via the portal circulation, where it undergoes significant first pass metabolism. Quercetin and its metabolites are distributed from the liver to various tissues in the body. Quercetin is strongly bound to albumin in the plasma. Peak levels of plasma quercetin occur from 0.7 to 7 hours following ingestion, and the elimination half-life of quercetin is approximately 25 hours.

Regarding pharmacokinetics of the quercetin glycoside conjugates, it appears that the main determinant of absorption of these conjugates is the nature of the sugar moiety. For example, quercetin glucoside is absorbed from the small intestine, whereas quercetin rutinoside is absorbed from the colon after removal of the carbohydrate moiety by bacterial enzymes.

It has been claimed that quercetin protects against heart attacks and stroke, but recent research found no support for this claim. Quercetin may, however, have benefit in some allergies, in conditions characterized by capillary fragility, in chronic prostatis and in some cancers. It may have beneficial effects on immunity and may have gastro-protective effects. It may also protect against the development of such diabetic complications as cataracts, retinopathy, neuropathy and nephropathy.

Epidemiological evidence has suggested for some time that dietary intake of flavonols and flavones is inversely associated with coronary heart disease. It has been hypothesized that the ability of flavonoids to inhibit lipid peroxidation, demonstrated both in vitro and in various animal models, might, at least partially, account for this association.

More recently, however, a double-blind, placebo-controlled study compared one-gram daily of oral quercetin with placebo in 27 healthy subjects. This dose was said to be about 50-fold greater than dietary intakes associated, epidemiologically, with reduced risk of coronary heart disease mortality. The study continued for 28 days, during which period subjects receiving quercetin achieved plasma quercetin levels 23-fold higher than levels in those on placebo.

The results showed no quercetin effect on serum total cholesterol, LDL-cholesterol, HDL-cholesterol or triglyceride levels. Nor was there any effect on other factors considered to be indicators of risk for cardiovascular/thrombogenic disease, including platelet aggregation, platelet thromboxane B2 production, blood pressure and resting heart rate. There was no effect on levels of (n-6) or (n-3) polyunsaturated fatty acids in serum or platelet phospholipids.

The researchers noted that the previous studies indicating that flavonoids, including quercetin, might be beneficial in heart disease were all performed in vitro or in animal models. They further observed that prior studies showing a quercetin platelet aggregation-inhibiting effect, for example, used doses far higher than those used in this study and thus amounts greatly in excess of those present in normal diet.

The researchers concluded: 'Our results suggest that any protective effect of foods containing quercetin may be mediated via effects on risk factors other than those we have measured.... Alternatively, the protective effect of quercetin-containing foods may be due to factors other than quercetin in those foods.'

Another recent study increased plasma quercetin concentrations in 18 healthy subjects, via increased intake of quercetin-rich foods, such as onions. But, again, there was no effect on platelet aggregation, thromboxane B2 production, factor VII or other hemostatic variables.

Quercetin is one of several flavonoids that have effects on mast cells and basophils; thus, some research suggests, it might be useful in some allergies, such as hay fever. Quercetin can help prevent the release of histamine and other mediators of allergic reactions, possibly by stabilizing cell membranes so that they are less reactive to allergens. Quercetin also exhibits anti-inflammatory properties, inhibiting formation of inflammatory prostaglandins and leukotrienes.

It was suggested in an open-label study that quercetin might be helpful in category III chronic prostatitis syndromes (nonbacterial chronic prostatitis and prostatodynia). Recently, this was confirmed in a prospective, randomized, double-blind, placebo-controlled trial.

Thirty men with these disorders received either placebo or 500 milligrams of quercetin twice daily for one month. Significant improvement was achieved in the treated group, as measured by the National Institutes of Health chronic prostatitis score. Some 67% of the treated subjects had at least 25% improvement in symptoms, compared with 20% of the placebo group achieving this same level of improvement.

In a follow-up, unblind, open-label study, 17 additional men received the same amount of quercetin for one month, but this time the quercetin was combined with bromelain and papain, which may enhance its absorption. In this study, 82% achieved a minimum 25% improvement score.

Several of quercetin's activities, including anti-inflammatory, anti-oxidant and immune-modulating actions, are believed to play roles in achieving these effects. There was evidence that quercetin prevented oxidative-mediated cellular injury, which the researchers suggested would apply whether the injury was due to infective, inflammatory or auto-immune mechanisms.

The researchers concluded: 'Few therapies have shown durable efficacy with these disorders. Quercetin is efficacious, inexpensive, well tolerated and safe.'

Fears that quercetin might be carcinogenic have not been supported by recent research. On the contrary, quercetin appears to have some anti-cancer effects, as demonstrated in vitro and in some animal models. Several processes that contribute to some cancers have been inhibited by quercetin. Some experiments have suggested a chemopreventive role for quercetin in colorectal carcinogenesis. Quercetin is reported to be one of very few substances that inhibit an animal model of basophil leukemia.

These preliminary results, as well as tentative findings that quercetin can have favorable immune-modulating and (in combination with vitamin C) some anti-viral activity (against picomaviruses) need more vigorous followup studies.

There are reports that quercetin may have some gastroprotective effects in animal models. High-dose quercetin promoted mucus production and helped diminish the severity of gastric lesions in animals injured with absolute thanol. Thiobarbituric acid reactive substances in gastric mucosa, a measure of lipid peroxidation, was significantly decreased by quercetin in ethanol-injured animals. Whether these findings will have relevance in humans remains to be seen.

Finally, quercetin has been shown to inhibit aldose reductase, the first enzyme in the polyol pathway. Experimental data link glucose metabolism via this pathway to long-term diabetic complications, such as cataract, nephropathy, retinopathy and neuropathy. Again, whether inhibition of aldose reductase will have any relevance in humans remains to be proven by well-controlled clinical trials.

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