Comprehensive Guide to Quercetin

QuercetinDr. James Meschino DC, MS, ND
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General Features
Quercetin is a flavonoid that serves as the backbone for many other flavonoids in nature, including the citrus flavonoids rutin, quercitin, and hesperidin. These derivates differ from Quercetin in that they have sugar molecules attached to their Quercetin backbone. Experimental studies reveal that many medicinal plants owe much of their biological activity to their high Quercetin content.1
This water-soluble pigment has been shown to inhibit histamine release at therapeutic doses, making it an attractive agent in the management of some allergic conditions, such as hay fever. It also demonstrates impressive anti-inflammatory and antioxidant properties, and like some other flavonoids (e.g. genistein, diadzein), appears to possess phytoestrogen effects. Under experimental conditions, its phytoestrogen effects have been shown to inhibit the proliferation of human breast cancer cells in test tube experiments. In one human trial, Quercetin supplementation demonstrated improvement in prostatitis, with 67% of patients experiencing significant improvement in the Quercetin group compared to only 20% of patients reporting improvement in the placebo group. This benefit may be due to its phytoestrogen activity and/or its anti-inflammatory properties.
Experimental studies also reveal that Quercetin may help to prevent cataracts in diabetics, by inhibiting the enzyme that forms sorbitol (aldose reductase) in the lens of the eye.
Clinical Application and Mechanism of Action
1. Anti-Inflammatory
Quercetin inhibits the manufacture and release of histamine and other allergic/inflammatory mediators, which gives Quercetin supplementation application in a variety of inflammatory and allergic reactions (arthritis, joint inflammation, asthma, hay fever, lupus).4-12
2. Diabetic Cataracts
Quercetin inhibits the aldose reductase enzyme, which converts glucose into sorbitol. Sorbitol build up in the lens of the eye (secondary to diabetes and hyperglycemia) is involved in the development of cataracts.
Elevated sorbitol may also contribute to the development of diabetic neuropathy and retinopathy. Thus, Quercetin may be beneficial in diabetes management for a number of reasons.13,14 Animal studies reveal that Quercetin effectively delays the onset of cataracts in diabetic animals.14
3. Anti-Tumor
Many flavonoids inhibit tumor formation, but Quercetin has demonstrated a very consistent effect in this regard. Under experimental conditions, Quercetin demonstrates a significant antiproliferative effect in regards to squamous cell carcinoma, leukemia, and cancers of the breast, ovaries, colon, rectum and brain.15-18 It is thought that Quercetin down-regulates enzymes that control the rate of cellular division (i.e. protein kinase, DNA-topoisomerase, ornithine decarboxylase), in addition to its mild phytoestrogen effects.
1. Anti-Inflammatory/Anti-Allergy: as a single agent; 200-400 mg, three times per day. Combinations of bromelain and Quercetin have been shown to potentiate or enhance each others anti-inflammatory activity.21 The amount of bromelain should be equal to the amount of Quercetin.1 Bromelain also enhances the absorption of Quercetin.1
2. Diabetes (neuropathy or cataract protection): consider 200-400 mg (three times per day).
Adverse Side Effects and Toxicity
Animal studies demonstrate that Quercetin is extremely non-toxic and non-carcinogenic.22,23,24 In rare cases, allergic reactions to Quercetin can occur in humans.
Drug-Nutrient Interactions
1. Estradiol (found in oral contraceptives and hormone replacement therapy)
Quercetin may slow down the detoxification of estradiol by liver enzymes. There are no human reports of Quercetin potentiating the effects of estradiol through this mechanism at this time.
2. Felodipine
Felodipine is a calcium channel blocker used in the treatment of congestive heart failure, hypertension and Raynaud’s Syndrome. Test tube studies indicate that Quercetin inhibits the enzyme in the liver that is responsible for the breakdown of felodipine. This may result in higher, more potentially dangerous levels of felodipine in the bloodstream, although no human reports of adverse effects have been reported to date.
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