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QUERCETIN
Physiology
and Clinical Effects
Quercetin belongs to a class of water-soluble plant pigments called
flavonoids. Quercetin acts as an antihistamine and has anti-inflammatory
activity. As an antioxidant, it protects LDL cholesterol (the “bad”
cholesterol) from becoming damaged. Cardiologists believe that damage to
LDL cholesterol is an underlying cause of heart disease. Quercetin blocks
an enzyme that leads to accumulation of sorbitol, which has been linked to
nerve, eye, and kidney damage in those with diabetes. However, no human
research has evaluated the possible beneficial effect of quercetin for
diabetics.
Food
Sources
Quercetin can be found in onions, apples, and black tea. Smaller amounts
are found in leafy green vegetables and beans.
Deficiency
Risk and Symptoms
No clear deficiency of quercetin has been established.
Recommended
Dosage
Common supplemental intake of quercetin is 400 mg two to three times per
day.
Contraindications
No clear toxicity has been identified. Early quercetin research suggested
that large amounts of quercetin could cause cancer in animals.1
Most, 2 3 4 but not all,5 current
research finds quercetin to be safe or actually linked to protection from
cancer.
Since
flavonoids help protect and potentiate vitamin C, quercetin is often taken
with vitamin C.
References:
1. Ishikawa M,
Oikawa T, Hosokawa M, et al. Enhancing effect of quercetin on
3-methylcholanthrene carcinogenesis in C57B1/6 mice. Neoplasma
1985;43:435–41.
2. Hertog MGL, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and
cancer risk in the Zutphen elderly study. Nutr Cancer
1994;22:175–84.
3. Castillo MH, Perkins E, Campbell JH, et al. The effects of the
bioflavonoid quercetin on squamous cell carcinoma of head and neck origin.
Am J Surg 1989;351–5.
4. Stavric B. Quercetin in our diet: from potent mutagen to probably
anticarcinogen. Clin Biochem 1994;27:245–8.
5. Barotto NN, López CB, Eyard AR, et al. Quercetin enhances pretumourous
lesions in the NMU model of rat pancreatic carcinogenesis. Cancer
Letters 1998;129:1–6.
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