Quercetin belongs to a family of naturally occurring, water-soluble plant compounds known as polyphenols, flavonoids, flavonols and bioflavonoids, and it appears to have both anti-inflammatory and antioxidant properties.
In 2003, the U.S. Department of Agriculture developed a database of the quercetin content of common foods. Apples, onions and teas are the main sources of quercetin in the diet, but it is also present in red wine, berries, seeds, leafy green vegetables, hot peppers, parsley and red grapes, and is available as a dietary supplement.1,2 (See sidebar for the quercetin content of specific foods.) Studies show that people normally consume about 23 mg to 34 mg of flavonoids each day, two-thirds of which is quercetin.3,4 A typical supplemental dose of quercetin is 200 mg to 400 mg taken three times a day.5
Quercetin's role in disease prevention
Increased consumption of fruits and vegetables, which are rich in flavonoids, is associated with a decreased incidence of cardiovascular disease, cancer and many other chronic conditions like cataracts and asthma.7 Because quercetin accounts for most of the flavonoids consumed, researchers theorize that it plays a role in disease prevention and, as a result, it has become one of the most studied flavonoids. However, few studies focus on quercetin alone, mainly because researchers, until recently, have lacked data on the quercetin content of foods.4
While studies on cancer yield contradictory results, several laboratory, animal and human studies suggest that flavonoids, including quercetin, may reduce the risk of developing several types of cancer, including cancer of the kidneys,8 colon,9 pancreas,10 lung7 and breast.7,11,12 A retrospective study of 183,518 men and women who were followed for eight years found that those with the highest dietary intakes of three flavonols —quercetin, kaempferol and myricetin —were 23 percent less likely to develop pancreatic cancer than those with the lowest intakes.10 A review of a case-control study in Italy concluded that the flavonoids in fruits and vegetables may reduce kidney-cancer risk.8 Protective effects have also been seen in animals. In one study, quercetin decreased the formation of colon-cancer precursors called aberrant crypts in rats exposed to a known carcinogen.13
The Iowa Women's Health Study of 34,492 postmenopausal women revealed a 38 percent lower risk of death from coronary heart disease among those who had the highest daily flavonoid intake (28.6 mg per day).16
Another large population study, the Zutphen Elderly Study out of the Netherlands, supported these findings.17 Researchers there found that among 805 men ages 65 to 84, those with the highest intake of flavonoids had a 58 percent lower risk of dying from CHD than those with the lowest intakes. The major sources of flavonoid intake were tea (61 percent), onions (13 percent) and apples (10 percent).17 Several studies have found that polyphenols, and specifically quercetin, inhibit the formation of blood clots, which helps prevent cardiovascular disease.18,19,20
Quercetin supplements also have been found to reduce high blood pressure.14 Researchers from the University of Utah conducted a randomized, double-blind, placebo-controlled, crossover study to test the effectiveness of 730 mg of quercetin a day for 28 days in 41 men and women with either pre-hypertension (systolic blood pressure between 120 and 139 mm Hg or diastolic blood pressure between 80 and 89 mm Hg) or stage 1 hypertension (systolic blood pressure between 140 and 159 mm Hg or diastolic blood pressure between 90 and 99 mm Hg). While the quercetin supplements had no effect on people with pre-hypertension, it had a significant blood-pressure-lowering effect on those with hypertension, lowering systolic blood pressure an average of 7 mm Hg, and lowering diastolic an average of 5 mm Hg.14 However, a previous study found that supplementing with 1 g of quercetin a day had no effect on total, LDL or HDL cholesterol, triglyceride levels, platelet aggregation (blood-clot formation) or blood pressure — all risk factors for heart disease, in a group of 27 healthy men and women.3
While studying rats, researchers found that diets with quercetin were effective at increasing the activity of nitric oxide synthase (the enzyme that produces nitric oxide) in the vascular tissue. Nitric oxide is known to relax restricted blood vessels.21
Quercetin may treat and prevent type 2 diabetes. A Finnish study of more than 10,000 men and women found that a lower risk of type 2 diabetes was associated with higher dietary intakes of quercetin.7 In rats, high doses of quercetin (10 mg per kg of body weight per day) were found to prevent diabetic nephropathy, a common complication that can cause kidney failure.22 Quercetin also reduces capillary fragility, and it might prevent diabetic cataracts, possibly by inhibiting aldose reductase, an enzyme that converts simple sugars to sugar alcohol, in the eye lens.23,24,25,26,27
The documented properties of quercetin make it a potential treatment for prostatitis, or inflammation of the prostate gland. In one study, 28 men with chronic pelvic-pain syndrome due to prostatitis were given either 500 mg of quercetin twice a day for one month, or a placebo. Researchers found that 67 percent of men receiving the quercetin supplements experienced improved symptoms, while only 20 percent of those receiving the placebo improved.30
High doses of quercetin (1 g per day) helped reduce illnesses in people who exercise extensively. Researchers from Appalachian State University in North Carolina gave 40 male cyclists either 1 g of quercetin or a placebo each day for three weeks. During this time, the cyclists spent three days training at maximum intensity for three hours a day. Researchers found that people taking quercetin experienced significantly fewer upper-respiratory-tract infections than those taking placebo. One person in the quercetin group had an upper-respiratory infection during the two-week post-exercise period, while nine people in the control group had upper-respiratory infections during the same period. 31
In a laboratory study, researchers found that quercetin inhibits thyroid cell growth in rats and inhibits iodide uptake by the thyroid cells. This suggests that quercetin may decrease thyroid function, and may potentially treat some thyroid diseases involving hyperactivity of the thyroid gland.32
How quercetin acts in the body
Once absorbed in the small intestine, enzymes metabolize quercetin. It is then further metabolized by the liver.33 While reviewing studies performed by Oregon State University, researchers concluded that, while lab studies show that flavonoids have three to five times the antioxidant activity of vitamins C or E, that antioxidant activity doesn't appear to happen in the body. Researchers observed that after people ate flavonoid-rich foods, only very low concentrations (from 0.7 to 7.6 mu moles per liter) of flavonoids appeared in their blood plasma. This low bioavailability suggests flavonoids are not the main source of the antioxidant activity that results when people eat fruits and vegetables. Rather, flavonoids like quercetin appear to influence cell signaling, which plays a role in cancer and heart disease.1 Cell signaling occurs when two or more cells coordinate their actions, and it influences processes like tissue repair, heart contraction and immunity.
When the body prepares to metabolize and dispose of flavonoids, it triggers enzyme systems that eliminate cancer-causing compounds. Once in the bloodstream, quercetin metabolites circulate up to 10 hours.1 A wide range of biological activities, including antibacterial, antithrombotic, vasodilatory, anti-inflammatory and anti-carcinogenic effects are mediated by flavonoid compounds, including quercetin.7 Quercetin also inhibits cholesterol synthesis in liver cells in laboratory studies;34 however, its effects in the body are less clear.33
In the 1970s, quercetin was reported to have cancer-causing properties; however, based on more recent studies, researchers have found the opposite effect.35 In 1999, the International Agency for Research on Cancer concluded that quercetin does not cause cancer in humans.35 There are no reports of quercetin having a negative effect on thyroid function, but the preliminary findings of a reduced thyroid-cell growth in rats32 should be considered.
The evidence for dietary quercetin and quercetin supplements suggests several health benefits from higher intakes of the flavonoid. Benefits include a reduced risk of many forms of cancer, a reduced risk of cardiovascular disease, a natural way to treat mild hypertension and prostatitis, a reduced risk of upper-respiratory tract infection in those who exercise extensively, a potential treatment for type 2 diabetes and a potential treatment for overactivity of the thyroid gland. Quercetin has no evident negative side effects. However, more research is needed to pinpoint specific dosages effective for treating or preventing different conditions.
Densie Webb, Ph.D., R.D., is a freelance writer and industry consultant based in Austin, Texas.
source: USDA Database for the Flavonoid Content of Selected Foods, 2003
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