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quercetin apple

Quercetin: An Apple a Day?

You’ve heard the old saying, “An apple a day keeps the doctor away.” Today, researchers in the field of antioxidants are beginning to explain why this old adage is more scientifically true than we may have thought. Apples, as well as many other colorful fruits and vegetables, contain a rich amount of a particularly powerful antioxidant called quercetin.

Quercetin is a flavonol type of flavonoid, found in plants, known for its potent antioxidant activity.1 Although quercetin is commonly found in many of the foods included in a healthy diet, supplements are usually needed for therapeutic dosages of this antioxidant.2 Known for its naturally occurring anti-histamine and anti-inflammatory effects, studies report that this powerful phytochemical works hard to prevent or help alleviate the symptoms of a variety of conditions.

Quercetin, like resveratrol, is also considered a potent xeno factor, a compound that can activate our “survival” genes when ingested. When the body is under stress, these genes produce enzymes that maintain cellular stability and promote longevity. 1

What Does Quercetin Do in the Body?

As a flavonoid, quercetin is suspected to work in the following three ways:2

  • Helping to rid the body of damaging free radical molecules.
  • Inhibits the formation of LDL (“bad”) cholesterol.
  • Inhibits various enzymes that cause inflammation in the body.

Quercetin may also inhibit the enzyme that the HIV virus needs to replicate itself (protease).

Recent in vitro studies in 2005 and 2006 suggest that the way quercetin inhibits cancer cell proliferation is by causing cell death and/or stopping the cells from growing at some point in the cellular reproductive cycle.3

Health Benefits of Quercetin

There is a large body of scientific evidence showing that the quercetin clearly has extremely potent antioxidant properties, as well as important cancer-fighting capabilities. Most information regarding quercetin is based upon in vitro and animal studies, but certainly suggests the potential for human benefits.

The problem is that in vitro effects of quercetin on cells may not be representative of their effects within the human body. Why? Because the form it naturally occurs in, as a glycoside, cannot be broken down and absorbed by the body’s enzymes, and instead is metabolized (hydrolyzed) by bacteria in the intestinal tract.4 However, the metabolites are still believed to maintain a weaker version of the positive effects the original glycoside form displayed in vitro.5

Tests have demonstrated that quercetin can maintain bioavailability longer in vivo longer when fused with sugar molecules.6

For example, blood plasma levels of quercetin were notably higher after consuming fried onions, which fuses sugar molecules to quercetin (quercetin 3-glucose) then after consuming quercetin glycoside (pure quercetin) by itself. The form of quercetin when consumed thus seems to influence its rates of absorption.6,7

Some in vivo human studies have confirmed the following benefits:

  • A recent human study involving both men and women with either pre-hypertension or stage 1 hypertension. Quercetin supplementation (730 mg) reduced blood pressure in stage 1 hypertensive individuals (but not in pre-hypertensive study participants). Interestingly, there was no corresponding reduction in systemic markers of oxidative stress (although in rat studies there was).8
  • A recent human study involving both men and women with either pre-hypertension or stage 1 hypertension. Quercetin supplementation (730 mg) reduced blood pressure in stage 1 hypertensive individuals (but not in pre-hypertensive study participants). Interestingly, there was no corresponding reduction in systemic markers of oxidative stress (although in rat studies there was).9
  • In vitro studies have shown that quercetin inhibits the growth of cancer cells in a variety of cancers (i.e., prostate, cervical, lung, breast, and colon), including a recent experiment where low dosages that matched peak blood levels of quercetin from food consumption on breast cancer cells.3
  • Human studies show that quercetin may reduce the risk of lung cancer.10

Adding More Quercetin to Your Diet

The antioxidant quercetin is responsible for that lovely red glow in your apple skin. It is also present in many forms of fruits and grains. Here is a list of foods that have relatively high levels of quercetin11:

mg/100 g.
Ancho peppers 27.60
Apple sauce, unsweetened 2.00
Apple, raw w/o skin 1.50
Apple, raw with skin 4.42
Apricot, raw 2.55
Bilberries, raw 3.04
Black grapes 2.54
Blueberries, frozen 3.93
Blueberries, raw 3.11
Bog whortleberries, wild frozen 17.70
Broccoli, cooked 1.06
Broccoli, raw 3.21
Buckwheat 23.09
Buckwheat flour, whole groat 2.72
Butterhead lettuce 1.19
Capers, canned 180.77
Celery, raw 3.50
Cherries, raw 1.25
Cherries, sweet canned 3.20
Cherry tomatoes, raw 2.77
Chives, raw 4.77
Chokeberries, frozen 8.90
Cocoa powder, dry, unsweetened 20.13
Coriander, raw 5.00
Corn poppy leaves 26.30
Cowberries, raw 21.00
Cranberries, raw 14.02
Cranberry juice, raw 16.41
Dill weed, fresh 55.15
Dock leaves, raw 86.20
European black currants, raw 5.69
Greek greens pie 12.40
Green beans, canned 1.49
Green beans, raw 2.73
Hot green chili peppers, raw 16.80
Hot wax yellow peppers, raw 50.63
Iceberg lettuce 2.47
Jalapeno, raw 5.07
Kale, canned 4.50
Kale, raw 7.71
Lemons, raw w/o peel 2.29
Lingonberries, raw 12.16
Lingonberry juice 1.02
Looseleaf lettuce 1.95
Lovage leaves, raw 170.00
Marinara sauce 0.91
Onions, boiled 19.36
Plums, raw 1.20
Queen Anne’s Lace leaves, raw 1.10
Red currants, raw 0.95
Red grapes 3.54
Red onion, raw 19.93
Rowanberries, frozen 7.40
Serrano peppers, raw 15.98
Spinach, raw 4.86
Spring onions, raw 14.24
Sweet potato leaves, raw 20.54
Tarragon, fresh 10.00
Tea, black brewed 2.07
Tea, decaf brewed 2.84
Tea, decaf green brewed 2.77
Tea, green brewed 2.69
Tomato juice, canned 1.46
Tomato puree, canned 4.12
Watercress, raw 4.00
White currants, raw 1.95
White sweet onion, raw 5.19
Yellow snap beans, raw 3.03

You may want to consider organically grown versions of fruits and vegetables when looking to maximize your quercetin intake. A recent study found that tomatoes which had been organically grown had more quercetin than their conventionally grown counterparts, although green peppers had no difference in quercetin levels.12

Possible Risks or Side Effects Associated with Quercetin

Although some DNA damage has been noted in high-dosage quercetin lab studies (in vitro), these genotoxic effects were not observed when tested in vivo on rats and mice.13 Memorial Sloan-Kettering Cancer Center notes that no adverse effects in humans have been reported taking doses up to 4 grams orally.14

Some precautions:

  • The use of quercetin may reduce the efficacy of some types of antibiotics, such as fluoroquinolones (also known as quinolones).2
  • May increase the effects and side effects of cyclosporin and other drugs metabolized in the liver.15
  • Quercetin may also inhibit the actions of the protein in cell membranes that transports a variety of drugs, thus making these medications less effective.15


  1. Maroon, Joseph. The Longevity Factor. New York : Atria Books, a division of Simon & Schuster, Inc., 2009. ISBN 978-1-4165-5107-2.
  2. Schulman, Robert A. and Dean, Carol A. Solve It With Supplements. New York : Rodale, 2007. ISBN-13 978-1-59486-819-1.
  3. Jeong, Jae-Hoon, et al. J Cell Biochem.: Effects of low dose quercetin: Cancer cell-specific inhibition of cell cycle progression. PubMed: U.S. National Library of Medicine and the National Institutes of Health. [Online] January 1, 2009. [Cited: November 2, 2009.]
  4. Bentz, Alexandra B. A Review of Quercetin: Chemistry, Antioxidant Properties, and Bioavailability. The Journal of Young Investigators: Appalachian State University. [Online] April 1, 2009. [Cited: November 2, 2009.]
  5. Bagchi, Debasis and Preuss, Harry G. Phytopharmaceuticals in cancer chemoprevention. s.l. : CRC Press, 2004. ISBN 0849315603, 9780849315602.
  6. Coates, Paul M. Encyclopedia of Dietary Supplements. s.l. : CRC Press, 2005. ISBN 0824755049, 9780824755041.
  7. Morand, Christine, et al. Respective bioavailability of quercetin aglycone and its glycosides in a rat model. Wiley InterScience: BioFactors. [Online] December 16, 2008. [Cited: November 2, 2009.]
  8. Edwards, Randi L., et al. The Journal of Nutrition: Quercetin Reduces Blood Pressure in Hypertensive Subjects. American Society for Nutrition: Nutrition and Disease. [Online] November 2007. [Cited: November 2, 2009.]
  9. Office of Media Relations. Arnold School study reveals benefits of quercetin. University of South Carolina. [Online] June 24, 2009. [Cited: November 2, 2009.]
  10. Neuhouser, M.L. Nutr Cancer: Dietary flavonoids and cancer risk: evidence from human population studies. PubMed: U.S. National Library of Medicine and the National Institutes of Health. [Online] 2004. [Cited: November 2, 2009.]
  11. Beltsville Human Nutrition Research Center. USDA Database for the Flavonoid Content of Selected Foods. U.S. Department of Agriculture Agricultural Research Service. [Online] March 2003. [Cited: November 2, 2009.]
  12. Mitchell, Alyson E. and Chassy, Alexander W. Antioxidants and the Nutritional Quality of Organic Agriculture. University of California: Mitchell Lab. [Online] [Cited: November 2, 2009.]
  13. Patri, Gianfranco, Silano, V. and Anton, Robert. Plants in Cosmetics: Potentially harmful components. s.l. : Council of Europe, 2006. ISBN 9287159122, 9789287159120.
  14. Memorial Sloan-Kettering Cancer Center. Quercetin. About Herbs. [Online] March 24, 2009. [Cited: November 2, 2009.]
  15. Therapeutic Research Faculty. Quercetin. WebMD: Find a Vitamin or Supplement. [Online] 2009. [Cited: November 2, 2009.]
  16. Murakami, A., Ashida, H. and Terao, J. Cancer Lett.: Multitargeted cancer prevention by quercetin. PubMed: U.S. National Library of Medicine and the National Institutes of Health. [Online] October 8, 2008. [Cited: November 2, 2009.]