With the increasing emphasis on disease prevention, you may know the term “bioflavonoid”. Recent human studies have shown that, within limits, the more bioflavonoids you consume, the lower your risk for heart and blood vessel disease.
Bioflavonoids are chemicals that are widely distributed in herbs and edible plants. The most common bioflavonoid in the human diet is quercetin. Red wine, cranberries, apples, cayenne pepper, cabbage, garlic, and black and green tea are all sources of quercetin but, more than any herb, red and yellow onions are exceedingly rich sources. In fact, their papery, outer skin—the peel that we usually discard—is the richest quercetin source of all.
For years, traditional herbal medicine has used onion to treat asthma, high cholesterol, weak capillaries, inflammation, viruses, and allergies. However, modern science has only recently begun to explore the role of quercetin in preventing and treating such diseases.
So far, laboratory and human studies of bioflavonoids as a group show that they:
• Reduce the risk of heart attack by lowering blood cholesterol.
• Stabilize and strengthen capillaries, the smallest blood vessels, thus reducing risk of stroke, hypertension, varicose veins, hemorrhoids, and bruising.
• Reduce inflammation.
• Fight viruses, allergies, and asthma.
• Prevent cell damage from oxidation by free radicals.
In 1995, researchers measured the distribution of quercetin in different rings of several colored onion (Allium cepa) cultivars. They found that it is most highly concentrated in the papery outer skin and least concentrated in the more palatable inner rings. The outer skin of ‘Red Bone’ onion contained the highest concentration of quercetin, while the outer skin of ‘Contessa’ possessed the least. ‘Kadavan’ had the highest quercetin concentration in the more edible fleshy rings, but it took more than two pounds of them to obtain just 345 mg of quercetin, while it took only 10 grams of onion peel to yield more than 300 mg of quercetin. Nutritionists recommend taking anywhere from 50 mg to 5,000 mg quercetin daily. If you don’t relish the idea of eating onion peels, try adding unpeeled onions to soups, or 500-mg capsules of quercetin are available in many health-food stores.
If you prefer your quercetin in fresh, rather than capsule, form, a 1995 study may be of interest. Researchers measured quercetin absorption in nine patients who ate controlled amounts of onions. The researchers concluded that the body more easily absorbs naturally occurring quercetin than the capsule form.
One word of caution: quercetin is safe and beneficial at dietary levels (up to 1,000 mg daily) but, like any substance, may be toxic at very high levels (50,000 mg).
Cook, N. C., and S. Samman. “Flavonoids— Chemistry, Metabolism, Cardioprotective Effects, and Dietary Sources”. Nutritional Biochemistry 1996, 7:66–76.
Hertog, M.G.L., and P.C.H. Hollman. “Potential Health Effects of the Dietary Flavonol Quercetin”. European Journal of Clinical Nutrition 1995, 50:63–71.
Patil, B. S., and L. M. Pike. “Distribution of Quercetin Content in Different Rings of Various Coloured Onion (Allium cepa L.) Cultivars”. Journal of Horticultural Science 1995, 70:643–50.
Reichert, R. “Quercetin Absorption”. Quarterly Review of Natural Medicine 1996, Spring: 15–16.
James Duke, a member of the Herbs for Health Editorial Advisory Board, spent thirty years with the U.S. Department of Agriculture. He serves on the board of trustees of the American Botanical Council.
C. Leigh Broadhurst holds a doctorate in geochemistry and is a nutrition consultant in Clovery, Maryland. She is currently researching antidiabetic herbs for the USDA.