Onions contain quercetin, a flavonoid (one category of antioxidant compounds). Studies have indicated that quercetin helps to eliminate free radicals in the body, to inhibit low-density lipoprotein oxidation (an important reaction in atherosclerosis and coronary heart disease), to protect and regenerate vitamin E (a powerful antioxidant) and to inactivate the harmful effects of chelate metal ions.
Several studies have shown quercetin to have beneficial effects against many diseases and disorders including cataracts, cardiovascular disease as well as cancer of the breast, colon, ovarian, gastric, lung and bladder.
Onions are also a source of vitamin C, potassium, dietary fiber and folic acid. They contain calcium, iron and have a high protein quality. Onions are low in sodium and contain no fat. "www.onions-usa.org"