Onions not only provide flavor; they also provide health-promoting phytochemicals as well as nutrients.

Onions contain quercetin, a flavonoid (one category of antioxidant compounds). Antioxidants are compounds that help delay or slow the oxidative damage to cells and tissue of the body. Studies have indicated that quercetin helps to eliminate free radicals in the body, to inhibit low-density lipoprotein oxidation (an important reaction in the atherosclerosis and coronary heart disease), to protect and regenerate vitamin E (a powerful antioxidant) and to inactivate the harmful effects of chelate metal ions. Major dietary sources of quercetin include tea, onions and apples.

Other studies have shown that consumption of onions may prevent gastric ulcers by scavenging free radicals and by preventing growth of the ulcer-forming microorganism, Helicobacter pylori.

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.

In addition to quercetin, onions contain the phytochemicals known as disulfides, trisulfides, cepaene, and vinyl dithiins. These compounds have a variety of health-functional properties, including demonstrated in vitro anticancer and anti-microbial activities.

Onions are also a source of vitamin C, potassium, dietary fiber and folic acid. They also contain calcium, iron and have a high protein quality (ratio of mg amino acid/gram protein). Onions are low in sodium and contain no fat.