Onions not only provide flavor, they also provide important nutrients and health-promoting phytochemicals. High in vitamin C, onions are a good source of 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.
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 apple, tea, and onion. Recent studies at Wageningen Agricultural University, the Netherlands, showed that the absorption of quercetin from onions is twice that from tea and more than three times that from apples. Based on studies conducted at The Queen's University at Belfast, Ireland and Wageningen Agricultural University, the content of quercetin in onions is estimated to be between 22.40 mg and 51.82 mg per medium-sized onion (100 gram). Further research at the Agricultural University of Wageningen showed that daily consumption of onions may result in increased accumulation of quercetin in the blood. Studies are in progress to determine whether the increased quercetin accumulation from eating onions translates into significant antioxidant benefit.
Other studies have shown that consumption of onions may be beneficial for reduced risk of certain diseases. Consumption of onions may prevent gastric ulcers by scavenging free radicals and by preventing growth of the ulcer-forming microorganism, Heliobacter pylori. University of Wisconsin-Madison researchers found that the more pungent onions exhibit strong anti-platelet activity. Platelet aggregation is associated with atherosclerosis, cardiovascular disease, heart attack, and stroke. A study in progress at the University of Wisconsin is determining the extent to which onion consumption and specific onion compounds affect the in vivo aggregation of blood platelets. "Using an in vivo model, we are beginning to investigate and, in some cases, confirm the potency of the onion as a blood thinner and platelet inhibitor. Onions may be among the vegetables that will be prized not only for their addition to our cuisine, but for their value-added health characteristics," said Irwin Goldman, Associate Professor of Horticulture, University of Wisconsin-Madison.
A recent study at the University of Bern in Switzerland showed that consumption of one gram dry onion per day for four weeks increased bone mineral content in rats by more than 17% and mineral density by more than 13% compared to animals fed a control diet. This data suggests onion consumption has the potential to decrease the incidences of osteoporosis.
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 anticancer and antimicrobial activities.