Chemical Constituents of Onion

Onions not only provide flavor; they also provide health-promoting phytochemicals as well as nutrients. Onion contains an acrid, volatile principle that stimulates the tear glands and the mucous membranes of the upper respiratory tract.

Of all the healthy compounds contained in onions, two stand out: sulfur and quercetin - both being strong antioxidants. They each have been shown to help neutralize the free radicals in the body, and protect the membranes of the body’s cells from damage.

“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”. 2

Major dietary sources of quercetin include tea, onions and apples. 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 on 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.4

White onions contain very little querctin, so it’s better to stick with the yellow and red varieties. Most health professionals recommend eating raw onions for maximum benefit, but cooking makes them more versatile and doesn’t significantly reduce their potency. In fact, unlike sulfur compounds, quercetin can withstand the heat of cooking as long as it is a low heat.

The strong smell of the onion and its relatives contain thioallyl compound or alliins, and alliins are an amino acid. When cut or crushed, the alliin within the onion is converted by an enzymatic reaction into allicin, this breaks down into sulfide compounds. Sulfide compounds are aromatic and this is what gives the onion, and all the plants in the onion family, their distinctive smell. The cysteine sulphoxides occurring in the genus Allium are precursors for a large number of compounds which are responsible for the typical aroma as well as for the health value of these plants.1

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.5

A recent study at the University of Bern in Switzerland showed that consumption of 1 g of dry onion per day for 4 weeks increased bone mineral content in rats by more than 17% and mineral density by more than 13% compared to animals fed a controlled 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.6

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.

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. They are low in calories with only 30 calories per serving, yet add abundant flavor to a wide variety of foods. Onions are also cholesterol free, and provide dietary fiber, vitamin C, vitamin B6, potassium, and other key nutrients.2