Onions contain the flavonoid quercetin and a raft of recent science has uncovered the role polyphenols, to which quercetin belongs, can play in preventing the onset of various diseases, notably certain cancers and cardiovascular disease, that annually kills 17 million people in the world.
Researchers at Cornell University found, in preliminary lab studies, that members of the onion family with the strongest flavour - particularly New York bold, western yellow and shallots - are the best varieties for inhibiting the growth of liver and colon cancer cells.
"No one knows yet how many daily servings of onions you would have to eat to maximise protection against cancer, but our study suggests that people who are more health-conscious might want to go with the stronger onions rather than the mild ones," said study leader Rui Hai Liu, a chemist with Cornell's department of food science in New York.
Liu and his associates analysed 10 common onion varieties and shallots for total antioxidant activity and their ability to fight the growth of cancer in human cell lines. Although shallots resemble onions, they are actually a separate, distinctive species. Fresh, uncooked samples were used, with extracts taken from the bulbs with the outer skin removed.
Shallots and onion varieties with the strongest flavour - western yellow, New York bold and northern red - had the highest total antioxidant activity, an indication that they may have a stronger ability to destroy charged molecules called free radicals, an excess of which are thought to increase the risk of disease, particularly cancer, the researcher says.
Onion varieties with the mildest flavour - empire sweet, western white, peruvian sweet, Mexico, texas 1015, Imperial Valley Sweet and Vidalia - had the lowest total antioxidant activity, reported Liu.
In tests against liver and colon cancer cells, onions were significantly better at inhibiting the growth of colon cancer cells than liver cancer cells, an indication that they are potentially better at fighting colon cancer, the researchers commented.
The strongest cancer-fighters tested were the New York Bold variety, Western Yellow and shallots. The sweetest tasting onions, including the beloved Vidalia, showed relatively little cancer-fighting ability, he notes. But at the same time cautioning that human studies are needed before any definitive links between onion consumption and cancer-prevention can be established.
World onion production has increased by at least 25 per cent over the past 10 years with current production at around 44 million tonnes, making it the second most important horticultural crop after tomatoes, according to researchers at the department of plant genetics and biotechnology, Horticulture Research International in the UK.
Because of their storage characteristics and durability for shipping, onions have always been traded more widely than most vegetables. Onions are versatile and used as an ingredient in a wide range of dishes, accepted by almost all traditions and cultures.
Horticulture Research International claims that onion consumption is increasing significantly, particularly in the USA, on the back of heavy promotion that links the vegetable to flavour and health benefits. Onions are rich in two chemical groups that have perceived benefits to human health.
Full findings for the recent US study will appear in the November 3 print issue of the Journal of Agricultural and Food Chemistry, the journal of the American Chemical Society.