Researchers have known for some time that onions may help fight cancer, but the current study is believed to be the first to compare cancer-fighting abilities among commonly consumed onion varieties.
Preliminary laboratory studies by a team from Cornell University found 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'd have to eat to maximize 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.
Liu and his associates analyzed 10 common onion varieties and shallots for total antioxidant activity and their ability to fight the growth of cancer in human cell lines. 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 free radicals, an excess of which are thought to increase the risk of disease, particularly cancer, the researchers will report in the 3 November issue of the Journal of Agricultural and Food Chemistry.
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, Liu said.
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 researcher added. The strongest cancer-fighters tested were the New York Bold variety, Western Yellow and shallots. The sweetest tasting onions showed relatively little cancer-fighting ability however.
Onions are rich in a flavour compound known as quercetin, a potent antioxidant, also found in apples, red berries and other vegetables, that has been linked to protection against cataracts and heart disease as well as cancer.
Green onions and cocktail onions were not tested in this study, nor did the researcher test whether cooking made a difference in terms of cancer-fighting ability. Liu cautions that human studies are needed before any definitive links between onion consumption and cancer-prevention can be established.
A European research network called Allium has been formed to promote better use of research results and facilitate future research on onions, the most widely cultivated horticultural crop in Europe.