"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," says study leader Rui Hai Liu, M.D., Ph.D., a chemist with Cornell's Department of Food Science in Ithaca, N.Y.
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. The new study will appear in the Nov. 3 print issue of the Journal of Agricultural and Food Chemistry, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.
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. 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 flavor -- 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 flavor -- Empire Sweet, Western White, Peruvian Sweet, Mexico, Texas 1015, Imperial Valley Sweet and Vidalia -- had the lowest total antioxidant activity, Liu says.
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 says. 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.
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.
While popular as fried "rings," onions are known mostly for their ability to add flavor to a variety of food dishes, including meats, pizza, soups and salads. But they are increasingly becoming known for their potential health benefits. Onions are rich in a flavor compound known as quercetin, a potent antioxidant that has been linked to protection against cataracts and heart disease as well as cancer. They are also sodium, fat and cholesterol free.
Onions are the third-most consumed vegetable crop in the United States, with a per capita consumption estimated around 19 pounds per year and a retail value estimated at $3 billion to $4 billion, according to the National Onion Association.
Onions can be part of a healthy diet. The National Cancer Institute recommends eating at least five servings of fruits and vegetables a day.
The New York State Department of Agriculture and Markets provided funding for this study.
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