University, Pa. -- July 2, 1997 -- In laboratory tests, garlic and onions block the formation of a potent carcinogen better than their milder cousin, the leek, a Penn State study has found.
Dr. John Milner, professor and head of the Department of Nutrition in Penn State's College of Health and Human Development, is leader of the study. He says the results are consistent with epidemiological evidence from China which shows that those who have a higher consumption of vegetables from the onion family have a reduced cancer risk.
The Penn State results are reported in the current issue (Vol. 28, No. 1) of the journal, Nutrition and Cancer, in a paper titled, "S-Allyl Cysteine Inhibits Nitrosomorpholine Formation and Bioactivation."
There is a growing body of evidence that plants from the garlic or Allium family, which includes onions, leeks and chives, are effective cancer fighters, says Milner. The Penn State study, however, is the first to show that not all Allium foods are equal in their ability to retard the formation of cancer causing compounds.
The researchers found that water extracts of garlic, deodorized garlic powder and onions each blocked the ability of two chemicals, nitrite and morpholine, to link to form N-nitrosomorpholine, a known liver carcinogen. The leeks' blocking ability, on the other hand, was minimal.
"Since deodorized garlic powder and garlic produced comparable results, the benefits cannot relate to odor causing constituents," Milner says.
N-nitrosomorpholine is a member of the nitrosamine family of chemicals, many of which are potent carcinogens. Most nitrosamines are produced naturally in the body from nitrates and nitrites in water and foods.
"Since garlic and onions can block the formation of N-nitrosomorpholine, it is likely it will reduce the formation of other nitrosamines, " Milner says.
The Penn State researchers also found that a water soluble, sulfur-containing compound found in processed garlic, called S-allyl cysteine, also depressed the formation of the carcinogen. In addition, the sulfur compound reduced N-nitrosomorpholine's ability to alter DNA, the genetic material present in cells.
"The benefits of eating garlic and onions is more than a blocking of nitrosamine formation but is also due to blocking their subsequent metabolism, Milner says.
"Well over 90 percent of nitrosamines are considered carcinogens," Milner noted, "anything you can do to counteract their effect should be important in reducing cancer risk." The study was supported, in part, with grants from the American Institute of Cancer Research and Wakunaga of America Co., Ltd.
Milner's co-authors are Mark E. Dion, who earned his master's degree at Penn State, and Melanie Agler, a Penn State undergraduate.