Flavonoids are plant chemicals that give a wide variety of fruits and vegetables their unique flavors.

Researchers led by Loic Le Marchand, MD, of the University of Hawaii?s Cancer Research Center, conducted interviews between 1992 and 1997 to compare in detail the food intake of 582 people diagnosed with lung cancer to the food intake of the same number of randomly-selected Hawaiians who did not have cancer.

The researchers found lung cancer risk was lowered by 40 to 50 percent among the people who ate the most apples, white grapefruit, and onions compared to those who ate the least amounts of those foods. Those foods contain high amounts of the flavonoids quercetin and naringin.

Like the best-known flavonoid, beta-carotene, which gives carrots their color and flavor, many flavonoids have antioxidant properties. Antioxidants are thought to reduce the risk of some cancers. But the researchers believe flavonoids may protect in another way ? by disarming cell enzymes that transform potentially dangerous chemicals into cancer-causing chemicals that damage a cell?s DNA.

"Many of the chemicals which can cause cancer don?t do so directly, but need activation by cell enzymes before they can start the process that leads to cancer," said Dr. Le Marchand. "And some flavonoids may suppress the activity of those enzymes."

It?s important to remember this study found the foods themselves were associated with lower cancer risk, not the individual flavonoids, said Dr. Le Marchand. More research will be needed to learn if the flavonoids alone might be responsible for the lower cancer risk.

"Many nutrition researchers believe that fruits and vegetables contain a number of protective [chemicals] that may act together to protect against disease," said Dr. Le Marchand. "And it may be that not all the flavonoids and their sources are known yet. For that reason, it?s important to eat a wide variety of fruits and vegetables, including apples, grapefruit and onions, on a regular basis," he added.

David Ringer, PhD, MPH, scientific program director at the American Cancer Society (ACS), agrees. "There are at least 4,000 known flavonoid compounds in fruits and vegetables, and to try to conclude that the beneficial effects of these compounds in fruits and vegetables are due to just one or two specific flavonoids is very difficult; the proof is just not there."

Both Dr. Le Marchand and Dr. Ringer suggest following the ACS nutrition guidelines, which call for eating at least five servings of fruits and vegetables a day to reduce risk of cancer. A medium-sized apple, banana, or orange equals one serving of fruit, as does three-quarters of a cup of fruit juice or a half-cup of chopped, cooked, or canned fruit. A cup of raw leafy vegetables equals one serving of vegetables ? and you can get a serving by having a half-cup of chopped vegetables, or three-quarters of a cup of vegetable juice.