Lead researcher Najla Guthrie, PhD, president of KGK Synergize Inc., studied the effects of 22 flavonoids on growth of prostate, lung, colon and skin (melanoma) cancer cells in laboratory dishes. The U.S. Department of Agriculture (USDA) collaborated in the research.

Two flavonoids, tangeretin and nobiletin, which are found in tangerine juice, "were the most effective inhibitors of human prostate cancer cells," according to Dr. Guthrie and John Manthey, PhD, of the USDA?s Agricultural Research Station in Winter Haven, Fla. Those compounds also appear to inhibit the growth of melanoma cells, according to Dr. Guthrie.

Another flavonoid found in tangerines and oranges, 5-desmethyl sinensetin, appears to inhibit lung cancer cells, according to Dr. Guthrie. In addition, a synthetic flavonoid known as quercetrin tetramethyl ether appeared to inhibit the growth of colon cancer cells in the studies.

"We looked at 22 flavonoids ? some naturally occurring, some synthetic ? and these are the most effective of them," Dr. Guthrie says. "The next step is to look at them in animals."

Although animal studies have begun, it is too early to report any findings, she says. Dr. Guthrie expects some results within about a year.

Experts know diets high in fruits and vegetables are beneficial in reducing a person?s risk for developing several types of cancer, but it?s not clear yet what part flavonoids play in that effect, says David Ringer, PhD, MPH, a scientific program director for the American Cancer Society (ACS). "There are more than 4,000 known, naturally occurring flavonoids, so there is quite a bit of research yet to be done in order to demonstrate which flavonoids are important and to what extent they account for the beneficial effects of a diet high in fruits and vegetables," he says.

The research by Dr. Guthrie and her colleagues is still at an early stage, Dr. Ringer says. "These studies are targeting one group of flavonoid compounds in citrus and are at an early stage of research investigation ? looking at effects on cells in culture."

Cancer risk can be reduced by an overall dietary pattern that includes a high proportion of plant foods (fruits, vegetables, grains and beans), limited amounts of high-fat foods (especially those from animal sources) and a balance of caloric intake and physical activity, according to ACS guidelines. The ACS recommends eating at least five servings of fruits and vegetables a day. These recommendations are based on many studies that correlate nutrition with cancer risk.

Much less is known about the role of diet on growth of cancers that have already formed. For most people with cancer, a balanced diet as outlined above is still the best choice. Eating large amounts of any one type of food is not recommended by the ACS.