The diet–cancer prevention connection is big news. The American Institute for Cancer Research (AICR), Washington, D.C., reports 30% to 40% of all cancers are directly related to diet, exercise and weight control. While many foods are being explored regarding their ability to help reduce the risk, scientists are keenly interested in the cancer-fighting potential of flavonoids, found in a wide variety of plant foods.
The AICR recommends people consume two-thirds or more of their meals in plant foods, such as vegetables, fruits, whole grains and beans, to reduce the risk of cancer. Populations with high dietary intake of fruits and vegetables are consistently associated with a reduced risk of common cancers, including cancers of the lung, breast, prostate and colon. The average American eats less than two servings per day of fruits and vegetables.
Understanding flavonoids
Flavonoids, a large family of polyphenolic compounds synthesized by plants with a similar structure, are divided into subclasses, including anthocyanidins, flavanols, flavanones, flavonols, flavones and isoflavones. Foods that contain flavonoids include tea, red wine, fruits, vegetables and legumes. The total average flavonoid intake in Western populations comes to about 150 to 200 mg per day.
Flavonoids occur in plants and most foods as glycosides, with the exception of flavanols. Most flavonoid glycosides reach the small intestine intact, even after cooking. Only flavonoid aglycones and flavonoid glucosides are absorbed in the small intestine, where they are quickly metabolized to form methylated, glucuronidated or sulfated metabolites. Beneficial bacteria in the colon play an important role in flavonoid metabolism and absorption. But the bioavailability of flavonoids is relatively low, because of limited absorption and rapid elimination.
The power of flavonoids
Although it is unclear how flavonoids work to protect against cancer, many of their biological actions have been attributed to their antioxidant activity. Of all the antioxidants found in fruits and vegetables, it is the flavonoids that people consume in greatest amounts, sometimes up to 1 gram per day.
Flavonoids scavenge free radicals in the test tube; but, even with high flavonoid intakes, concentrations in humans are probably 100 to 1,000 times lower than concentrations of other antioxidants. Flavonoid metabolites have even lower antioxidant activity than the parent flavonoid. Thus, antioxidant activity is not considered the only mechanism at play.
Many in vitro and animal model studies suggest flavonoids influence important cellular and molecular mechanisms related to carcinogenesis, but it is still unclear how high flavonoid intake can help prevent cancer in humans.
Flavonoids inhibit the development of induced lung, esophageal, stomach, colon, skin, prostate and breast cancers in animals. Yet, epidemiological studies do not support a link between high intakes of dietary flavonoids and significant decreases in human cancer risk.
Favorite flavonoids
Some rising stars can be found within the flavonoid family. Consistent evidence from studies indicates that quercetin, mainly from apples, might reduce the risk of lung cancer. Flavonoids found in green and black tea also show cancer preventive activity in a variety of animal cancers, including skin, lung, mouth, esophagus, stomach, colon, pancreas, bladder and prostate. David Heber, M.D., Ph.D., director, UCLA Center for Human Nutrition, Los Angeles, reports much of the research in the flavonoid field has centered upon cruciferous vegetables that contain glucosinolates, including broccoli, Brussels sprouts, cabbage, Chinese cabbage, wasabi, horseradish and kale.
Flavonoid functionality
Because scientists believe the benefits of flavonoid- rich foods may come from a number of compounds working together in the whole food, the jury is still out on whether isolated flavonoid supplement extracts can provide the same benefits. According to a study published in the
Journal of Food Science (2006; 4:S358-S363), significant synergistic antiproliferative effects were demonstrated in animal and human cancer cell lines when flavonoids were provided in combination treatments. This suggests the combinations of flavonoids, which are naturally present in whole fruits and vegetables, are more effective in cancer-cell growth inhibition than the individual flavonoids.
Popular flavonoid supplements include bilberry, elderberry, black currant, blueberry, red grape and mixed berry extracts rich in anthocyanins; numerous tea extracts that may be labeled as tea catechins or tea polyphenols; citrus bioflavonoid supplements; and the flavonol aglycone, quercetin and its glycoside rutin.
Sharon Palmer is a registered dietitian with 16 years of experience in health-care and foodservice management. She writes on food and nutrition for newspapers, magazines, websites and books. Palmer makes her home in Southern California and can be reached at info@sharonpalmer.net.