Eating lots of fruits and vegetables appears to confer some protection against many common cancers, including malignancies of the breast and lung. For years, researchers have been trying to understand why. A new study of 10,000 Finns now finds support for the notion that a substantial share of those benefits may trace to a family of plant pigments known as flavonoids.
The new report provides especially good news for apple lovers. It linked lung-cancer protection most strongly to diets high in this flavonoid-rich fruit. The study was launched in 1966. Throughout a 6-year period, it enrolled roughly 10,000 healthy men and women, age 15 years or older. The volunteers completed an extensive questionnaire on their medical history, smoking status, and food consumption patterns. Over the next 24 years, government researchers followed these persons via the national Finnish Cancer Registry to identify who developed disease. In all, 997 malignancies occurred, including 151 lung cancers.
Knowing that oxidative reactions in the body contribute to the development of many diseases and features of aging, Paul Knekt of the National Public Health Institute in Helsinki and his coworkers used the dietary data to estimate the volunteers consumption of antioxidants -- especially vitamins C and E, beta-carotene, and flavonoids. In the August 1 American Journal of Epidemiology, they now report finding that the individuals who developed cancer tended to be male, smokers, older, and leaner than those who didnt.
Diets also set cancer patients apart. In general, they had reported eating more saturated fats and fewer vegetables. But what most distinguished the eating patterns of people who went on to develop cancer was their significantly lower consumption of flavonoids. The new analysis showed that the more flavonoids a volunteer had consumed daily, the less likely he or she was to develop cancer.
Though the study found signs of a link between high flavonoid consumption and protection from cancers of the breast, nervous system, urinary tract, colon, and rectum, the tie proved statistically significant only for cancer of the lung. This association held even after accounting for consumption of other antioxidants, smoking, dietary fat, body weight, and other factors that might affect cancer risk.
Though flavonoid consumption within this Finnish group averaged about 4 milligrams per day, some individuals downed up to 10 times that much. Quercetin proved the most common, accounting for roughly 95 percent of all of flavonoids consumed. Together, apples and onions contributed two-third of that quercetin -- with berries, sweetened juices, jams, and vegetables adding much of the rest.
Because apples are not a good source of other antioxidants, "flavonoids are thus likely to provide the protection," Knekts group notes. Onions do contain other classes of anticancer compounds (Cancers do not savor garlic). However, because diets high in onions did not appear as protective in this population as those high in apples, the researchers conclude that its unlikely that anything other than flavonoids in onions are the primary source of cancer protection seen.
Many studies have shown that other flavonoids in tea and red wine also possess potential anticancer benefits. However, because consumption of both of these drinks tended to be low in this population -- which prefers coffee, beer, and hard liquor -- apple-derived quercetin remains the leading anticancer candidate.
Though apples and onions appeared to offer potent benefits in this population, other dietary constituents also seem to fend off cancer. In a second article in the same journal this week, scientists with the National Cancer Institute (NCI) and National Center for Health Statistics, both in Maryland, correlated diets rich in the more popular antioxidants -- vitamins C, E, and beta-carotene -- with reduced risk of lung cancer among roughly 10,000 U.S. men and women who were followed for 19 years.
People with the diets highest in these nutrients were only one-third as likely to develop the cancer as were those with diets low in them. Many fresh fruits and vegetables are good sources of C and beta-carotene. Grains, nuts, and vegetable oils generally contain lots of vitamin E. Though many people take supplements of one or more of these antioxidants, there appeared to be no additional anticancer benefit from doing so -- at least in this study. Not surprisingly, smoking largely erased the protective effect attributable to these nutrients.
Lee-Chen Yong of NCIs cancer-prevention studies branch and her coworkers conclude that while smoking cessation is the most important factor in lowering lung cancer risk within the population, "a daily diet consisting of a variety of fruits and vegetables that provide a natural source of these nutrients and other potential protective factors may offer the best dietary protection." Finally, these recommendations fit well with those made to persons interested in fighting heart disease, because antioxidants not only appear to inhibit the formation of artery-clogging plaque but also to maintain the health of blood vessels generally.
Knekt, P. et al. 1997. Dietary flavonoids and the risk of lung cancer and other malignant neoplasms. American Journal of Epidemiology 146(Aug. 1):223.
Yong, Lee-Chen, et al. Intake of vitamins E, C, and A and risk of lung cancer. American Journal of Epidemiology 146(Aug. 1):231.
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This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.