Study Undercuts Cancer-Protective Role for Fruits And Vegetables

      Consuming plenty of fruits and vegetables is still a healthy way to eat, but using colon cancer protection as a motive may no longer be justified, according to new research.

      The study from Harvard researchers generated both controversy and consternation recently by disputing long-held assumptions that fiber from fruits and vegetables provides some protection against colon cancer. According to their research, which analyzed the self-assessed eating habits of over 136,000 health care workers, overall there is neither a positive nor a negative connection between colon cancer and consumption of fruits and vegetables.

      Several top cancer prevention specialists were quick to downplay the report as "a single set of findings," calling it contradictory "to the bulk of available evidence." Yet other recent studies have failed to prove fiber has any effect against precancerous growths in the colon (see a related story ).

      The new study, published in a recent issue of the Journal of the National Cancer Institute, did turn up some intriguing correlations. For example, prunes were associated with an elevated risk of colon cancer, while legumes and multivitamins were connected with a reduced risk of the disease.

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      "Our finding of an elevated relative risk of colon cancer associated with prune consumption was unexpected and merits examination in other studies," wrote the researchers. But this surprising finding, they said, may be the result of "reverse causality"; people experiencing constipation due to an intestinal blockage from an undiagnosed tumor may eat more prunes, which have laxative properties, to encourage bowel movements.

      "However," the researchers added, "neither constipation nor laxative use was associated with colon cancer incidence in the Nurses Health Study." The NHS was one of two large groups of health care workers whose diets were analyzed.

      In addition to the unexpected finding about prunes, the Harvard report said legumes, according to smaller studies, might be associated with reduced colon cancer risk.

      "In a recent incidence analysis [of Seventh-Day Adventists] higher consumption of cooked green vegetables or of salad was not associated with a statistically significant protection from colon cancer, but consumption of legumes more than twice a week was associated with a reduced relative risk of colon cancer," the researchers wrote.

      What''s the Explanation?

      The researchers offered several possible reasons for their negative findings on fruits and vegetables. They noted that people who eat more fruits and vegetables also tend to eat less red meat, exercise more, take vitamins, and are less likely to drink and smoke. All or some of these co-factors, alone or in combination, may offer some colorectal cancer prevention.

      Or, they postulate, early fiber intake -- 15 or more years before a colon cancer diagnosis

      -- may be the key. Or perhaps, they offered further, Americans now get enough folate, which may fight cancer, from breakfast cereals, vitamins and orange juice, making fruits and vegetables less important as a source of this B vitamin.

      Another possibility is that it''s the vitamins in fruits and vegetables, not the fiber, that offers protection against cancer. They cited one study of health care professionals that linked long-term multivitamin supplement usage with a reduced risk of colon cancer.

      "The fact that multivitamins may have an effect does suggest that it is the vitamins themselves and not some other constituent of vitamin-rich foods," said Dartmouth epidemiologist John A. Baron, M.D. "However, this is a central question that has not been answered yet." He downplayed the study''s results about legumes and prunes, suggesting they may be chance findings.

      As for the study itself, Baron said it probably could have benefited from more participant diversity. However, he added, the wide range of fruits and vegetables reportedly consumed among participants may compensate somewhat for that shortcoming.

      Baron agreed with the report where it suggested that the length of time fruits and vegetables are eaten and their variety may be the key to any colon cancer prevention, but disagreed where it suggested that foods containing folate may replace their importance in the diet. "Of course it is conceivable that longer-term fruit and vegetable intake would matter," he said. "But it certainly looks as though fruits and vegetables are doing it."

      The Harvard researchers said there are still good reasons to eat plenty of fruits and vegetables. "A diet rich in these foods remains advisable," they said," because it conveys protection against other diseases, such as cardiovascular disease and possibly other cancers."

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