Folk wisdom that eating fruits and vegetables is healthy found scientific backing when several bodies of research, some now more than 30 years old, showed that people whose diets were high in fruits and vegetables seemed to have a reduced risk of some kinds of cancer, most notably lung and colorectal cancers.
The evidence appeared so strong that in 1991 the National Cancer Institute started the "5 A Day for Better Health" program to increase consumption of fruits and vegetables in the general population. (That program is currently undergoing a review, and a report is scheduled to be out this week.) By 1997, when a group of epidemiologists convened by the American Institute for Cancer Research endorsed vegetables as part of a diet that would reduce risk of colorectal cancer with the "highest degree of scientific certainty," it seemed clear that produce had an important role to play in cancer prevention.
But now the messages about the benefits of diets high in fruits and vegetables are being clouded by recent studies of stronger scientific design that have found that such a diet may not be protective against cancer. Earlier this year, two studies (New England Journal of Medicine, April 20) slammed the theory that high-fiber diets (which are often high in fruits and vegetables) guard against that disease, finding instead that patients with high-fiber diets had no reduction in colorectal polyps.
And two new prospective studies from the Nurses Health Study and Health Professionals Follow-up Study cohorts have also yielded unexpected results. In the first, which appears in this issue of the Journal (p. 1812), a group of investigators led by Diane Feskanich, Sc.D., of Harvard Medical School, found that diets high in fruits and vegetables were not associated with a substantial reduction in the risk of lung cancer; risk was reduced only in women and only by 21% in the highest intake group.
Another study of the same cohorts appeared in the Nov. 1 issue of the Journal. In this report, Karin B. Michels, Sc.D., of Harvard Medical School, led a group of researchers in a look at the relationship between fruit and vegetable consumption and risk of colorectal cancer. They found that a diet rich in fruits and vegetables also was not associated with a reduced risk of that disease. "It was a bit of a shock and a disappointment," said Michels. "We all expected to see an important relationship between diet and cancer, and we havent."
Recent findings have left many experts scratching their heads and many consumers wondering what to eat. But its not time to throw out the veggies just yet. "No one study can discount a lot of other studies," said Regina Ziegler, M.D., a senior investigator in the Epidemiology and Biostatistics Program at NCI, and a co-author of the lung cancer study. "A lot of good studies have shown a protective effect for fruits and vegetables and we cant discount that."
What they can do is take a closer look at the design of diet studies. "It may be that fruits and vegetables have no impact on cancer, or it may be that we dont have the proper measurement tools to find the impact," said Arthur Schatzkin, M.D., of NCIs Division of Cancer Epidemiology and Genetics, who wrote an editorial that accompanied the colorectal cancer study.
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In addition, many of earliest studies on diet and cancerand those that showed the greatest risk reduction from eating fruits and vegetableswere casecontrol studies, which are widely recognized as being less reliable for diet assessment, noted Walter Willett, M.D., Dr.P.H., a co-author of both Harvard cohort studies.
"If you ask people with cancer to recall what they ate, their answers are biased by the fact that they have cancer," he said. "By the same token, the control subjects tend to be more health conscious than the average person, so they might not be ideal controls either." It is possible that this fact alone could have resulted in overestimates of the risk reduction possible from dietary changes.
Another limitation lies in the amount of fruits and vegetables that study subjects eat; in both Feskanichs study and Michels study, the overall consumption was not high. Only 2% of the Nurses cohort ate more than 4.5 servings of produce a day, and only 3% of the Health Professionals group ate more than 3.5 servings a day. "These studies cannot rule out the possibility that more substantial intake would be associated with lower risk," wrote Schatzkin.
Some investigators also think that they could be looking at the wrong time points in the development of cancer. "Its possible that we could be assessing diet too close to the point of disease," said Michels. If cancer development starts early in life, then beginning diet assessment in middle age (or later) might not give an accurate picture of the relationship between diet and cancer. For example, women in the Nurses cohort were 30 years old and older, and the men in the Health Professionals group were 40 years old and older when their fruit and vegetable consumption was assessed.
Although the results of recent studies have allowed new doubt to creep in about the role of fruits and vegetables in cancer prevention, the idea that they might lower cancer risk "is still very much alive and worthy of continued investigation," Schatzkin said in his editorial. In an interview he added that the ideal investigation would make use of biomarkers of dietary intake. A few good biomarkers exist nowurinary nitrogen to measure dietary protein, for examplebut there are not enough to create a real gold standard.
Until enough tools exist, there are still good reasons to eat a diet rich in fruits and vegetables. There is evidence that such eating patterns are associated with reduced risk of heart disease, stroke, and high blood pressure, as well as obesity. High intake of fruits and vegetables is one of the few consistent health and nutrition messages in the United States, embraced by groups from the American Heart Association to the U.S. Department of Agriculture.
"Even though these new studies have been somewhat disappointing, we shouldnt change our recommendations because there are so many other clear health benefits to a diet rich in fruits and vegetables," said Willett.
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