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Data Review Shows Fruits and Vegetables Can Block Major Cancers

Data Review Shows Fruits and Vegetables Can Block Major Cancers

When organizers asked Tim Byers, MD, MPH, to speak at the American Cancer Society's National Conference on Cancer Prevention and Early Detection, he liked their proposed title for his talk--"Nutrition: The Data Are There."

"I thought that was pretty fair," said Dr. Byers, of the University of Colorado School of Medicine, Denver. He then proceeded to drive the point home as he reviewed some 150 studies of nutrition's role in upper gastrointestinal (including oral, esophageal, and gastric), colorectal, breast, prostate, and lung cancers. The diseases included in his review of large case-control and prospective studies account for almost two-thirds of the cancer mortality in the United States.

British and US studies, he noted, indicate that nutrition and tobacco contribute equally to cancer's toll, with about one-third due to nutrition and one-third resulting from tobacco. "Amongst the three quarters of Americans that don't smoke, nutrition becomes the most important preventable cause of cancer," Dr. Byers said.

"I haven't tried to be selective in terms of the outcome, only selective in terms of the study size and design," he told the conference, sponsored, in part, by the Centers for Disease Control and Prevention.

Upper Gastrointestinal: Thirty-four studies--done at different times, by different investigators, in different countries, in different populations, using different methods of dietary intake--"consistently show a 30% to 40% reduction in risk for those who eat five servings of fruits and vegetables a day vs those who eat less than two a day," Dr. Byers said.

These results are independent of tobacco and alcohol use, he noted. "So there is something healthful about fruits and vegetables."

Colorectal: His analysis of 27 "of the better studies" suggested a risk reduction from a high fruit and vegetable diet also on the order of 30% to 40% on average. "In fact, the risk reduction for colon cancer may be substantially more than 50%, but conservatively, I think, there's going to be a halving of risk," Dr. Byers said.

Conversely, "the strongest studies" suggest a doubling of risk from a diet high in fat. "Epidemiologic studies, despite their uncertainties, paint for colorectal cancer a fairly compelling picture of a high-fat diet that generates increased risk and a high fruit and vegetable diet associated with decreased risk," he said. "Fruits and vegetables do reduce cancer risk at all the GI sites, from the mouth to the anus."

Breast: Clearly, the issue of whether a high-fat diet increases breast cancer risk stands today as one of the most controversial nutritional questions. A summary of 22 studies reveals a mixed picture, with some showing an increased risk on the order of 50% and some showing a reduced risk, he reported. "The best evidence to date is that modest reduction in fat intake, of the types we recommend for colon cancer, heart disease, and weight control, will not appreciably affect the breast cancer risk," he said.

However, this does not mean dropping fat intake to 20% of calories, as in the National Institutes of Health's Women's Health Initiative trial, will have no effect, he stressed. "There are treatment trials of very low fat diets that I think are very reasonable experiments to do."

As for fruits and vegetables, the data suggest a slight protective effect for breast cancer, but much weaker than for colon and lung cancer.

Alcohol intake presents quite a different story. An updated meta-analysis showed a 40% increase in breast cancer risk for women who consume alcohol, Dr. Byers said, although a number of studies suggest that moderate drinking helps protect against coronary heart disease.

"Despite the fact that one of my studies is one of the negatives [in showing an alcohol-cancer risk], I am convinced that alcohol is a risk factor for breast cancer, but I am equally convinced it's a protective factor for heart disease," he said. "I think women who are at high-risk of breast cancer could reasonably make an informed decision not to drink alcohol. For women who are at average risk for breast cancer and heart disease, it's a trade-off."

Prostate: In his review of 12 prostate studies, Dr. Byers found them inconsistent and indicative that fruits and vegetables do little or nothing to ward off prostate cancer.

With regard to dietary fat, however, "the strongest studies suggest about a 30% to 40% elevation in risk for prostate cancer for being in the upper quintile compared to the lowest quintile of dietary fat intake. Dietary fat is a risk factor for prostate cancer and is probably mediated through fat's effects on testosterone metabolism."

Lung: His analysis of 31 studies concluded a link exists between diet and lung cancer. All studies that controlled for tobacco exposure revealed "a fairly consistent lower risk for those in the upper quintile compared to those in the lower quintile of fruit and vegetable intake in the diet," he said. "And, indeed, there is a relationship as well with fat, but I don't think it's nearly as strong."

A number of substances in food have been suggested as cancer preventers. But Dr. Byers noted that studies to date have failed to identify any single one as a "magic bullet." He cited six major studies of beta-carotene. Only one, carried out in China in a population with high gastric cancer rates and micronutrient deficiencies, showed any benefit--and it used a combination that included selenium and vitamin E as well as beta-carotene. Two studies actually suggested that beta-carotene may have increased lung cancer risk.

Dr. Byers believes that these apparent ill effects of beta carotene confirm "that there is something beneficial in the natural goodness of whole foods and when you disturb that balance, you actually may trigger cancer to occur."

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