MEDICINE / BREAST CANCER : Study Finds No Link of Fat in Diet to Malignancy Risk

The amount of fat in a middle-aged woman’s diet does not affect her risk of developing breast cancer, researchers report today in a study that is rekindling a longstanding debate over the effect of diet on cancer.

Other recent studies have hinted that fat consumption is probably not a factor in breast cancer, but today’s study in the Journal of the American Medical Assn. is the largest and longest-running to cast doubt on a popular theory linking dietary fat and breast cancer.

Moreover, the new study included a group of women on low-fat diets and concludes that even those diets do not protect against breast cancer.

But the issue of dietary fat and breast cancer will not be resolved with this latest finding, a number of physicians said in interviews Tuesday.

“I really don’t understand what is happening in (that) study. I suspect there is a serious methodological problem with the way we’re going about diet and cancer studies,” said Dr. Ross Prentice of the Fred Hutchinson Cancer Research Center in Seattle, who is coordinating another large study that will look at the health effects of a low-fat diet among women.

The suggestion that fatty diets contribute to breast cancer stems from observations that women in countries where low-fat diets are common have lower rates of breast cancer. Various other studies of dietary fat and breast cancer have produced ambiguous results--some finding a connection, others not.

“I think this study has swung the evidence in favor of a lack of association,” said Geoffrey R. Howe of the University of Toronto, who previously found a weak connection between dietary fat and breast cancer. But Howe said the issue remains unresolved because other studies, including his, have shown a weak correlation.

In an editorial accompanying the study, Howe suggested that even a weak association might contribute to a substantial number of breast cancer cases. North American women have about a 10% lifetime risk of developing the disease.

But the new study, directed by Dr. Walter C. Willett of Brigham and Women’s Hospital in Boston, found no relationship between breast cancer and either dietary fat or the amount of fiber in the diet.

The analysis included 89,494 women who are part of the Nurses’ Health Study, a long-term study examining various components of the diet on health. The women, ages 34 to 59, completed questionnaires about their diets in 1980 and 1984 and also responded to inquiries between 1982 and 1988 about whether they had developed breast cancer.

Over the eight-year period, 1,439 women developed breast cancer, including 774 post-menopausal women.

But, said Willett: “We could find no differences in fat intake between this group and the rest of the women that would account for breast cancer.”

Based on the questionnaires completed in 1984, the amount of fat in the women’s diets ranged widely, with a mean of 33%. On average, Americans consume about 36% of their calories from fat. Public health officials, however, suggest that fat consumption should not exceed 30% of the diet. A low-fat diet has been linked to a reduced risk of cardiovascular disease and colon cancer.

“We don’t feel that these data say that dietary fat isn’t relevant to health,” said David Hunter of the Harvard School of Public Health, a co-author of the nurses study. “If you’ve adopted a low-fat diet, there are still many very good reasons to maintain that diet. It’s just that, for breast cancer, we’ll have to look elsewhere in terms of dietary and lifestyle factors.”

But other experts have suggested that diets of less than 25% fat, or even 20%, are required to reduce breast cancer risk.

“It is possible that both the Willett study and the many studies showing an adverse effect of dietary fat on breast cancer are correct,” said Dr. Peter Greenwald, director of the division of cancer prevention and control at the National Cancer Institute. “For women to reduce their risk of post-menopausal breast cancer by dietary measures, a reduction of dietary fat to 20% to 25% of total calories may be necessary.”

The nurses study included 7,000 women consuming diets of less than 25% fat but still found no association, Hunter said. There was not enough to data to reach a conclusion on diets of less than 20% fat, he said.

“We couldn’t totally exclude that an ultra-low-fat diet might not influence breast cancer risk,” he said. “But we found no evidence of any trend. We couldn’t even find a hint of a reduction among women consuming about 25% of their calories from fat.”

It is thought that fat intake might increase breast cancer risk by raising estrogen levels. But Brigham researchers suggested that women in countries with low breast cancer rates might be protected from the disease for reasons other than low dietary fat, including reproductive factors such as giving birth to their first child at a relatively young age.

Other studies looking at the diets of women who have already developed breast cancer have also found a link to a high-fat diet. But Hunter said those studies might be subject to “recall bias” in that the women are unable to accurately recall the specifics of their diets in the years preceding the development of breast cancer.

Prentice and Greenwald said the new findings should not thwart plans for the multimillion-dollar Women’s Health Initiative, a government-funded study that will explore, among other things, the health effects of a diet of 20% of total calories from fat.

But, Hunter said: “I guess our view is we shouldn’t wait for the results of that trial to become available before we vigorously pursue other hypotheses on breast cancer.”