CHICAGO -- Contradicting current medical theories, a 14-year study of nearly 89,000 women found no evidence that a high-fat diet promotes breast cancer or that a low-fat diet protects against it.
Experts were quick to note that a low-fat diet is still good for the heart and other aspects of health. They said the study indicates a need to look more carefully at how diet may affect the risk of breast cancer.
"We should just accept that good scientists can't tell you yet what to eat to minimize your breast-cancer risk," said Dr. John A. Glaspy of the University of California at Los Angeles, who was not involved in the research.
The Harvard study was published in Wednesday's Journal of the American Medical Association.
Doctors have theorized that eating lots of fat increases breast-cancer risk. They have based their thinking on animal studies, international comparisons and studies of women who developed breast cancer and women who didn't.
Others have suggested that the key is the type of fat consumed, rather than the amount; that fish oil offers some protection from breast cancer; or that contaminants stored in fat trigger breast cancer.
The new study tracked 88,795 women in the continuing Nurses' Health Study. The women, ages 30 to 55, completed detailed questionnaires about their eating habits every four years from 1980 to 1994.
Researchers compared the diets of the women without breast cancer and the 2,956 women whose breast cancer was discovered during the course of the study.
Breast cancer was found to be no more common among women who ate lots of fat, or among those who ate a large proportion of animal fat, polyunsaturated fat (vegetable fat) or trans-unsaturated fat (partially hydrogenated oils, such as those used in margarine and to cook doughnuts and french fries).
Nor was breast cancer any less common in women who got a high proportion of their fats from fish oil or who got less than 20 percent of their total calories from fat.
"Our research indicates it's highly unlikely that women who consume a low-fat diet are protected against breast cancer," said the study's lead author, Dr. Michelle D. Holmes, an instructor of medicine at Harvard Medical School and Brigham and Women's Hospital in Boston. "Equally, it appears a high-fat diet also poses no increased risk for the disease."
Most surprisingly, women who ate the least fat appeared to have a 15 percent higher rate of breast cancer, the researchers said. But Holmes said she's not ready to conclude that a low-fat diet increases breast cancer risk, because the finding was based on fewer than 1,000 women who ate less than 20 percent of calories as fat.
UCLA's Glaspy said the new research "is something that we're going to need to explain."
He said fat intake may need to be very low -- as little as 10 percent of total calories -- to reduce breast-cancer risk. He also noted that only some kinds of fish oil have appeared beneficial in previous studies. The new study did not distinguish between types.
Nor did it run long enough to explore the possibility that dietary effects may take longer than 14 years to emerge, he said. Glaspy believes that diet interacts with environmental factors in subtler ways than scientists have been able to identify.
The theory that dietary fat is linked to breast cancer arose from the observation that breast-cancer rates are far lower in traditional Asian cultures, where diets are generally low in fat.
Compared with women in Western cultures, traditional Asian women start menstruating later, give birth at a younger age and gain far less weight in adulthood -- all factors that decrease breast-cancer risk, Holmes noted.