Medical Instructor Michelle Holmes. Photo by Jon Chase.

Go ahead, lady, have another doughnut. A high-fat diet may not be healthy but it won't increase your risk for breast cancer, according to the largest single study to date on this subject.

Researchers at Harvard Medical School followed 88,795 nurses for 14 years and concluded that intake of fat -- animal, vegetable, saturated, polyunsaturated, monounsaturated, trans-unsaturated, or cholesterol -- doesn't increase your chances of getting the cancer.

"Women should make decisions about their fat intake based on preventing other diseases such as heart disease," says Michelle Holmes, lead author of a report on the study published March 10 in the Journal of the American Medical Association.

A high intake of fat was believed to increase breast cancer risk on the basis of animal studies and comparisons of diets in different countries, particularly the low rates of breast cancer in Asian nations, where relatively little fat is consumed. However, the evidence did not convince Holmes and her colleagues at Harvard and Brigham and Women's Hospital in Boston.

They analyzed information on fat intake and breast cancer obtained from periodic questionnaires filled out by nurses from 1980 to 1994. During this time, 2,956 of the women got the disease, but these cases could not be tied to what they ate.

For a low-fat diet to protect against breast cancer, some experts insist that women must consume no more than 20 percent of their calories in fat. Most women in the United States consume closer to 30 percent, so a reduction in risk would not be detectable. However, 20 women in the study who ate 20 percent or less of fat calories each day did get breast cancer, a high enough number to demonstrate a lack of protection.

Holmes noted that there was one "completely unexpected result. We found an increased risk of breast cancer associated with omega-3 [fat] from fish," she notes. "Animal studies have indicated the opposite, so we're going to have to study this further."

Lack of evidence that a low-fat diet decreases risk of breast cancer suggests that reductions in dietary fat during the middle years of life "are unlikely to prevent the disease and should receive less emphasis," the team reported. "Rather, women's decisions about fat intake should be guided primarily by risk of heart disease, which is strongly influenced by the type, not the total amount, of fat."

Holmes notes that "saturated fat found in meats and dairy foods, and trans-unsaturated fat found in margarine, packaged cookies, and fast foods increase the risk of heart disease. But polyunsaturated and monounsaturated fats from vegetable oils actually decrease the risk."