Unsaturated fats play yin-yang cancer role.

Five years ago, researchers at the Harvard School of Public Health in Boston proclaimed that dietary fat appears to play no direct role in breast cancer risk (SN: 10/24/92, p. 276). That was then.

This week, the same researchers, along with a group of Swedish colleagues, came to a different conclusion. When they distinguished between fats, they found that monounsaturated fats, characteristic of olive and canola oils, appear to protect against breast cancer, while polyunsaturated fats in vegetable oils seem to enhance risk. Saturated fats--a major risk factor for heart disease--had no effect.

After collecting detailed dietary data from more than 61,000 Swedish women between the ages of 40 and 76, the researchers followed the women for 4 years. During that time, 674 developed invasive breast cancer. To determine any influence of foods, the scientists compared the diets of the cancer patients to those of the cancerfree women.

Even after accounting for standard breast cancer risks, such as having a family history of the disease, the effects of unsaturated fats stood out, the researchers report in the Jan. 12 Archives of Internal Medicine. For each 10 grams of monounsaturates that a woman consumed daily, the risk of breast cancer fell by 55 percent. However, that risk increased by almost 70 percent for each 5 grams of polyunsaturates downed per day.

The take-home message, says study leader Alicja Wolk of the Karolinska Institute in Stockholm, "should not be to add monounsaturated fats to one's current diet, but rather to substitute them for another fat [already being eaten]." Indeed, her group found that as calorie consumption climbed, so did cancer risk, regardless of the types of fats eaten.

Studies over the past 4 years have shown that breast cancer rates are low among Mediterranean women who eat a lot of olive oil, but they haven't determined whether there is something unusual about this oil. Because the Swedish women derived most of their monounsaturates from dairy products and meat, Wolk says, "we can now say monounsaturates are protective--whatever their source." Such animal products, though rich in saturates, can be major sources of monounsaturates.

That monounsaturates might inhibit some cancers and polyunsaturates spur them has long been evident from studies by Leonard A. Cohen of the American Health Foundation in Valhalla, N.Y., and others. "But [because they were] done in animals, few people paid attention to them," he says. In fact, he had trouble getting a review published in 1990 outlining the mechanisms by which monounsaturates may prevent breast cancer. It was "too far out," he says.

Lenore Kohlmeier of the University of North Carolina at Chapel Hill and her colleagues have turned up evidence of another risky fat--the trans fatty acids in margarines and vegetable shortening (SN: 5/21/94, p. 325). In the September 1997 Cancer Epidemiology, Biomarkers & Prevention, the researchers report that these fats also appear to increase breast cancer risk.