Diet’s Role in Breast Cancer Disputed

A 14-year study of nearly 89,000 American women has found no consistent evidence that a high-fat diet causes breast cancer or that a low-fat diet prevents it.

Appearing today in the Journal of the American Medical Assn., the study takes a surprisingly hard line in a heated debate that has long vexed cancer researchers and women. Breast cancer is the most common cancer among women and the second deadliest, after lung cancer.

The study, which tracked more women for a longer time than any breast cancer study has done so far, appears to undermine recent best-selling books that have aroused controversy by blithely touting a low-fat diet as part of a strategy for reducing breast cancer risk.

“These findings suggest that reductions in total fat intake during midlife are unlikely to prevent breast cancer and should receive less emphasis,” wrote the team of eight Harvard-affiliated researchers, led by Dr. Michelle Holmes of Brigham and Women’s Hospital in Boston.

At the same time, other medical researchers caution that fat intake may have a cancer causing role that the Harvard study was not able to detect.

Dr. John Glaspy, a physician and scientist at UCLA’s Jonsson Comprehensive Cancer Center, said the study was based partly on dietary questionnaires, which can be misleading. And, he said, it only considers a decade and a half of dietary habits, while fat may have a more insidious, lifelong role.

“We should just accept that good scientists can’t tell you yet what to eat to minimize your breast cancer risk,” he said.

Despite the controversy over fat’s role in breast cancer, medical researchers tend to agree that minimizing fat intake and maintaining a healthy weight reduce the risk of other serious afflictions, including heart disease, stroke and diabetes.

Breast cancer struck about 178,700 women in 1998 and killed about 43,000, according to the American Cancer Society.

The theory that dietary fat may play a role in breast cancer is based on animal studies, analyses of breast cancer rates in different nations, and U.S. comparisons of women who did and did not develop breast cancer.

Taking the idea even further, other researchers have suggested that the key to the proposed fat link is the type of fat consumed, rather than the total amount. For instance, some studies have suggested that the omega-3 fats found in fish oils may offer some protection.

The Harvard study involved 88,795 health professionals in the ongoing Nurses’ Health Study. The women completed detailed dietary questionnaires in 1980, 1984, 1986 and 1990.

Analyzing the data, the researchers divided the women into eight groups, depending on their fat intake, ranging from more than 50% of total calories to 20% or less. Public health authorities generally recommend a total fat intake of 20% to 30% of calories.

Overall, 2,956 women developed breast cancer, and the rate of the disease was virtually the same regardless of fat intake.

Nor did the researchers find that the type of fat consumed had any impact on breast cancer risk, whether animal fat, polyunsaturated fat (vegetable fat) or trans-unsaturated fat (also known as partially hydrogenated oils).

Similarly, breast cancer was no less common among women who got a high proportion of their fats from fish oil or who got less than 20% 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,” Holmes said. “Equally, it appears a high-fat diet also poses no increased risk for the disease.”

Surprisingly, women who ate the least fat appeared to have a 15% higher rate of breast cancer, the researchers said. But Holmes said she is not ready to conclude that a low-fat diet increases breast cancer risk, because the finding was based on the fewer than 1,000 women who ate less than 20% of calories as fat.

The new data need explaining, Glaspy said. “The stakes are very high.”

Previous studies have shown that women in traditional Asian societies had low breast cancer rates--until they moved to the West and started eating fattier diets.

The Associated Press contributed to this story.