A popular theory claims low-fat diets reduce the risk of cancer and heart disease. But results of a vast new research project suggest that at least for older women, it’s not so.

The research, which involved the largest long-term study of a low-fat diet in the United States, found that simply eating less fat won’t generally lower a woman’s risk of cancer or heart disease, scientists said.

The study involved nearly 49,000 women between ages 50 and 79, who were tracked over the course of about eight years on average. It was part of a federally funded research project, the Women’s Health Initiative.

The study showed a modest reduction in breast cancer among the women who started with the highest fat intake before cutting back, said a researcher at the Stanford University School of Medicine in Stanford, Calif., who helped direct the work. It also suggested a health benefit for women who ate less of two types of fats, called saturated and trans fats.

“Just switching to low-fat foods is not likely to yield much health benefit in most women,” said Marcia Stefanick of Stanford, chairman of the steering committee for the initiative. “Rather than trying to eat ‘low-fat,’ women should focus on reducing saturated fats and trans fats.”

She also suggested women eat more vegetables, especially dark, leafy greens and cruciferous vegetables, though the trial did not specifically study these foods. Cruciferous vegetables include cabbage, broccoli, radishes, turnips, cauliflower, brussel sprouts, collards, and kale.

The findings are reported in the Feb. 8 issue of the Journal of the American Medical Association.

The research showed that women on a low-fat diet had a 9 percent lower incidence of breast cancer, but this reduction was too small to be considered statistically significant, scientists said. For instance, in a group of 10,000 women, 42 in the low-fat diet group developed breast cancer each year compared with 45 in the group maintaining their regular eating patterns.

The dietary changes also made no appreciable difference in the rates for colorectal cancer and cardiovascular disease, according to the researchers.

Forty percent of the participants were assigned to the low-fat diet, in which they were asked to reduce fat intake to 20 percent of their total calories while eating five or more daily servings of vegetables and fruits, and six servings of grains. The other 60 percent served as a comparison group and did not change their diet.

Although the theory that low-fat diet would prevent breast and colorectal cancer failed the test, the researchers noted, most of the women assigned to the low-fat diet didn’t meet the 20-percent fat goal. On average, they cut fat intake to 24 percent in the first year, then slowly increased it to 29 percent by the end.

The study found women with the highest initial fat intake showed more evidence for reducing their breast cancer risk on the diet, researchers said. And there was a suggestive trend, they added, of lower breast cancer risk for women who initially had the lowest vegetable and fruit intake, then raised that by one daily serving as part of the diet.

Stefanick noted that the women weren’t asked to differentiate between “good fats”—the so-called unsaturated fats, found in fish, nuts and vegetable oils—and “bad fats,” which are the saturated fats and “trans fats” found in processed foods, meats and some dairy products. This distinction, related to the molecular structure of the fats, is emphasized in current guidelines for preventing heart disease.

“This shows that you can’t rely on using low-fat substitutes to make a difference,” Stefanick said. “You really need to think about what kinds of fats you’re eating and the foods that should be part of your diet, such as vegetables, for instance.”

Stefanick advised women to follow a diet low in saturated and trans fats, and rich in vegetables and fiber rather than to strive to eat “low-fat” foods. She also advised them to pay attention to total caloric intake regardless of diet composition and to get regular exercise. Women should also get routine mammograms and screenings for colorectal cancer and heart disease risk, including checking their cholesterol profile, blood pressure, blood sugar and body weight, she added.


Courtesy Stanford University Medical Center
and World Science staff

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.