Low-fat diet has fewer benefits than expected, at least for women

Older women who simply reduce the amount of total fat in their diet won’t generally lower their risk of breast or colorectal cancer or heart disease, according to the latest findings from the federally funded Women’s Health Initiative.

But a Stanford University School of Medicine researcher who helped direct the WHI work said the study showed a modest reduction in breast cancer among the women who started with the highest fat intake before cutting back. And it also suggested a health benefit for women who reduced their consumption of saturated and trans fats.

“Just switching to low-fat foods is not likely to yield much health benefit in most women,” said Marcia Stefanick, PhD, professor of medicine at the Stanford Prevention Research Center and chair of the WHI steering committee. “Rather than trying to eat ‘low-fat,’ women should focus on reducing saturated fats and trans fats.” She also recommended that women eat more vegetables, in particular dark, leafy greens and cruciferous vegetables, though the trial did not specifically study these foods.

The dietary findings are reported in three studies to be published in the Feb. 8 issue of the Journal of the American Medical Association. The studies showed that women on the low-fat diet experienced a 9 percent reduction in the incidence of breast cancer – but this was not statistically significant. (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 made no appreciable difference in the rates for colorectal cancer and cardiovascular disease.

The findings are the latest clinical results from the WHI, a 15-year, broad-based look at the causes and prevention of diseases affecting older women. Previous WHI studies have involved hormone therapy, heart disease and osteoporosis.

The low-fat diet study involved nearly 49,000 postmenopausal women between the ages of 50 and 79 who were tracked over the course of a little more than eight years on average, making it the nation’s largest long-term study of a low-fat diet. The goal was to test a widely held theory that low-fat diets helped reduce the risk of cancer and heart disease.

Forty percent of the participants were assigned to the low-fat diet, in which they were asked to reduce their 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 remaining 60 percent served as a comparison group and did not change their diet.

Although the primary hypotheses of protective effects of a low-fat diet on breast and colorectal cancer failed the test, the WHI researchers pointed out that the majority of women assigned to the low-fat diet didn’t meet the 20 percent fat goal: On average, the women reduced their fat intake to 24 percent in the first year, but slowly increased their fat intake to 29 percent by the eighth year.

Furthermore, the study showed that women who had the highest fat intake at the study’s outset showed greater evidence for reducing their breast cancer risk on the diet program. There was also a suggestive trend of breast cancer risk reduction for women who initially had the lowest consumption of vegetables and fruits and then increased their intake by one serving per day as part of the diet.

Regarding the heart disease findings, Stefanick noted that the women weren’t asked to differentiate between “good fats” (the unsaturated fats found in fish, nuts and vegetable oils) and “bad fats” (saturated fats and trans fats found in processed foods, meats and some dairy products), which is emphasized in current guidelines for heart disease reduction. “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.”

For women who want to maintain their health, Stefanick advised them 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 adequate 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.

The low-fat diet study is one of three randomized clinical trials that are part of the WHI. A previous trial on hormone therapy showed that estrogen and a combination of estrogen and progestin increased risks in such diseases as stroke, blood clots and breast cancer among postmenopausal women. The third study on the effects of calcium and Vitamin D on bone fractures and colorectal cancer will be released later this month.

From Stanford University

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