The hypothesis that low-fat diets could help reduce the risk of certain diseases had been assumed, but never tested. But, do the findings mean that what we eat doesn't matter?
"Nutrition knowledge has progressed dramatically since the study began," said Mara Vitolins, Dr.Ph., associate professor of public health sciences at Wake Forest University Baptist Medical Center. "Today, we know that reducing total fat may not be enough – we need to focus on the types of fat we eat."
Vitolins, a registered dietician, is an author on the three papers that report the results in the Feb. 8 edition of the Journal of the American Medical Association. Wake Forest was a Vanguard Center, one of 16 university sites chosen to launch the WHI.
The study compared a group of women who followed their normal eating patterns with a group who followed a study diet designed to reduce total fat. At the end of the first year, the low-fat diet group was consuming about 24 percent of calories from fat, compared to 35 percent in the normal-diet group.
At the end of the study's sixth year, the low-fat diet group consumed about 29 percent of calories from fat, compared to 37 percent in the normal-diet group. The low-fat diet group also increased their consumption of vegetables, fruits and grains.
Researchers found no difference between the two groups in terms of risk of breast cancer, colon cancer, heart disease or stroke.
Vitolins said one explanation for the results is that the low-fat diet was designed to reduce total fat and didn't make a distinction between good fats, such as those found in nuts, fish, and vegetables oils, and bad fats, such as the saturated fat in meats and the trans fat used in baked goods and potato chips.
"The study was testing the belief that lowering total fat would reduce the risk of cancer," said Vitolins. "Since the study began, we've learned a lot more about how the types of fats we consume make a difference."
Vitolins and others said the study's findings should not change recommendations for staying healthy.
"The results of this study do not change established recommendations on disease prevention," said National Heart, Lung, and Blood Institute Director Elizabeth G. Nabel, M.D., in an NIH press release. "Women should continue to get regular mammograms and screenings for colorectal cancer, and work with their doctors to reduce their risks for heart disease including following a diet low in saturated fat, trans fat and cholesterol."
Current dietary guidelines call for keeping saturated fats to less than 10 percent of calories, with most fats coming from fish, nuts and vegetable oils. They also call for limiting levels of trans fat. Vitolins said that only recently has trans fat become recognized as harmful – it must now be listed on food labels.
Vitolins also said it's important to remember that participants began the low-fat diet later in life. Women were 50 to 79 years old when the study began and were followed for an average of 8.1 years.
"Our diets start when we are born and it makes sense that what you eat over a lifetime will make a difference," she said.
About 3,300 women in the Piedmont Triad were participants in the Women's Health Initiative, according to local principal investigator Denise E. Bonds, M.D., M.P.H., assistant professor of public health sciences at Wake Forest University Baptist Medical Center. Of those, 988 participated in the diet study, which was one of three WHI trials. The other trials evaluated hormone therapy and the effects of calcium and vitamin D on prevention of osteoporosis-related bone fractures and colon cancer.
Nationally, the WHI enrolled 157,000 women between 50 and 79 years old at 40 clinical centers, making it the largest clinical trial ever undertaken in the United States.
Wake Forest University Baptist Medical Center is an academic health system comprised of North Carolina Baptist Hospital and Wake Forest University Health Sciences, which operates the university's School of Medicine. The system comprises 1,187 acute care, psychiatric, rehabilitation and long-term care beds and is consistently ranked as one of "America's Best Hospitals" by U.S. News & World Report.
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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