By LEE BOWMAN
Scripps Howard News Service
February 12, 2006
The three studies, published Wednesday in The Journal of the American Medical Association, were done through the Women's Health Initiative (WHI), a 15-year look at the causes and prevention of diseases affecting women after menopause.
The studies included 48,835 American women, ages 50 to 79. Forty percent were randomly assigned to go on a low-fat diet, with only 20 percent or less of their daily calories coming from fat. The other women were not asked to make any dietary changes. The dieting women were also asked to eat five or more daily servings of fruits and vegetables and six servings of grains.
Although the research is considered the most valid of the many done on the effects of a low-fat diet, the scientists noted that most of the women put on the diet didn't meet the goal of 20 percent. On average, the women reduced fat intake to 24 percent in the first year they were in the study, but crept up to 29 percent by the eighth year. The fat share of their diets at the start of the study was between 35 percent and 38 percent.
"If we had achieved an even higher adherence rate, I believe the study's results would have been more dramatic," said Ross Prentice, a biostatistician at the Fred Hutchinson Cancer Research Center in Seattle. He was the lead author of the study on breast cancer.
Over an average follow-up period of eight years, the women in the low-fat-diet group reduced their overall rate of breast cancer by 9 percent compared to women in the control group, a difference not considered statistically significant.
For colorectal cancer, the rate in the diet group was 0.13 percent per year, virtually the same as the 0.12 percent per year in the control group. The low-fat group's heart-disease rate was only 3 percent less than the control group's, and the dieters' low-density lipoprotein (LDL) or "bad" cholesterol was only 2.4 percent lower.
But researchers say they did see benefits from the low-fat diet, particularly regarding some breast-cancer rates among women who were eating very-high-fat diets to start with. And the scientists noted recent research that suggests cutting certain fats from the diet, especially saturated and trans fats, may be more important than curbing total fat consumption.
"Nutrition knowledge has progressed dramatically since the study began," said Mara Vitolins, a registered dietitian and co-author of all three studies. "Today, we know that reducing total fat may not be enough, we need to focus on the types of fat we eat," added Vitolins, an associate professor of public health at Wake Forest University Baptist Medical Center.
Marcia Stefanick, a professor at the Stanford University Prevention Research Center and chair of the WHI steering committee, said the dieting women were not asked to distinguish between "good fat" - unsaturated fats found in fish, nuts and vegetable oil - and the saturated fats and trans fats found in processed foods, meats and some dairy products.
Current dietary guidelines call for keeping saturated fats to less than 10 percent of daily calories and limiting consumption of trans fats.
"Just switching to low-fat foods is not likely to yield much health benefit in most women," Stefanick said. "You can't rely on using low-fat substitutes to make a difference."
There were marked responses among some women in the breast-cancer study. For instance, the low-fat group had a 30 percent risk reduction for a type of breast cancer - tumors that are progesterone-receptor negative. Prentice said that while such tumors are rare, they are also more deadly because they don't respond to hormone-blocking drugs.
There was a 15 percent to 20 percent overall reduction in breast cancer among women who were eating the most fat at the start of the study, and who most rigorously adhered to their diets, he noted.
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