Even so, scientists say the results from the government study of 48,835 women don't mean dieters should just throw up their hands and eat cake.
Researchers suggested that the women in the long-running study — with an average age of 62 — may have started their healthy eating too late. They also didn't reduce fats as much as the diet demanded, and most remained overweight, a major risk factor for cancer and heart problems.
"These results do not suggest that people have carte blanche to eat fatty foods without health problems," said Dr. JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital, a co-author of the study and respected nutrition authority.
But the scientists declined to call the $415 million venture a failure, pointing to signs of less breast cancer in women who cut out the most fat, and in less heart disease in women who ate low amounts of the worst kinds of fats.
Heart and cancer specialists said the overall results were not surprising since scientific thinking on the role fats play in disease prevention has evolved since this study was designed. That is especially true when it comes to good and bad fats and heart disease.
The research involved postmenopausal women who either cut overall fat consumption and increased vegetables, fruits and grains, or who continued their usual eating habits. The researchers said the dieters may not have cut out enough fat for a meaningful comparison. Cancer and heart disease incidence was similar in both groups.
"The results, of course, are somewhat disappointing. We would have liked this dietary intervention to have a major impact on health," Manson said.
The study, appearing in Wednesday's Journal of the American Medical Association, is part of the Women's Health Initiative, a landmark government project involving tens of thousands of postmenopausal U.S. women. An earlier WHI study linked long-term use of hormone pills with breast cancer and heart disease risks.
One of the women in the study, 66-year-old Judy LaCour of Kent, Wash., began the low-fat diet more than 10 years ago.
"I was raised in a farm family where high-fat food was the norm," LaCour said. "It was a real culture shock for me when I first started."
But she said she has stuck with the changes and is disease-free. She also thinks the diet has helped keep her weight down while her friends have gotten heavier with age.
The study was designed mainly to investigate breast cancer risk. Dietary fat was initially thought to be implicated because breast cancer rates are high in Western countries with fatty diets, but recent studies have failed to show any relationship, said Dr. Michael Thun of the American Cancer Society.
Recent research also has suggested that for breast cancer in particular, earlier eating habits may have the most influence on risk.
Another target was colon cancer, which some studies have linked with red meat.
Thun said the results aren't surprising because fat in the diet "is no longer center stage" when it comes to cancer risk. While the Cancer Society recommends limiting fats, that's primarily because of the calories, Thun said.
Breast cancer rates in both groups were about 3 percent, marginally higher than for postmenopausal women in the general U.S. population, probably because these women got routine mammograms, said study investigator Ross Prentice of the Fred Hutchinson Cancer Research Center. Colon cancer rates in both groups were similar to national rates for similarly aged women — roughly 1 percent in both groups.
The researchers assumed that lowering fats would help prevent heart disease, too, but specialists now stress the differences in fats. Some, like the kind in olive oil and nuts, are healthier than the saturated fats and trans fats found in processed and fried foods.
Study participants filled out food questionnaires but might not have reduced the right kinds of fat, said Dr. Robert Eckel, president of the American Heart Association.
"It would be easy to misinterpret the results of this study," he said.
Both groups had relatively low rates of heart disease, about 2.5 percent compared with just over 4 percent among postmenopausal women nationally, Prentice said.
Both groups started out with about 37 percent of daily calories from fat. The goal was to cut that to 20 percent for the low-fat group; the women managed about 24 percent on average in the first year, but it climbed to about 29 percent later on, said Dr. Jacques Rossouw, WHI project officer at the National Institutes of Health.
Most dietary guidelines recommend about 20 percent to 35 percent of daily calories from fat.