A low-fat diet did not reduce older women's risk of breast cancer, colorectal cancer, or heart disease, according to a long-awaited $415 million government-funded study that creates uncertainty about exactly what Americans should eat to prevent disease.
Researchers had expected the eight-year study of nearly 49,000 women ages 50 to 79 -- the largest and longest ever of whether a low-fat diet prevents cancer and heart disease -- to show that reducing fat has significant health benefits. This belief has existed for decades, based on smaller studies and the observation that people in Asian and other countries consume far less fat than Americans and have lower rates of breast and colorectal cancer and heart disease.
But the study, published in three separate papers in today's Journal of the American Medical Association, had disappointing results. When researchers compared women on a low-fat diet with those eating their regular diet -- where 38 percent of their daily calories were from fat -- there was virtually no difference in the percentage who developed breast cancer, colorectal cancer, or had coronary heart disease or strokes.
Several of the study's authors and officials from the National Heart, Lung and Blood Institute, which paid for the research, seemed uncertain during a question-and-answer session with journalists yesterday about what message they should send to women.
''These results are not definitive enough to make a recommendation that most women out there should follow a low-fat diet," said Dr. Jacques Rossouw, an institute scientist and one of the authors. He said there is no reason to believe that the results would be any different for men and younger women.
But other scientists and health advocacy groups rushed to say that Americans should not abandon low-fat diets. They said the women in the study may have started eating low-fat diets too late in life, that it could take longer for a low-fat diet to show benefits, or that steeper cuts in fat consumption than were achieved by the study participants may be necessary to reduce the risk of serious disease.
Dr. Elizabeth Nabel, director of the institute, defended the modest findings despite the high hopes for the study and its cost of more than $400 million. Women who ate a low-fat diet had a slightly lower rate of breast cancer than women on a normal diet, although the difference was statistically insignificant, meaning it could have occurred by chance. But Nabel said the difference nevertheless could signal a trend that emerges with longer follow-up. Cancer can take years or even decades to develop in the body, and researchers will follow most of the women in the study for another five years.
Additionally, women who were eating a very high-fat diet when they enrolled in the study reduced their risk of breast cancer significantly after switching to a low-fat diet, suggesting that this group of women can benefit from a low-fat diet.
In another positive finding, even though cutting back on fat overall did not reduce a woman's risk of heart disease and stroke, women who cut back slightly on their consumption of saturated fat and trans fatty acids did have fewer heart attacks and strokes. Researchers said this indicates that public health officials should increasingly focus education efforts on reducing these specific types of fats, which are found in processed and fast-food, and increasing consumption of fruits, vegetables, whole grains, and fish. The study was designed in the late 1980s, before researchers knew as much as they do today about good fats, such as those in canola oil and nuts, and harmful fats.
''I tend to look at the cup as being half-full," Nabel said. ''If there are lifestyle changes one can make to promote overall health and there's no risk to them, then people should engage in those behaviors."
The study was done with postmenopausal women, who were randomly assigned to a low-fat diet or their regular diets. Over the eight years, the women filled out periodic questionnaires about what they ate and visited clinics where researchers conducted blood tests for cholesterol levels and other indicators of how changes in diet affected their body chemistry. The research is part of the Women's Health Initiative, a landmark study that led the government to warn women that taking hormone-replacement therapy increases one's risk of breast cancer and heart disease.
While both groups of women were eating about 38 percent of their calories from fat as the study began, the women in the low-fat diet group reduced that amount to 24 percent to 29 percent over the course of the study. They not only cut their fat intake, but also slightly increased their consumption of fruits and vegetables.
One of the more interesting and instructive lessons in the study is that most women found it impossible to stick to the low-fat diet that researchers originally created for the study. The goal was to have women in the low-fat group get 20 percent of their daily calories from fat, similar to women in countries with lower cancer rates. To achieve this, the researchers from 40 medical centers provided the women with an intensive behavioral-modification program, including 18 group sessions the first year to teach the women about low-fat diets and how best to alter their intake. Quarterly sessions occurred in following years. Still, the women were able to reduce their fat intake to only 24 percent of daily calories the first year; that increased to 29 percent by the sixth year.
''Despite intensive dietary counseling and a highly motivated and committed study population, the intervention did not achieve the goal," said Dr. JoAnn Manson, one of the authors and chief of preventive medicine at Brigham and Women's Hospital in Boston. ''It really does not appear feasible to implement this type of low-fat diet on a populationwide basis, without major societal, environmental, and food industry changes. These women achieved what has to be considered a best-case scenario."
Many of the women had jobs, giving them little time to prepare low-fat meals, she said. Restaurants and prepared food from grocery stores provide few options low enough in fat to meet the study's goal. In other cases, even if a woman wanted to cook low-fat meals, her spouse and children did not want to eat them. Even the government's dietary guidelines recommend that up to 35 percent of calories a day come from fat.
Researchers expected a much greater difference between the two groups in terms of cancer risk, she said, but that's because they expected a much greater difference in terms of fat consumption.
''Everybody is probably disappointed," said Dr. Michael Dansinger, an obesity researcher at Tufts-New England Medical Center in Boston who helped the journal review the portion of the study on heart disease and stroke. ''Low-fat diets seemed so promising 15 years ago, when they were designing these studies and thinking them through. This diet was the best bet for improving overall health."
Dansinger agreed with Manson that the expectations of the trial were too high.
''Your average American can't get lower than 25 percent of their daily intake from fat without it taking over their lives. You need special cooking techniques without oil. You have to be vigilant at every meal. You can't just order something off of the menu. You have to ask the chef to make it special. It's hard work."
Dr. Dean Ornish, who is nationally known for his very low-fat diet and research into the impact of lifestyle on disease, called the study ''deeply flawed" because the ''difference between the groups wasn't very much." His research has shown that much steeper reductions of fat consumption -- to 10 percent of daily calories -- are necessary to reverse hardening of the arteries.
Doctors said people should not give up on eating healthy. Alcohol is a risk factor for breast cancer and esophageal cancer, and there is evidence that red meat contributes to colon cancer and heart disease, while obesity is a risk factor for many diseases.
The American Dietetic Association issued a statement urging Americans to stick with fat intake between 20 percent and 35 percent of calories to keep off excess weight. ''It's not easy, but people should not be discouraged about adhering to a healthy diet," Manson said.