The $415 million federal study involved nearly 49,000 women aged 50 to 79 who were followed for eight years. In the end, those assigned to a low-fat diet had the same rates of breast cancer, colon cancer heart attack and stroke as those who ate whatever they pleased, researchers are reporting today.
"These are three totally negative studies," said Dr. David Freedman, a statistician at the University of California at Berkeley, who is not connected with the study but has written books on clinical trial design and analysis. And, he said, the results should be taken seriously for what they are — a rigorous attempt that failed to confirm a popular hypothesis that a low-fat diet can prevent three major diseases in women.
And the studies were so large and so expensive that they are "the Rolls Royce of studies," said Dr. Michael Thun, who directs epidemiological research for the American Cancer Society. As such, he said, they are likely to be the final word.
For decades, many scientists have been saying, and many members of the public have been believing, that what you eat — the composition of the diet — determines how likely you are to get a chronic disease. But it has been hard to prove. Studies of dietary fiber and colon cancer failed to find that fiber was protective. Studies of vitamins thought to protect against cancer failed to show an effect.
The new studies, reported in the Feb. 8 issue of the Journal of the American Medical Association, found that women who were randomly assigned to follow a low-fat diet ate significantly less fat over the next eight years. But they had just as much breast and colon cancer and just as much heart disease.
And, confounding many popular notions about fat in the diet, the different diets did not make much difference in anyone's weight. The common belief that carbohydrates in the diet lead to higher insulin levels, higher blood glucose levels and more diabetes was also not confirmed. There was no such effect among the women eating low-fat diets.
As for heart disease risk factors, the only one affected was LDL cholesterol, which increases heart disease risk. The levels were slightly higher in women eating the higher fat diet, but not enough to make a noticeable difference in their risk of heart disease.
The current study, asking if a low-fat diet could protect women from breast cancer in the first place, had findings that fell short of statistical significance, meaning they could have occurred by chance. In essence, there was no solid evidence that a low-fat diet helped in prevention.
Although all the study participants were women, the colon cancer and heart disease results also should apply to men, said Dr. Jacques Rossouw, the project officer for the Women's Health Initiative. He explained that the observational studies that led to the colon cancer-dietary fat hypothesis included both men and women. As for heart disease, he said, researchers have consistently found that women and men respond in the same way to dietary fat.
The results, the study investigators agreed, do not justify recommending low-fat diets to the public to reduce their heart disease and cancer risk.
As for the cancer society, Dr. Thun said, with these results that he describes as "completely null over the eight-year follow-up for both cancers and heart disease," his group has no plans to suggest that low-fat diets are going to protect against cancer.
While cancer researchers say they were disappointed by the results, heart disease researchers say they are not surprised that simply reducing total fat made had no effect.
"The problem is that this study was designed two decades ago when the fad was low fat," Dr. Libby said. Now, he said, he and others are persuaded that a so-called Mediterranean diet is best — not necessarily low in fat but low in saturated fats, like butter and cream cheese. That, with exercise, should help prevent heart disease, he says.
The low-fat diet was not easy, Dr. Chlebowski notes. Women were told to aim for a diet that had just 20 percent of its calories as fat. Most substantially cut their dietary fat, but most fell short of that 20 percent goal. The women in the study had 18 sessions of meeting in small groups with a trained nutritionist in the first year and four sessions a year after that.
In the first year, the women on the low-fat diets reduced the percentage of fat in their diet to 24 percent of daily calories and by the end of the study their diets contained 29 percent of their calories as fat. In the first year, the women in the control group were eating 35 percent of their calories as fat and by the end of the study their dietary fat content was 37 percent.
"What we are saying is that a modest reduction of fat and a substitution with fruits and vegetables did not do anything for heart disease and stroke or breast cancer or colorectal cancer," said Dr. Nanette Wenger, a cardiologist and professor of medicine at Emory University Medical School. "It doesn't say that this diet is not beneficial," she added.
But the overall lesson, said Dr. Freedman, is clear; "We, the scientific community, tend to go off the deep end giving dietary advice based on pretty flimsy evidence."