The theory that eating less fat can help prevent disease suffered a setback today with the release of a large-scale study that failed to show such a diet lowered older women's risk of breast and colon cancer, and heart disease.
Health researchers are not giving up on the low-fat message, however, and say the new study has too many shortcomings to provide a clear answer on the health benefits of eating less fat.
The new study was conducted by researchers with the Women's Health Initiative, a $415 million National Institutes of Health study of nearly 49,000 postmenopausal women.
They analyzed the health of women who ate a diet lower in fat and higher in vegetables, fruits and grains over an eight-year period. The low-fat group was compared with women on a regular diet.
At the end of the study, the low-fat group did not show a significantly lower risk of developing breast cancer, colon cancer or heart disease and strokes.
Results of the study were published in several articles in today's issue of the Journal of the American Medical Association.
In e-mails and phone interviews that ABC News conducted with more than 50 specialists in heart disease, cancer and nutrition, questions were raised about the shortcomings of the study. Among the concerns:
The study was too short in duration to detect reduction in breast and colon cancer from fat reduction. Eight years is not enough time to see a reduction in cancers that often take decades to develop, experts said.
The amount of fat reduction was too small. The researchers hoped to get the low-fat dieters to reduce fat intake by 20 percent, but they only achieved, on average, a less than 10 percent reduction. Heart researchers say such a small decrease is unlikely to produce meaningful benefits.
The researchers simply looked at fat intake in general and did not distinguish between healthy fats like olive oil and unhealthy fats like trans fats, which increase cholesterol.
Whatever the concern, the lackluster findings do not mean people should add more fat to their diets, because some small health benefits were seen in the low-fat group, said Dr. Jacques Rossouw, the Women's Health Initiative project officer.
Notably, the low-fat eaters had lower body weight, blood pressure and "bad" cholesterol levels, he said.
"I hope it [the study's findings] doesn't say to the public that oh, fat doesn't matter," he said.
Also, Dr. George L. Blackburn, the associate director of nutrition at Harvard Medical School, said that the breast cancer data was promising.
At the end of eight years, the incidence of breast cancer was 9 percent lower in the women on the reduced-fat diet; the finding was not statistically significant but "provides suggestive evidence that the risk of breast cancer my be modified by changing an individual diet," he said.
Because one of the chief concerns raised about the study was that it was too short, one Women's Health Initiative researcher said they will follow the women for another five years to see if stronger patterns emerge, especially since diseases like cancer can take years to develop.
"There is some evidence that with longer follow-up we might come to some different conclusions," said Shirley Beresford, a Women's Health Initiative investigator and professor of epidemiology at the School of Public Health and Community Medicine at the University of Washington in Seattle.