But they quickly added that the study results should not be interpreted as a license to "rush out to the nearest fast-food store and stuff your face," as one researcher put it.

"There's a stay-tuned message here," said Dr. Henry Black, a preventive cardiologist at Rush University Medical Center in Chicago and one of the study investigators.

Some Hopeful Signs

The federally funded study, reported in a series of three papers in Wednesday's issue of the Journal of the American Medical Association, involved nearly 50,000 women ages 50-79 and was part of the landmark Women's Health Initiative. The participants were recruited in the 1990s and followed for an average of eight years.

Some women continued their usual eating habits. Others were randomly assigned to a low-fat diet group and took part in group sessions and other behavior-modification techniques to help them cut down on dietary fat and eat more vegetables, fruit and grains. But they were not asked to avoid saturated fat, lose weight or exercise -- which some experts believe might have made a difference. Most of the women were overweight.

The data did offer contain some hopeful signs: Although the trend was not statistically significant, the women who were watching their fat intake had a slightly smaller chance of getting breast cancer; their average cholesterol was lower; and their risk of developing colon polyps was lower. So it's possible continued follow-up will reveal benefits, Black said.

It's also possible that "eating right your whole adult life, not just after menopause, would have made a difference," he said.

'Very Complex Science'

Another study investigator, research nutritionist Linda Van Horn of Northwestern University, said that if the study had been designed today, it probably would not have looked just at total fat intake. Instead, it would have examined overall dietary patterns, such as increasing fruits, vegetables and whole grains while replacing saturated fats with unsaturated ones and cutting salt.

"Reducing total fat alone isn't enough," said Van Horn. "Nutrition is a very complex science."

Van Horn said the researchers would continue tracking the women and analyzing the data in an attempt to find out what factors may make a difference. Among other things, she said, they will compare women who lowered fat in their diet in part by shifting to fat-free cookies (which contain a lot of sugar) to women who shifted to "apples and oranges and whole-grain bread." They will also look at weight loss, physical activity, age and adherence to the diet.

The goals of the diet were to reduce fat intake to 20 percent of daily calories, to increase intake of fruits and vegetables to at least five servings a day, and to increase grains to at least six servings a day. Although the dieters did better than the control group, none of those goals was met.

Both investigators called the results of the study disappointing. "We expected and hoped a low-fat diet would prevent breast cancer," said Black.

Small Decreases in Fat Consumption?

One possible explanation for the findings, Black said, is that the women who volunteered for the study were not "eating huge amounts of fat" to begin with, and those in the diet group were able to reduce their fat consumption by only 8 percent when compared with the control group. There were even smaller decreases in consumption of saturated fat and trans fat, and only tiny increases in consumption of fruits, vegetables and grains.

Another possibility is that the women in the control group were also eating more healthfully, even though they weren't asked to. Public knowledge about healthy eating was increasing in the period after the study began, Black said, and the country's average cholesterol level was coming down.

"Research is always filled with surprises," said Van Horn. "That's why we do it."

She added: "If you ask me if I still think diet is important -- absolutely. Do I think the nutrients in fruits, vegetables and grains are important for the immune system? Absolutely. But we didn't design the study to answer those questions."

Some Significant Reductions

Biostatistician Ross Prentice of the Fred Hutchinson Cancer Research Center in Seattle, lead author of the breast cancer paper, said the women on the low-fat diet decreased their risk of getting that disease by 9 percent, which "could have been due to chance." (For every 10,000 women in the study, an average of 42 in the low-fat group and 45 in the control group developed breast cancer each year.)

The researchers did find statistically significant reductions in blood levels of a form of estrogen and in a subtype of breast cancer that is sensitive to estrogen but not to progesterone. There were also significant reductions in breast cancer incidence among the women with the highest fat consumption at the start of the trial and among those who adhered most strictly to the diet.