Low-fat diet falls short; it's not enough to stop cancers, heart disease.

Reducing fat consumption after menopause offers most women little if any protection against breast cancer or several other diseases, according to three reports from a massive prevention trial. No significant differences in rates of colorectal cancer, heart disease, or stroke emerged during the trial.

But little snippets of information" from the Women's Health Initiative (WHI) trial suggest that cutting back on fats may protect some women from breast cancer, says epidemiologist and study investigator Shirley Beresford of the University of Washington in Seattle. She was among nearly 50 investigators who worked on the trial.

For example, one segment of the new data suggests that switching and sticking closely to a low-fat diet prevents breast tumors in women who previously are especially large amounts of fat. People with "bad diets" have the most to gain, says epidemiologist Barbara Howard of the nonprofit MedStar Research Institute in Hyattsville, Md., another of the trial's investigators.

The data also suggest that the dietary switch may block a particularly aggressive form of breast cancer.

Past studies had suggested that people who eat relatively little fat and plenty of fruits, whole grains, and vegetables are at reduced risk for breast and colorectal cancers and heart disease.

The gold standard of diet research is an intervention trial, in which some volunteers are directed to change the way they eat and, for comparison, others maintain their established eating habits. The WHI is the largest dietary-intervention trial ever conducted. It ran from 1993 to 2005 and included nearly 50,000 postmenopausal women, ages 50 to 79, who initially consumed an estimated 32 percent or more of their calories in fat.

Two-fifths of the volunteers were asked to switch permanently from their normal diet to one low in fat and high in fruits, vegetables, and grains. Nutritionists then saw those women at least four times per year and encouraged them to adhere to the regimen.

The average fat intake was 8.1 percent lower among women in the low-fat group than among women maintaining a normal diet. Even so, only 14 percent of women in the low-fat-diet group met the researchers' target of reducing fat consumption to 20 percent or less of total caloric intake.

During the 8 years, on average, that women spent on the low-fat diet, 0.42 percent of that group developed breast cancer each year. In the regular-diet group, cancer incidence was nearly identical, 0.45 percent per year.

However, women in the low-fat group were 24 percent less likely, compared with other women, to get the aggressive form of breast cancer, says biostatistician Ross Prentice of the Fred Hutchinson Cancer Research Center in Seattle, who was a trial leader.

Furthermore, says Prentice, the women who adhered most closely to the diet had a 15 percent reduced risk of breast cancer overall, compared with the normal-diet group.

The current results appear in three reports in the Feb. 8 Journal of the American Medical Association.

The researchers will monitor the volunteers' health through at least 2010.

Recommending a low-fat diet and providing nutritional counseling to postmenopausal women aren't enough to alter the risk of breast cancer, colorectal cancer, and cardiovascular disease in women who may have had an above-average fat intake for more than 40 years, Beresford says. She adds, "Changing dietary behavior at younger ages makes a lot of sense."

By emphasizing cuts in total fat intake, the intervention diet "also reduced what you might think of as protective fats," such as unsaturated oils, comments epidemiologist Cheryl Anderson of the Johns Hopkins Medical Institutions.

Although they weren't instructed to lose weight, women who changed diet weighed on average 0.8 kilogram (2 pounds) less after 6 years than the other women did. "Had weight come down more [among dieting women], we might have seen some effects on cardiovascular disease," says Anderson.