Three new studies from the Women's Health Initiative (WHI) suggest that eating a low-fat diet doesn't have a big an impact on cancer or heart disease in women past menopause -- contrary to what many people believe and what many researchers had hoped. But experts say the findings are no reason to abandon a healthy eating plan.

The studies are published in this week's issue of the Journal of the American Medical Association. One deals with the effect of a low-fat diet on breast cancer, a second looks at the diet's impact on colon cancer, and the third examines heart disease.

They are the latest entries in the debate over whether controlling fat consumption can have an impact on cancer. Although many people assume it can, the idea has actually been very controversial in the medical community, said Michael Thun, MD, vice president of epidemiology and surveillance for the American Cancer Society.

That's because some other large studies have found no such relationship, explained Thun, who was not involved in the research. And nowadays there's more evidence that being overweight or obese is much more important to cancer risk than how much fat a person eats.

Still, the results are unlikely to close the book on the subject.

"I wouldn't see our trial as a reason for women to abandon a low-fat diet or to lose interest in the potential of a low-fat diet," said Ross L. Prentice, PhD, lead author of one of the studies and a co-author on the others. He is a researcher at the Fred Hutchinson Cancer Research Center in Seattle.

He said there are hints in the study results that low-fat eating may have an effect, even though the studies didn't find a statistically significant benefit.

The studies included nearly 49,000 postmenopausal women between the ages of 50 and 79.

About 40% of the women were assigned to follow a diet that got 20% of calories from fat and had at least 5 daily servings of fruits and vegetables and 6 daily servings of grains. These women took part in nutritional and behavioral counseling to help them stick with the diet. The remaining 60% of the women were told to follow their regular diets and didn't get any special counseling. The women were followed for about 8 years.

The women assigned to the low-fat diet did manage to trim some fat out of their eating routine -- though most didn't get it down to 20%. On average, women on the diet were getting about 24% of their calories from fat in the first year, while women who weren't dieting got about 35% of their calories from fat. That gap was a little smaller by the sixth year.

But the changes made virtually no difference in colon cancer rates between the 2 groups. This held true even when the researchers looked at specific types of colon cancer, and divided the women into smaller groups based on age, race, and other risk factors for colon cancer. Likewise, the researchers found no significant difference in heart disease and stroke rates.

The results were a little better for breast cancer. Women on the low-fat plan had a 9% lower risk of breast cancer than women eating their regular diet. That reduction was not statistically significant -- but Prentice said his group found hints that the effect might be real.

For instance, the drop in risk was greater -- around 15% -- for women who made the biggest change in their fat intake and for those who stuck most closely to the diet plan, he said. The researchers also saw a similar reduction in risk when they looked at the hormone status of the tumors. The low-fat diet did seem to lower the risk of tumors that were negative for progesterone receptors (PR-) and tumors that were negative for progesterone receptors and positive for estrogen receptors (ER+/PR-).

Several factors could help explain why the studies didn't find a bigger impact from a low-fat diet, Prentice said.

For one thing, the follow-up period was relatively short. It may simply take longer than 8 years for a low-fat diet to affect cancer risk. That's particularly true of colon cancer, which can take 10 years or more to develop. Prentice said his group will continue tracking the women in the study to see if cancer rates change with more time.

Furthermore, the low-fat plan was designed only to cut fat, not calories. Most of the women in the study were overweight when it began, and those on the low-fat plan only lost a few pounds on average. But the link between obesity and cancer wasn't as clear in 1993 when the WHI began, so it wasn't factored into the study. Prentice said a diet designed to cut calories and help women lose weight might have gotten better results.

"That's quite possibly why we didn't see any reduction in colon cancer," he said.

Prentice also said the study design may have been a factor. Women overall were eating less fat when the study started than the researchers had counted on. That made the differences between the low-fat group and the regular diet group smaller than the study had planned for -- and may have hidden any benefit from the low-fat plan.

The new studies won't have an impact on ACS nutrition guidelines, Thun said. Those recommend limiting fat because that's a good way to maintain a healthy body weight and prevent obesity.

"Fat is the most calorie-dense food," Thun explained.

He said women who want to lower their risk of breast cancer should try to maintain a healthy body weight, avoid gaining weight in adulthood, and get regular physical activity. They should also avoid hormone use after menopause if possible, and avoid excess alcohol. Regular mammograms -- though they can't prevent breast cancer -- can help find tumors when they're small and easier to treat.

Keeping at a healthy weight and getting regular exercise are also ways for women to reduce their risk of colon cancer, Thun said. In addition, women should limit how much red and processed meat they eat, and avoid heavy alcohol use. Regular screening can also help prevent colon cancer by finding polyps before they turn cancerous -- and can find cancers early, when they're most treatable.

Citations: "Low-Fat Dietary Pattern and Risk of Invasive Breast Cancer: The Women's Health Initiative Randomized Controlled Dietary Modification Trial." Published in the Feb. 8, 2006 Journal of the American Medical Association (Vol. 295, No. 6: 629-642). First author: Ross L. Prentice, PhD, Fred Hutchinson Cancer Research Center.

"Low-Fat Dietary Pattern and Risk of Colorectal Cancer: The Women's Health Initiative Randomized Controlled Dietary Modification Trial." Published in the Feb. 8, 2006 Journal of the American Medical Association (Vol. 295, No. 6: 643-654). First author: Shirley A. A. Beresford, PhD, University of Washington.

"Low-Fat Dietary Pattern and Risk of Cardiovascular Disease: The Women's Health Initiative Randomized Controlled Dietary Modification Trial." Published in the Feb. 8, 2006 Journal of the American Medical Association (Vol. 295, No. 6: 655-666). First author: Barbara V. Howard, PhD, Medstar Research Institute.