First solid data on health effects of a low-fat diet

Adopting a low-fat diet in later life and following such a regimen for nearly a decade does not appear to have a significant impact on reducing the overall risk of breast cancer, colorectal cancer or heart disease, according to a Women's Health Initiative study that involved nearly 50,000 postmenopausal women across the United States.

The results of the federally funded dietary modification study will be published in a series of three papers - two with lead authors at Fred Hutchinson Cancer Research Center and all three involving co-authors from the Hutchinson Center - in the Feb. 8 issue of the Journal of the American Medical Association, or JAMA.

The study - the first attempt to test the health impact of a low-fat diet in a randomized, controlled trial, considered the gold standard of clinical and public-health study design - did, however, uncover some encouraging trends, according to Hutchinson Center biostatistician Ross L. Prentice, Ph.D., lead author of the JAMA paper that describes the impact of a low-fat diet on breast-cancer risk, one of the primary goals of the study.

"Women in the low-fat-diet group reduced their overall rate of breast cancer by about 9 percent as compared to the women who didn't change their eating patterns, but that difference was not statistically significant; it could have been due to chance. So at this point we're not able to say with certainty that a low-fat diet reduces the risk of breast cancer," said Prentice, member and former director of the Hutchinson Center's Public Health Sciences Division. A 9 percent reduction in breast-cancer incidence means that, out of 10,000 women, 42 in the low-fat-diet group and 45 in the comparison group developed breast cancer each year.

Prentice and colleagues did find, however, that a low-fat diet was associated with a statistically significant 15 percent reduction in estradiol, a form of blood estrogen that increases the risk of breast cancer.

Women in the low-fat group also experienced a 30 percent risk reduction for a certain subtype of breast cancer: tumors that were progesterone-receptor negative. "This finding provides an interesting hypothesis for further development and reinforces that breast cancer is multifaceted; it is not a single disease," Prentice said. PR-negative tumors, while relatively rare, are difficult to treat and associated with a higher mortality rate because they are unresponsive to hormone-blocking drugs such as tamoxifen.

Significant results were seen also among women in the low-fat-diet group who began the study with the highest baseline fat consumption and among women who most strictly adhered to the study's dietary-fat goals. Women in these categories experienced a 15 percent to 20 percent overall reduction in breast-cancer incidence.

"The bottom line is that changing to a low-fat diet may reduce breast-cancer risk, especially among women who have a relatively high-fat diet to begin with, but we don't view our data as strong enough at this time to make a broad recommendation that all women initiate a low-fat diet for that purpose," Prentice said. "Additional follow up with these women may yield a stronger, statistically significant conclusion."