Several studies have examined the effects of low-fat diets on the risk of breast cancer. Some have appeared to show benefits of reducing fat intake while others have not.

However, now, more rigorously controlled studies are providing important new information, especially for postmenopausal women who have already had breast cancer and want to do everything possible to prevent it from coming back. Here's what we've learned.

The Women's Intervention Nutrition Study enrolled 2,437 postmenopausal women who had surgery for early-stage breast cancer. The women were randomly assigned either to a control group who consumed their usual diet or to a low-fat group whose diet contained just 15 percent of calories from fat. The low-fat group consumed 33 grams of fat per day, compared with 51 grams per day for the control group.

Not only did the women in the low-fat group lose an average of six pounds during the five-year trial, they also had a lower rate of breast cancer recurrence (9.8 percent) compared to the women who followed their usual diets (12.4 percent). But the differences were small, so a final conclusion on the value of the low-fat diet will require analysis of results from a longer follow-up period. The benefit of a low-fat diet in preventing breast cancer recurrence was more clear-cut, however, in women who had estrogen-receptor-negative breast cancer.

Can a low-fat diet also prevent breast cancer from developing in the first place? The results are less clear on this score. One study of nearly 49,000 postmenopausal women looked at the effects of a low-fat diet on the risk of first developing invasive breast cancer. After about eight years, there was a trend toward a reduced risk among the women on the lower-fat diet, but the difference was not statistically significant. The risk reduction was greatest among women who strictly followed the low-fat diet and those who had been eating a higher-fat diet at the time the study began. And in this study the risk of breast cancer was lowest in women who eventually developed estrogen-positive and progesterone-negative breast cancer.

If you are or know a postmenopausal woman who had breast cancer, this information should be enough to consider switching to a low-fat diet - not just to control weight and prevent heart disease but also to prevent breast cancer recurrence, especially if the cancer was estrogen-receptor-negative. And, even if we don't yet know if a low-fat diet prevents an initial invasive breast cancer, this diet will help you lose weight and reduce your risk of heart disease, which poses a greater threat than breast cancer, anyway.