New data suggest that long-term and intense physical exercise may help protect women against some types of breastcancer.

"Previous studies have linked physical activity with a reduction in breast cancer, but few studies have examined risk separately for invasive breast cancer and noninvasive breast cancer," said Leslie Bernstein, professor of preventive medicine at USC and primary investigator of the California Teacher's Study. "This study is unique because it provides the first prospective study data documenting that a woman's long-term exercise habits are important in determining her future breast cancer risk."

As part of the study, beginning in 1995 researchers analyzed data from 110,599 female current or former California teachers and public school professionals with no history of breast cancer. The data included detailed information on exercise histories and current exercise habits.

The study focused on the impact of strenuous as well as moderate activities, collecting information on the amount each woman exercised per week from high school through age 54. During the 6.5 year follow-up period, 2,649 women were diagnosed with invasive breast cancer and 593 were diagnosed with non-invasive breast cancer.

Published in the February 26, 2007 issue of the Archives of Internal Medicine, the results show that invasive breast cancer risk was lower for women participating in strenuous activity, such as swimming, jogging or other aerobic activities for more than five hours per week annually, compared to the least active women. Similar results were seen for non-invasive breast cancer risk.

A unique finding of this study is that the benefit of long-term strenuous activity was only seen among those with what are called estrogen receptor (ER)-negative tumors. These belong to a category in which the hormone estrogen does not contribute to tumor progression.

"This finding was somewhat unexpected; however, if it is repeated in future studies of physical activity and breast cancer risk, in offers a promising complement to our current approaches to preventing breast cancer, namely the use of drugs that block estrogens, like tamoxifen or raloxifene. These drugs seem to be effective in reducing risk of ER-positive cancers, but do not impact the occurrence of ER-negative cancers," says Bernstein.

"This study confirms that breast cancer risk is influenced by consistent participation in strenuous exercise. However, it may require substantial commitment of time to achieve this lower risk," concludes Bernstein. "This study showed that risk was lower for women who exercised at least 5 hours per week and our prior studies of other groups of women have suggested a minimum of 3-4 hours per week."