(Ivanhoe Newswire) -- A regular dose of strenuous exercise may keep you from developing breast cancer.

New research from the University of Southern California, Los Angeles, reveals women who regularly do strenuous physical activity, like swimming laps, aerobics and running, can lower the risk of both invasive breast cancer and in situ (early-stage) breast cancer more than women who do not.

Researchers looked at 110,599 women ages 20 to 79. Among the participants, 2,649 were diagnosed with invasive breast cancer, and 593 had in situ breast cancer. Results show women who did strenuous activity for more than five hours per week over the long term had a 20-percent lower risk of invasive breast cancer and 31-percent lower risk of in situ breast cancer than those who were strenuously active less than 30 minutes a week.

Also examined was the link between strenuous physical activity and the risk of breast cancer by the type of hormone receptor. Some cancers are more affected by hormones than others, which means treatments need to target the specific kind of tumor. Strenuous activity is associated with a lower risk of the more difficult to treat estrogen-receptor-negative breast cancers. Exercise did not appear to affect estrogen-receptor-positive tumors, which accounts for approximately 75 percent of breast cancer diagnoses.

"In summary, these results provide additional evidence supporting a protective role for long-term strenuous recreational physical activity on risk of invasive and in situ breast cancer, whereas the beneficial effects of moderate activity are less clear," the authors write. "For invasive breast cancer, strenuous and moderate physical activity affect risk of estrogen-receptor-negative tumors, but neither affects risk of estrogen-receptor-positive tumors."

SOURCE: Archives of Internal Medicine, 2007;167:408-415