Strenuous exercise - such as lap swimming, aerobics and running - appears to cut the risk of some breast cancers in women, a study said on Monday.

While it is still not clear how hard or long women need to exercise, the study adds to a growing body of evidence that rigorous activity lowers breast cancer risk.

The team at the University of Southern California in Los Angeles said exercise may reduce cancer risk through changes in metabolism and the immune system, and by reducing weight gain.

The study, appearing in the Archives of Internal Medicine, involved 110,599 women in California whose health histories were tracked from 1995 onward.

Women who said they engaged in strenuous activity for more than five hours a week had a 20 percent lower risk of invasive breast cancer and a 31 percent lower risk of early stage breast cancer, compared to women who participated in less than 30 minutes of such activity every week.

Through 2002, a total of 2,649 of the women were diagnosed with invasive breast cancer, and 593 with early-stage forms of the disease.

The study is the first to look at the cumulative effect of exercise over time, as opposed to women’s reports of whether they had exercised shortly before being diagnosed with breast cancer.

“These results provide additional evidence supporting a protective role for long-term strenuous recreational physical activity on risk of invasive and (early stage) breast cancer, whereas the beneficial effects of moderate activity are less clear,” the study concluded.