October 31 (HealthDay News) -- Vigorous activity can reduce the risk of breast cancer by about 30 percent in normal-weight women, according to an 11-year U.S. study of 32,269 postmenopausal women.

For the study, vigorous activity was defined as heavy housework (scrubbing floors, washing windows, demanding yard work, digging, chopping wood) and strenuous sports or exercise, such as running, fast jogging, competitive tennis, aerobics, bicycling on hills, and fast dancing.

While vigorous activity reduced breast cancer risk in normal-weight women, it had no effect in women who were overweight or obese, according to study leader Michael F. Leitzmann and colleagues.

They also found that non-vigorous activity, such as light housework (vacuuming, doing laundry, painting, general gardening) and light sports or exercise (walking, hiking, light jogging, recreational tennis, bowling) offered no protection against breast cancer.

The findings were published in the journal Breast Cancer Research.

"Possible mechanisms through which physical activity may protect against breast cancer that are independent of body mass include reduced exposure to growth factors, enhanced immune function, and decreased chronic inflammation, variables that are related both to greater physical activity and to lower breast cancer risk," the study authors wrote.

"An alternative explanation for the stronger apparent effect of vigorous activity among lean over heavy women is that heavier women may misreport non-vigorous activities as vigorous activities," the researchers added.

SOURCE: BioMed Central, news release, Oct. 30, 2008