Normal-weight women who carry out lots of vigorous exercise are approximately 30% less likely to develop breast cancer than those who don't exercise vigorously. A study of more than thirty thousand postmenopausal American women, reported in the journal Breast Cancer Research, has revealed that a sedentary lifestyle can be a risk factor for the disease - even in women who are not overweight.

While an Investigator at the National Cancer Institute of the U.S. National Institutes of Health, Michael F. Leitzmann led a team of researchers who followed the 32,269 women for eleven years and found that vigorous activity may protect against breast cancer, independent of body weight control. Vigorous activity was judged to include things like heavy housework (scrubbing floors, washing windows, heavy yard-work, digging, chopping wood) and strenuous sports or exercise (running, fast jogging, competitive tennis, aerobics, bicycling on hills, and fast dancing).

Leitzmann said, "Notable strengths of our study include its large sample size, prospective design, high follow-up rate, and availability of relevant known or suspected breast cancer risk factors. These features enabled us to minimize any effects from other factors apart from exercise."