Vigorous exercise reduces a woman’s breast cancer risk by about 30 percent, according to a new National Cancer Institute study. The effect of vigorous activity, such as heavy housework (scrubbing floors, washing windows, demanding yard work, digging, chopping wood), running, aerobics, was complete only in women who were neither overweight nor obese.

Non-vigorous activity and light exercise had no effect against breast cancer.

The 11-year study involved 32,269 postmenopausal women who were given questionnaires about their exercise habits. Participants completed health surveys from 1987 to 1989, 1993 to 1995, and 1995 to 1998.

Examples of activities classified as "non-vigorous" included: light housework, vacuuming, washing clothes, painting, home repairs, general gardening, light sports or exercise, walking, recreational tennis, bowling, golf. Activities rated as "vigorous" ranged from heavy housework, heavy yard work, digging in the garden to running, fast jogging, aerobics, bicycling on hills, strenuous sports or exercise. Previous studies have shown that less intense exercise can also protect against breast cancer in the long run.

"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 authors wrote in the study published in the Oct. 31 issue of the journal Breast Cancer Research.

Researchers say they need to do more research to find out exactly how and why exercise affects breast cancer risk.

After all, you can choose to lead an active lifestyle for a variety of reasons that may benefit your health. You can see the world with fresh eyes when breast cancer is not a part of it.