Ian Sample

London (GUARDIAN NEWS SERVICE): Scrubbing floors, cleaning windows and vigorous dancing can reduce the risk of breast cancer by almost a third, according to a major study.

The protective effect of strenuous exercise was only apparent in women who were lean or of normal weight, it found.

The findings help tease apart previous research that suggested exercise had a mildly protective effect against breast cancer, by revealing the benefits are almost entirely confined to women who maintained a healthy weight in older age.

Doctors at the US National Cancer Institute in Maryland monitored the lifestyles and health of 32,269 postmenopausal women from 1987 to 1998. They recorded 1,506 new cases of breast cancer.

The researchers asked women about their typical levels of daily activity. Light housework, such as vacuuming and clothes washing, was regarded as moderate activity, while heavy housework, such as scrubbing floors, or "fast dancing" was regarded as vigorous activity.

The study found that high levels of any activity reduced cancer risk by 24% in women who were regarded as having a normal weight, or a body-mass index of 25 and under. Vigorous exercise reduced cancer risk by 32%, according to the study in the journal Breast Cancer Research.

Surprisingly, exercise appeared to have no effect on the cancer risk of overweight and obese women. Obesity is known to increase cancer risk in postmenopausal women by up to 30%.