The risk of breast cancer death was 50 percent lower for women with hormone-responsive tumors who engaged in 9 or more MET-hours per week of activity compared with women with hormone-responsive tumors who engaged in less than 9 MET-hours per week.

There is reason to believe that physical activity might extend survival in women with breast cancer, according to background information in the article.

Physical activity has been linked to lower levels of circulating ovarian hormones, which may explain the relationship between physical activity and breast cancer. Lower estrogen levels among physically active women with breast cancer potentially could improve survival, although there is little data to support this hypothesis.

Higher Levels of Activity Linked to Longer Survival

Michelle D. Holmes, M.D., Dr.P.H., of Brigham and Women's Hospital and Harvard Medical School, Boston, and colleagues conducted a study to examine whether higher levels of physical activity after a breast-cancer diagnosis would be associated with longer survival.

The study was based on responses from 2,987 female registered nurses in the Nurses' Health Study who were diagnosed with stage I, II, or III breast cancer between 1984 and 1998 and who were followed up until death or June 2002, whichever came first.

Physical activity was measured as metabolic equivalent task (MET) hours. Three MET-hours is equivalent to walking at average pace of 2 to 2.9 mph for 1 hour.

The researchers found that compared with women who engaged in less than 3 MET-hours per week of physical activity, the adjusted relative risk of death from breast cancer was as follows:

Tendency to Reduce Activity After Diagnosis

The benefit of physical activity was apparent particularly among women with hormone-responsive tumors.

The risk of breast cancer death was 50 percent lower for women with hormone-responsive tumors who engaged in 9 or more MET-hours per week of activity compared with women with hormone-responsive tumors who engaged in less than 9 MET-hours per week.

Compared with women who engaged in less than 3 MET-hours per week of activity, the absolute unadjusted risk of death reduction was 6 percent at 10 years for women who engaged in 9 or more MET-hours per week.

"It has been estimated that women decrease their levels of physical activity by 2 hours per week after a breast cancer diagnosis, with greater decreases among obese women, and that less than one third of breast cancer survivors participate in levels of activity recommended by government agencies," the authors note.

"Women with breast cancer who follow the Centers for Disease Control and Prevention recommendations for all individuals in the United States to exercise at moderate intensity for 30 or more minutes per day for 5 or more days per week may survive longer," they conclude.