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 could potentially improve survival, although few data exist to support this hypothesis.
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 20 percent lower for 3 to 8.9 MET-hours per week; 50 percent lower for 9 to 14.9 MET-hours per week; 44 percent lower for 15 to 23.9 MET-hours per week; and 40 percent lower for 24 or more MET-hours per week. The benefit of physical activity was particularly apparent 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. 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," the authors conclude.
(JAMA. 2005;293:2479-2486. Available pre-embargo to the media at www.jamamedia.org)
Editor's Note: The research for this article was funded by a grant from the National Institutes of Health.