by ecancer reporter Janet Fricker

Undergoing brisk walking for five hours each week can reduce the risk of breast cancer in post menopausal women by nearly 10%, reports the latest publication from the Nurses’ Health Study. The study, published in Archives Internal Medicine, found  also that women who increase their activity levels after menopause experience benefits.

Prospective studies have shown that women with the highest levels of physical activity have a 10 to 30 % lower risk of developing breast cancer than those with the lowest levels. Indeed, physical activity has been shown to decrease circulating oestrogen levels in post-menopausal women, and lower circulating oestrogen levels are associated with lower breast cancer risk.  There are, however, still gaps in knowledge concerning the timing, type and intensity of the activity required to achieve  reductions in  breast cancer risk, and whether the association varies according to different study groups.

In the current study, Heather Eliassen and colleagues, from Brigham and Women’s Hospital (Boston, MA, USA), used data from the Nurse’s Health Study to investigate the importance of long-term activity, changes in activity, and specific types of activity. Although the Nurses’ Health study began in 1976, it was only in 1986 that participants started to report their average levels of weekly physical activity, in 10 categories ranging from zero minutes to greater than 11 hours. Categories recorded included walking, running cycling, tennis, aerobic exercise, squash and racquet ball. The participants also reported their usual walking pace (less than 2 mph, 2-2.9 mph, 3 to 3.9 mph or greater than 4 mph) and the number of stairs climbed daily. Answers were translated into hours of metabolic equivalent task values (MET-h).

During follow-up, investigators documented 4,782 cases of breast cancer among 95,396 post menopausal women.  Results showed that women who underwent  27 MET-h/week (equivalent to 1 hour of brisk walking a day) had significantly less risk of breast cancer than women who underwent less than 3 MET-hours per week (equivalent to less than one hour walking  a week) (HR 0.85; 95% CI, 0.78-0.93; P<.001 for trend).

When women who had increased their levels of activity from less than 9 MET-hour/week at menopause to at least 9 MET-h/week during follow-up  were compared with the least active women (less than 9 MET-h/week) it was found  they had reduced their breast cancer risk (HR, 0.90, 95% CI, 0.82-0.98).

When all the individual types of activities were included in a single statistical model, only brisk walking  for 20 MET-hour/ week (equivalent to 5 hours per week) was found to be significantly associated with a lower brisk cancer risk (HR, 0.91, 95% CI. 0.84-0.98, P=.01). The study demonstrated similar risk reductions among active lean and overweight women regardless of their hormone therapy status.

“Our finding that brisk walking is associated with lower breast cancer risk suggests that women do not need to engage in intense activities to appreciate a benefit,” wrote that authors, adding that the strengths of the study included its large cohort size and long follow-up, with the limitations including the fact that physical activity was self-reported.



Article:

A H Eliassen SE Hankinson, B Rosner, et al. Physical Activity and Risk of Breast Cancer Among Postmenopausal Women. Arch Intern Med 2010; 170, 1758-1764.