A new prospective study has reported that regular exercise, especially brisk walking, is associated with reduced risk of breast cancer among postmenopausal women. Furthermore, an increase in the level of physical activity after menopause for those previously sedentary also appears to be beneficial. The study included 95,396 postmenopausal participants in the Nurses' Health Study. Physical activity was assessed every two to four years since the beginning of the study in 1986. A total of 4,782 invasive breast cancer cases were diagnosed during 20-year follow-up period (through 2006).

Women who participated in approximately one hour per day of brisk walking or the equivalent were found to have a 15% lower risk of breast cancer than those who engaged in less than one hour per week. The results were adjusted for other factors known to influence breast cancer risk (e.g., alcohol consumption and body weight). Compared to women who were least active at menopause and throughout the follow-up period, women who increased their level of activity during follow-up (to at least 30 minutes of walking at an average pace on most days of the week) were at 10% lower risk of breast cancer than those who did not increase their level of activity after menopause. In other words, less active women who start exercising get similar benefits to those who exercise all along. Among specific activities, brisk walking was found to be associated with lower risk than swimming, hiking, or jogging. However, walking was also the most common type of exercise, which may have skewed the findings, according to study author Dr. A. Heather Eliassen in a separate interview with Reuters Health.

Regular exercise was associated with reduced risk of both hormone receptor positive (ER+/PR+) and hormone receptor negative (ER-/PR-, including triple negative) tumors. Physical activity reduces circulating estrogen, and this has been proposed as a mechanism by which exercise reduces breast cancer risk. But the fact that exercise also reduced the risk of hormone receptor negative breast cancers (which are not promoted by estrogen) suggests that there are other explanations. "Physical activity also has an effect on chronic inflammation, and insulin sensitivity," Eliassen said. "So there are other ways in which physical activity could be acting" on breast cancer risk.