Exercise, including walking, can substantially reduce a woman’s risk of developing breast cancer by changing how her body metabolizes estrogen.  We have known for some time that regular physical activity lowers the risk of many types of cancer, including breast cancer.  However, the physiological mechanisms involved have been ambiguous.  In addition, the type and amount of exercise necessary for protection have not been well characterized. A new study published this month in Cancer Epidemiology, Biomarkers & Prevention, researchers with the Epidemiology Research Program at the American Cancer Society combed a trove of data maintained by the cancer society.  This included detailed health and medical information from more than 73,600 postmenopausal women, age 50-73, who enrolled in the study in the early 1990s.  For almost two decades, they completed follow-up questionnaires every two years.  They were asked, among other topics, for detailed descriptions of how leisure time was spent and whether or not they exercised.  About 9% reported never exercising.  A few said that they exercised vigorously and often, usually by participating in running, swimming or playing singles tennis.  However, what did most of the women report doing for exercise?  They walked, usually at a pleasant speed of about 3 miles per hour.  About half of this group reported that this was their only form of exercise. Over the course of the study, 4760 of the women developed breast cancer.  When the researchers analyzed the data, they found that those women who walked at least seven hours per week, usually about an hour per day, had a 14 percent reduced risk of developing breast cancer than those who walked fewer than three hours per week.  Meanwhile, the small number of women who were the most active – engaging in vigorous exercise  for up to 10 hours per week - enjoyed a 25 percent reduced risk of developing breast cancer than those who exercised the least.  These risk reductions held true whether or not the women were overweight, or using hormone replacement therapy. Another interesting study that looked at younger women, published in May in Cancer Epidemiology, Biomarkers & Prevention, furthers comprehension of how exercise may reduce breast cancer risk.  For this experiment, scientists from the School of Public Health at the University of Minnesota divided several hundred sedentary, premenopausaal women into two groups.  One group remained sedentary, while the other began a moderate aerobic exercise program 5 times per week for 16 weeks.  At the beginning and end of the 4 months, the researchers tested urine for levels of estrogen and various estrogen metabolites.  Past studies have found that a particular ratio of these metabolites in a woman’s urine indicates a heightened risk of breast cancer over a woman’s lifespan.  In this case, the volunteers who remained sedentary showed no changes in the ratio of their estrogen metabolites after four months.  However, among the exercising group, the levels of one metabolite fell while another rose, shifting the ratio in ways that are believed to indicate a reduced probability of developing breast cancer.  The women also lost body fat and gained muscle, which may have implications for older women whose estrogen production resides in fat cells as opposed to the ovaries. Overall, it would be foolhardy to believe that regular exercise is a panacea for all ills.  Some women who have always exercised develop breast cancer, and some women who have never exercised do not.  However, these two studies add to a robust literature on the power of regular exercise to reduce breast cancer risk, and can serve to motivate sedentary women to incorporate more physical activity into their daily lives. References: Hildebrand, JS et al. Recreational Physical Activity and Leisure-Time Sitting in Relation to Postmenopausal Breast Cancer Risk.  Cancer Epidemiology, Biomarkers & Prevention, October 2013 22; 1906. Reynolds, G. How Walking May Lower Breast Cancer Risk.  New York Times Well Blog, retrieved 06/11/2013.