The study group consisted of a large group of postmenopausal women, approximately 73,615 women who took part in a study begun by the American Cancer Society in 1992 called the CPS-II nutritional cohort study. During the seventeen year study 4,760 women were diagnosed with breast cancer and the researchers specifically investigated breast cancer status and levels of exercise.
“Our results clearly support an association between physical activity and postmenopausal breast cancer, with more vigorous activity having a stronger effect,” said Alpa Patel, PhD, leader of the study and American Cancer Society strategic director of Cancer Prevention Study-3. “Our findings are particularly relevant, as people struggle with conflicting information about how much activity they need to stay healthy. Without any other recreational physical activities, walking on average of at least one hour per day was associated with a modestly lower risk of breast cancer. More strenuous and longer activities lowered the risk even more.”
The findings were independent from weight and women also benefited whether or not they were taking menopausal hormone therapy. Exercise reduced the risk of both estrogen receptor positive and estrogen receptor negative cancers. (In estrogen receptor-positive breast cancer, the female hormone estrogen promotes the growth of the cancer. Hormone therapy for breast cancer works by blocking the effects of estrogen or lowering estrogen levels. Estrogen receptor-negative breast cancer is often harder to treat because it is not likely to respond to hormone therapy.)
Hildebrand JS, Gapstur SM, Campbell PT, Gaudet MM, Patel AV. Recreational physical activity and leisure-time sitting in relation to postmenopausal breast cancer risk. Cancer Epidemiology, Biomarkers, and Prevention, October 2013