Abstract
BACKGROUND: Exercise is a modifiable factor that is inversely related to risk for breast cancer. To determine if physical activity has a preventative effect on development of premalignant breast lesions, we examined the association between exercise and the incidence of proliferative benign breast disease. METHODS: In 1997, the Nurses' Health Study II cohort reported levels of physical activity during adolescence and adulthood using a validated recall instrument. We followed 40,318 participants free from benign breast disease (BBD) or cancer prospectively for four years and confirmed 232 proliferative benign breast lesions by centralized pathology review. Cox proportional hazards models estimated the age-adjusted and multivariable-adjusted relative risks for physical activity and proliferative benign breast disease. RESULTS: We observed a significant inverse association for walking and incidence of BBD, risk was reduced by 9% per hour of walking (95% CI 0% to 17%), (p trend = 0.05). Despite a small number of cases, risk of columnar cell lesions also suggested an inverse association with strenuous activity (RR for 4 or more hours of strenuous activity per week = 0.62; 0.31-1.22 compared to < 1 h per week). CONCLUSIONS: This study suggests that exercise may be inversely associated with the risk of developing proliferative benign breast disease, one of the earliest steps in the development of breast cancer.