Childhood and teenage physical activity and breast cancer risk

Adult physical activity is associated with reduced breast cancer risk, but few studies have evaluated activity before adulthood. Early life may be an important period because of rapid breast development and hormonal changes. This study contributes new information by examining childhood (ages 5-12) and teenage (ages 13-19) activity separately and overall. The Sister Study is a cohort of 50,884 women aged 35-74. Women reported age 5-19 sports/exercise activities and age 10 and 16 unstructured activities. Both hours and MET-hours of activity were considered in association with breast cancer overall, by ER status, and by menopausal status. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated with Cox proportional hazards models. 2416 cases were diagnosed during follow-up (mean = 6.4 years). Participation in 7+ hours (vs < 1 h) per week of sports/exercise during ages 5-19 was associated with reduced breast cancer risk (HR = 0.75; 95% CI 0.57-0.99). 7+ hours (vs < 1 h) per week of unstructured physical activity at age 16, but not age 10, was inversely associated with breast cancer (HR = 0.81; 95% CI 0.70-0.95). Associations were more pronounced for ER+ tumors, especially for activity during the childhood (ages 5-12) period. Due to low correlation between childhood/teenage and adulthood activity in this study (r = 0.1), it is unlikely that recent activity explains our results. Findings from this large cohort indicate higher levels of physical activity during ages 5-19 are inversely associated with breast cancer risk, supporting early life as a window of susceptibility for breast cancer development.