Adolescent physical activity in relation to breast cancer risk.

Adolescent physical activity may protect against premenopausal breast cancer. Whether it also prevents postmenopausal breast cancer, and whether associations are independent of adult activity, is unclear. We evaluated this association among 75,669 women in the Nurses' Health Study II. In 1997, participants reported strenuous, moderate, and walking activity (hours/week) at ages 12-13, 14-17, 18-22, and 23-29 years. We estimated metabolic equivalent task hours (MET-h)/week. Participants also reported current physical activity over follow-up. Breast cancer diagnoses (n = 2,697; premenopausal = 1,351; postmenopausal = 965) through 2011 were reported by participants and confirmed with medical records. We additionally stratified analyses by median age at diagnosis. In Cox proportional hazards models adjusted for adolescent characteristics, physical activity from ages 14-22 was modestly inversely associated with premenopausal breast cancer [e.g., hazard ratio (HR) comparing 72+ to <21 MET-h/week 0.81 (95 % confidence interval (CI) 0.69-0.95; p-trend = 0.10) for ages 14-17 and 0.85 (95 % CI 0.71-1.02; p-trend = 0.06 for ages 18-22]. However, adjustment for adult activity and additional breast cancer risk factors attenuated the associations [ages 14-17: 0.85 (95 % CI 0.73-1.00; p-trend = 0.33)]. Associations were stronger among women diagnosed at younger ages [e.g., ages 18-22, HR 0.77 (95 % CI 0.60-0.99; p-trend = 0.05) for women diagnosed before 46.9 years; HR 1.02 (95 % CI 0.79-1.32; p-trend = 0.94) for those diagnosed at/after 46.9 years]. Early life physical activity was not associated with postmenopausal breast cancer. Overall, adolescent physical activity was not associated with breast cancer risk. However, we observed a suggestive inverse association of physical activity at ages 14-22 years with premenopausal breast cancer.