Physical Activity during Adolescence and Early Adulthood and Breast Cancer Risk before Age 40 Years

Background: Breast cancer incidence is increasing in women under age 40, underscoring the need for research on breast cancer risk factors for younger women. Methods: We used data from an international family cohort (n ¼ 26,348) to examine whether recreational physical activity (RPA) during adolescence and early adulthood is associated with breast cancer risk before age 40. The cohort includes 2,502 women diagnosed with breast cancer before age 40, including 2,408 diagnosed before study enrollment (68% within 5 years of enrollment). Women reported their average hours per week of moderate and strenuous RPA during adolescence (12–17 years) and early adulthood (25–34 years), which were converted to total age-adjusted metabolic equivalents per week and categorized into quartiles. We conducted attained age analyses until age 40 (follow-up time began at age 18) using Cox proportional hazards regression models adjusted for study center, race and ethnicity, and education. Results: Being in the highest versus lowest quartile of RPA during adolescence and early adulthood were respectively associated with 12% [HR (95% confidence interval, or CI), 0.88 (0.78–0.98)] and 16% [HR (95% CI), 0.84 (0.74–0.95) lower breast cancer risks before age 40. Being in the highest quartile of RPA during both adolescence and early adulthood (Pearson correlation ¼ 0.52) versus neither time point was associated with a 22% lower risk [HR (95% CI), 0.78 (0.68–0.89)]. Conclusions: Findings suggest that RPA during adolescence and early adulthood may lower breast cancer risk before age 40. Impact: Policies promoting physical activity during adolescence and early adulthood may be important for reducing the growing burden of breast cancer in younger women.