Abstract PD4-03: Physical Activity, Reproductive Factors, and Breast Cancer Risk: A Prospective Cohort Study using data from the UK Biobank

Abstract Introduction: Reproductive factors such as early menarche, late menopause, nulliparity, and exogenous hormone use are well-established risk factors for breast cancer. Physical activity is associated with a lower risk of breast cancer, but whether this protective effect varies across reproductive risk factors is not well characterized. Objective: To investigate whether reproductive risk factors modify the association between physical activity and breast cancer risk. Methods: Females ≥40 years without a history of cancer from UK Biobank provided self-reported data on moderate-to-vigorous intensity physical activity and reproductive factors (2007-2010). Physical activity levels were categorized based on reported metabolic equivalent hours per week (MET-hrs/week), with high levels of physical activity defined as ≥50 MET-hrs/week, moderate levels of physical activity as 10 to 0.05 for interaction terms and RERI estimates). Conclusions: Physical activity is a useful breast cancer risk reduction strategy for women, regardless of their reproductive history. Citation Format: H. Harsanyi, A. Harper, H. Freisling, N. Slot, S. Lupichuk, A. Toriola, L. Yang. Physical Activity, Reproductive Factors, and Breast Cancer Risk: A Prospective Cohort Study using data from the UK Biobank [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD4-03.