Sedentary Time and Breast Cancer Risk in the Sister Study Cohort

Abstract Background: Previous research investigating sedentary behavior and breast cancer risk has shown mixed results. We investigated the association between sedentary time and breast cancer incidence overall and by time-dependent menopausal status. Methods: The Sister Study recruited 50,884 women aged 35 to 74 years from all 50 states and Puerto Rico who had not been diagnosed with breast cancer but had at least one affected sister. Sedentary time was collected at the first detailed follow-up from 2008 to 2012 and categorized as ≤5 hours/day (referent), 6 to 9 hours/day, and ≥10 hours/day. Breast cancer cases were reported annually. We used multivariable Cox proportional hazards regression to estimate the HR and 95% confidence intervals (CI) for the association of sedentary time with overall breast cancer incidence, with age as the primary time scale and adjusted for relevant covariates. Participants were followed through September 2021. We evaluated effect measure modification by menopausal status. Results: Among the 39,111 eligible women with information on sedentary behavior and covariates, sedentary time [6–9 vs. ≤5 adjusted HR (aHR) = 1.18; 95% CI, 1.08–1.28; ≥10 vs. ≤5 aHR = 1.19; 95% CI, 1.07–1.32] was associated with higher breast cancer incidence. The association varied by menopausal status (P heterogeneity = 0.002), with sedentary time inversely associated with breast cancer among premenopausal women (≥10 vs. ≤5 aHR = 0.69; 95% CI, 0.50–0.95) and positively associated with breast cancer among postmenopausal women (6–9 vs. ≤5 aHR = 1.22; 95% CI, 1.11–1.33; ≥10 vs. ≤5 aHR = 1.28; 95% CI, 1.14–1.43). Conclusions: Increased sedentary time was associated with breast cancer incidence, but the direction of this association varied by time-dependent menopausal status. Impact: The impact of sedentary time on cancer risk may vary by menopausal status.