Sedentary time and postmenopausal breast cancer incidence.

PURPOSE: The objective of this study was to evaluate the prospective association between sedentary time and postmenopausal breast cancer incidence, and whether associations differ by race/ethnicity, physical activity levels, and body measurements.

METHODS: The Women's Health Initiative Observational Study is a prospective cohort among women ages 50-79 years at baseline (1994-1998) (analytic cohort = 70,233). Baseline questionnaire data were used to estimate time spent sitting and total sedentary time. Associations between time spent sitting and invasive breast cancer incidence overall (n = 4,115 cases through September 2015), and by hormone receptor subtypes, were investigated using Cox proportional hazards regression. Analyses were replicated stratified by race/ethnicity, body measurements, and physical activity.

RESULTS: Among women in this study, 34.5% reported ≤ 5 h/day sitting, 40.9% reported 6-9 h/day and 24.7% reported ≥ 10 h/day. Time spent sitting (≥ 10 vs. ≤5 h/day adjusted HR = 1.00, 95% CI 0.92-1.09) was not associated with breast cancer incidence, regardless of hormone receptor subtype. Associations did not differ by race/ethnicity, physical activity, or body measurements.

CONCLUSIONS: Results from this study do not support an association between sedentary time and breast cancer incidence.