A physical activity and sedentary time balance index and all-cause, cancer, and cardiovascular mortality risk: UK Biobank accelerometry findings

Abstract Background Physical activity and sedentary time are co-dependent behaviours, yet are usually studied separately. We investigated the joint associations of objectively measured physical activity and sedentary time, quantified by the Physical Activity and Sedentary Time Balance Index (PASTBI), with all-cause, cancer, and cardiovascular disease (CVD) mortality. Methods This prospective cohort study included 92,656 UK Biobank participants (aged 40–69 years) who provided valid wrist-accelerometry data between 2013 and 2015. The PASTBI was calculated as total daily moderate-to-vigorous physical activity (MVPA; minutes/day) divided by total daily accelerometry-derived sedentary time (hours/day), expressed as minutes of MVPA per sedentary hour (min/h). Mortality outcomes were ascertained through linkage to national death registries. Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). Results Over a mean follow-up of 7.8 years (719,056 person-years), 3,979 deaths occurred. A graded association was observed across PASTBI quartiles. Compared with Q1, Q4 was associated with lower all-cause mortality (HR 0.52, 95% CI 0.47–0.57), cancer mortality (HR 0.62, 95% CI 0.55–0.71), and CVD mortality (HR 0.48, 95% CI 0.39–0.59) in the primary model. Conclusions In this UK Biobank cohort, higher accelerometry-derived PASTBI values were associated with lower all-cause, cancer, and CVD mortality. These observational findings extend prior self-report-based evidence but do not establish that changing PASTBI would reduce mortality. PASTBI may serve as an epidemiological summary measure of movement-behaviour balance, pending further validation in more representative populations.