Association of changing physical activity intensity and bout length with mortality: a study of 79,507 participants in UK Biobank

BackgroundSpending more time active (and less time sedentary) is associated with many health benefits such as improved cardiovascular health and lower risk of all-cause mortality. However, it is unclear whether these associations differ depending on whether time spent sedentary or in moderate-vigorous physical activity (MVPA) is accumulated in long or short bouts. In this study we used a novel analytical approach, that accounts for substitution (i.e. more time in MVPA means spending less time sleeping, in light activity or being sedentary), to examine whether length of sedentary and MVPA bouts associates with all-cause mortality. Methods and findingsWe used data on 79,507 participants from UK Biobank. We derived the total time participants spent in activity categories - sleep, sedentary, light activity and MVPA - on average per day. We also derived the time spent in sedentary and MVPA bouts of short (1-15 minutes), medium (16-40 minutes) and long (41+ minutes) duration. We used Cox proportion hazards regression to estimate the association of spending 10 minutes more average daily time in one activity or bout length category, coupled with spending 10 minutes less time in another, with all-cause mortality. Those spending more time sedentary had higher mortality risk if this replaced time spent in light activity (hazard ratio 1.02 [95% confidence interval (CI): 1.01, 1.03]), and an even higher risk if this replaced time spent in MVPA (hazard ratio 1.08 [95% CI: 1.06, 1.10]). Those spending more time in MVPA had lower mortality risk, irrespective of whether this replaced time spent sleeping, sedentary or in light activity. We found little evidence to suggest that mortality risk differed depending on the length of sedentary or MVPA bouts. A limitation of our study is that we cannot assume that these results are causal, though we adjusted for key confounders. ConclusionsUsing our novel analytical approach, we uniquely show that time spent in MVPA is associated with reduced mortality, irrespective of whether it replaces time spent sleeping, sedentary or in light activity. This emphasizes the specific importance of MVPA. We found little evidence to suggest that the impact of MVPA differs depending on whether it is obtained from several short bouts or fewer longer bouts, supporting recent policy changes in some countries. Further studies are needed to investigate causality and explore health outcomes beyond mortality.