Do the associations of sedentary behaviour with cardiovascular disease mortality and cancer mortality differ by physical activity level? A systematic review and harmonised meta-analysis of data from 850 060 participants

ObjectiveTo examine whether the associations between sedentary behaviours (ie, daily sitting/TV-viewing time) and mortality from cardiovascular disease (CVD) and cancer differ by different levels of physical activity (PA).DesignHarmonised meta-analysis of prospective cohort studies. Data on exposure variables were harmonised according to a predefined protocol and categorised into four groups for sedentary behaviours and into quartiles of PA (MET-hour/week).Data sourcesPubMed, PsycINFO, Embase, Web of Science, Sport Discus and Scopus.Eligibility criteria for selecting studiesIndividual level data on both sedentary behaviours and PA and reported effect estimates for CVD or cancer mortality.ResultsNine studies (n=850 060; deaths=25 730) and eight studies (n=777 696; deaths=30 851) provided data on sitting time and CVD and cancer mortality, respectively. Five studies had data on TV-viewing time and CVD (n=458 127; deaths=13 230) and cancer (n=458 091; deaths=16 430) mortality. A dose–response association between sitting time (9%–32% higher risk; p for trend ConclusionPA modifies the associations between sedentary behaviours and CVD and cancer mortality. These findings emphasise the importance of higher volumes of moderate and vigorous activity to reduce, or even eliminate these risks, especially for those who sit a lot in their daily lives.