Research in context
Evidence before this study
Compelling evidence from many observational studies shows that lack of physical activity increases the risks of many non-communicable diseases such as type 2 diabetes, cardiovascular disease, stroke, some cancers, and premature mortality. Sedentary behaviour, on the other hand, has emerged as a potential risk factor for many chronic conditions and mortality during the last decade. A recent meta-analysis suggested that prolonged TV-viewing time was associated with increased risk for type 2 diabetes, cardiovascular disease, and all-cause mortality. Two other meta-analyses had examined the associations of sitting time with non-communicable disease incidence and mortality. One of these concluded that prolonged sitting time was associated with increased risks of deleterious health outcomes regardless of physical activity level, whereas the other concluded that physical activity (no details on the amount of activity were provided) seemed to attenuate the increased risk of all-cause mortality due to high sitting. No previous systematic review had directly compared the joint effects of different, specified levels of physical activity and sitting time, to investigate the associations of different amounts of sitting time and physical activity in relation to all-cause mortality. Such information is required for the development of public health guidelines targeting sedentary behaviour.
We performed a systematic literature search in six databases (PubMed, PsycINFO, Embase, Web of Science, Sport Discus, and Scopus) from database inception until October, 2015, following the PRISMA guidelines. We identified 8381 articles, of which 16 were identified as eligible for inclusion. We also identified two studies in which the pertinent data were available but unpublished. We then contacted the principal author/investigator of these 18 studies and asked whether they were willing to reanalyse their data according to a harmonised protocol. In total, 16 studies were analysed according to a predefined protocol and included in this harmonised meta-analyses (details on the two excluded studies are provided in the text).
Added value of this study
This is the first meta-analysis to use a harmonised approach to directly compare mortality between people with different levels of sitting time and physical activity. Examining the joint effects of these two behaviours is important, because most people engage in both behaviours every day, so the effects of both should be considered in public health guidelines.
Implications of all the available evidence
These results provide further evidence on the benefits of physical activity, particularly in societies where increasing numbers of people have to sit for long hours for work or transport. Our findings indicate that increased sitting time is associated with increased all-cause mortality; however, the magnitude of increased risk with increased sitting time is mitigated in physically active people. Indeed, those belonging to the most active quartile and who are active about 60–75 min per day of moderate intensity physical activity seem to have no increased risk of mortality, even if they sit for more than 8 h a day.