Elsevier

The Lancet

Volume 388, Issue 10051, 24–30 September 2016, Pages 1302-1310
The Lancet

Articles
Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women

https://doi.org/10.1016/S0140-6736(16)30370-1Get rights and content

Summary

Background

High amounts of sedentary behaviour have been associated with increased risks of several chronic conditions and mortality. However, it is unclear whether physical activity attenuates or even eliminates the detrimental effects of prolonged sitting. We examined the associations of sedentary behaviour and physical activity with all-cause mortality.

Methods

We did a systematic review, searching six databases (PubMed, PsycINFO, Embase, Web of Science, Sport Discus, and Scopus) from database inception until October, 2015, for prospective cohort studies that had individual level exposure and outcome data, provided data on both daily sitting or TV-viewing time and physical activity, and reported effect estimates for all-cause mortality, cardiovascular disease mortality, or breast, colon, and colorectal cancer mortality. We included data from 16 studies, of which 14 were identified through a systematic review and two were additional unpublished studies where pertinent data were available. All study data were analysed according to a harmonised protocol, which categorised reported daily sitting time and TV-viewing time into four standardised groups each, and physical activity into quartiles (in metabolic equivalent of task [MET]-hours per week). We then combined data across all studies to analyse the association of daily sitting time and physical activity with all-cause mortality, and estimated summary hazard ratios using Cox regression. We repeated these analyses using TV-viewing time instead of daily sitting time.

Findings

Of the 16 studies included in the meta-analysis, 13 studies provided data on sitting time and all-cause mortality. These studies included 1 005 791 individuals who were followed up for 2–18·1 years, during which 84 609 (8·4%) died. Compared with the referent group (ie, those sitting <4 h/day and in the most active quartile [>35·5 MET-h per week]), mortality rates during follow-up were 12–59% higher in the two lowest quartiles of physical activity (from HR=1·12, 95% CI 1·08–1·16, for the second lowest quartile of physical activity [<16 MET-h per week] and sitting <4 h/day; to HR=1·59, 1·52–1·66, for the lowest quartile of physical activity [<2·5 MET-h per week] and sitting >8 h/day). Daily sitting time was not associated with increased all-cause mortality in those in the most active quartile of physical activity. Compared with the referent (<4 h of sitting per day and highest quartile of physical activity [>35·5 MET-h per week]), there was no increased risk of mortality during follow-up in those who sat for more than 8 h/day but who also reported >35·5 MET-h per week of activity (HR=1·04; 95% CI 0·99–1·10). By contrast, those who sat the least (<4 h/day) and were in the lowest activity quartile (<2·5 MET-h per week) had a significantly increased risk of dying during follow-up (HR=1·27, 95% CI 1·22–1·31). Six studies had data on TV-viewing time (N=465 450; 43 740 deaths). Watching TV for 3 h or more per day was associated with increased mortality regardless of physical activity, except in the most active quartile, where mortality was significantly increased only in people who watched TV for 5 h/day or more (HR=1·16, 1·05–1·28).

Interpretation

High levels of moderate intensity physical activity (ie, about 60–75 min per day) seem to eliminate the increased risk of death associated with high sitting time. However, this high activity level attenuates, but does not eliminate the increased risk associated with high TV-viewing time. 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 and may also inform future public health recommendations.

Funding

None.

Introduction

In a seminal 1953 Lancet paper, J N Morris and colleagues1 reported an increased risk of coronary heart disease in London bus drivers compared with conductors. Since then, many observational studies have shown that lack of physical activity is a major risk factor for morbidity and premature mortality.2, 3, 4 Indeed, estimates from 2012 indicated that not meeting physical activity recommendations is responsible for more than 5 million deaths globally each year.4

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.

Nowadays, sedentary behaviours are highly prevalent, and data from adults in high-income countries suggest the majority of time awake is spent being sedentary.5, 6 Further, high amounts of sedentary behaviour, usually assessed as daily sitting time or time spent viewing TV, have been associated with increased risks for several chronic conditions and mortality.7, 8, 9 A crucial question is: if one is active enough, will this attenuate or even eliminate the detrimental association of daily sitting time with mortality?8, 9

We therefore did a systematic review and meta-analysis to examine the joint and stratified associations of sedentary behaviour and physical activity with all-cause mortality, using data from studies that were analysed according to a standard protocol.

Section snippets

Data sources, literature search, and study selection

Following PRISMA guidelines,10 we identified 16 published11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 studies through a systematic review of six databases (PubMed, PsycINFO, Embase, Web of Science, Sport Discus, and Scopus) from database inception until Oct 30, 2014, updating the search up to Oct 10, 2015 (a detailed search description is provided in the appendix). We also obtained data from two additional studies,27, 28 when the pertinent data were available but not

Results

We identified 8381 articles by searching six different databases. We retrieved 98 papers for full text review, of which 16 studies11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 were identified as eligible for inclusion (figure 1). We also obtained data from two additional studies.27, 28 We used publicly available data29 for the follow-up of one of the studies.26 Therefore, we analysed individual data from 16 studies13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 27, 28, 29

Discussion

These analyses, including data from more than 1 million individuals, indicate that high levels of physical activity, equivalent to 60–75 min of moderate intensity physical activity per day, seem to eliminate the increased mortality risks associated with high total sitting time. Indeed, those in the highest physical activity quartile (about 60–75 min/day) who sat for more than 8 h daily had a significantly lower risk of dying during follow-up than did those who sat for less than 4 h in the least

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