Physical Activity, Sitting Time, and Mortality Risk
Does physical activity attenuate the mortality risk associated with prolonged sitting?
Studies for this meta-analysis were identified through six databases (PubMed, PsycINFO, Embase, Web of Science, Sport Discus, and Scopus) for published studies reporting both daily sitting or TV-viewing time and physical activity through October 2015. Studies needed to report effect estimates for all-cause mortality, cardiovascular mortality, and breast, colon, and colorectal cancer mortality. Study designs included were prospective cohort studies that had participant level information on physical activity and sitting behaviors and mortality data. A total of 16 studies were included, of which two were additional unpublished studies where pertinent data were available. Reported daily sitting time and TV-viewing time were categorized into four standardized groups each and then divided into quartiles (in metabolic equivalent of task [MET]-hours per week). Physical activity was handled in a similar manner. The primary outcomes of interest were all-cause mortality.
Of the 16 studies included in this meta-analysis, 13 reported sitting time and all-cause mortality. These studies included 1,005,791 participants who were followed up between 2 and 18.1 years. A total of 84,609 (8.4%) of the participants died during the follow-up period. The referent group consisted of those who sat <4 hours/day and were in the most active quartile (>35.5 MET-hours per week) for physical activity. Compared to the referent group, mortality rates were 12–59% higher in the two lowest quartiles of physical activity (from hazard ratio [HR], 1.12; 95% confidence interval [CI], 1.08-.16, for the second lowest quartile of physical activity [<16 MET-hours per week] and sitting <4 hours/day; to HR, 1.59; 95% CI, 1.52-1.66, for the lowest quartile of physical activity [<2.5 MET-hours per week] and sitting >8 hours/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 group, there was no increased risk of mortality among those who sat for >8 hours/day, but who also reported >35.5 MET-hours per week of activity (HR, 1.04; 95% CI, 0.99-1.10). By contrast, those who sat the least (<4 hours/day) and were in the lowest activity quartile (<2.5 MET-hours per week) had a significantly increased mortality risk during follow-up (HR, 1.27; 95% CI, 1.22-1.31). Six studies had data on TV-viewing time, which included 465,450, of which 43,740 died. Watching TV for 3 hours 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 hours/day or more (HR, 1.16; 95% CI, 1.05-1.28).
The investigators concluded that high levels of moderate-intensity physical activity (i.e., about 60-75 minutes/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.
This large meta-analysis examines an important question about the relationship of sedentary time and physical activity on all-cause mortality. The data support the need for regular physical activity for those with sedentary lifestyles, and the need for interventions to reduce sedentary time, in particular TV-viewing time.
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