High daily levels of physical activity can eliminate the harmful effect prolonged sitting

Background

Over time, many observational studies have shown that lack of physical activity is a major risk factor for morbidity and premature mortality. In modern life, sedentary behaviour accounts for the majority of time awake in most adults in high-income countries.

High amounts of sedentary behaviour, often assessed as daily sitting time or time spent viewing TV, have been associated with increased risks for several chronic conditions and mortality [1-3]. But the remaining questions is whether being active enough can attenuate or even eliminate the detrimental association of daily sitting time with mortality.

This is a systemic review and meta-analysis to examine the joint and stratified associations of sedentary behaviour and physical activity with all-cause mortality. Individual data from 16 studies were analysed according to a predefined protocol and included in a harmonised meta-analysis. All studies were prospective cohort studies that had individual exposure and outcome data, and that provided data on both daily sitting or TV-viewing time and physical activity.

Main results

• A clear dose-response association was seen for higher risk for all-cause mortality with increased sitting time in combination with lower levels of activity. As compared with the referent group (sitting 8 h/day of sitting time (HR: 1.59, 95%CI: 1.52-1.66).
• In the most active group, there was no significant relation between amount of sitting and mortality rates (HR: 1.00, 95%CI: 0.96-1.04 for 4-6 h/day, HR: 1.01, 95%CI: 0.97-1.06 for 6-8 h/day, HR: 1.04, 95%CI: 0.98-1.10).
• When comparing HRs of different groups, those in the most active quartile, but who also reported long sitting time (>8h/day), had a significantly lower risk (P<0.0001) of dying during follow-up (HR: 1.04, 95%CI: 0.99-1.10) than did the least active group who sat the least (<4h/day: HR: 1.27, 95%CI: 1.22-1.30).
• Analyses were also performed with TV-viewing time instead of sitting time (sample sizes were smaller). In those who watched TV for >5h/day, risk for all-cause mortality was increased by between 16% and 93% across activity quartiles.
• Among those most active, only TV-viewing for longer than 5 h/day was associated with increased mortality risk (HR: 1.16, 95%CI: 1.05-1.28), while in the least active quartile watching TV for less than 1 h/day was already associated with an increased mortality risk (HR: 1.32, 95%CI: 1.20-1.46).
• Analyses of a subset of studies showed that the results for cardiovascular disease mortality were similar to those observed for all-cause mortality, with rates 23-74% higher in the two lowest quartiles of physical activity as compared with those sitting
• The effect on cancer mortality was smaller, with increased risks of between 12% and 22% with more sitting time only seen in the least active quartile

Conclusion

References

Find this article online at The Lancet