Prof Juan Carlos Kaski, St George's, University of London, UK

How to increase physical activity to mitigate the increased mortality risk associated with sedentary behaviour at home and at work – A major challenge confronting individuals and institutions.

It is known that sedentary behaviour, extremely common nowadays in the developed world, is associated with increased risks for the development of several chronic conditions and increased cardiovascular mortality. The Lancet’s initiative to highlight the importance of physical activity regarding disease prevention is truly worth praising. Among the important articles published in The Lancet series on physical activity, the paper by Ekelund et al (1), published on-line on 27th July 2016, is worth mentioning.

The authors aimed at assessing whether physical activity attenuates the detrimental effects of prolonged sitting. They looked at the association among sedentary behaviour, physical activity and all-cause mortality.  To that end Ekelund et al. carried out a systematic review of prospective cohort studies that had reported specifically on the following: “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”. 16 such studies fulfilled the pre-established criteria and were analysed as appropriate. The main focus of this investigation, as stated by the authors, was “to assess the association of daily sitting time and physical activity with all-cause mortality, and estimated summary hazard ratios using Cox regression”.

Of the 16 studies included in the meta-analysis, 13 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. The main findings were as follow: comparing those sitting <4 h/day v. 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. Daily sitting time, however, was not associated with increased all-cause mortality in subjects belonging in the most active quartile of physical activity. 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 mortality risk during follow-up (HR=1·27, 95% CI 1·22–1·31). 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).

This study has shown that high levels of moderate intensity physical activity (ie, about 60–75 min per day) appear to eliminate the increased mortality risk associated with high sitting time. Albeit not completely understood why, this high activity level attenuates, but does not eliminate the increased risk associated with high TV-viewing time. Without any doubt, the Ekelund study (1) has confirmed the beneficial effects of physical activity, and its ability to attenuate the risk derived from having to sit for long hours for work during the day, a common practice in most European countries and many more around the World. These findings will have implications regarding public health strategies for disease prevention and will represent a major challenge for employers worldwide, who will have to implement suitable logistic measures to mitigate the deleterious effects of sitting for long hours for work.

References

Ekelund F et al. 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 DOI: http://dx.doi.org/10.1016/S0140-6736(16)30370-1