High daily levels of physical activity can eliminate the harmful effect prolonged sitting
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.
- 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
These analyses on data of over 1 million individuals indicate that high levels of physical activity (60-75 min of moderate intensity physical activity per day) appear to eliminate the increased mortality risks associated with prolonged sitting time. Those most active who sat for more than 8 hours daily showed a lower risk of dying during follow-up than those in the least active quartile 9
The effect of TV-viewing on all-cause mortality seemed stronger than that of sitting, and its effect was only attenuated, but not eliminated by high physical activity. One of several plausible explanations may be that TV-viewing typically occurs in the evening (at least for the generation represented in the studies), which may reflect prolonged postprandial sedentary time, which might be particularly harmful for glucose and lipid metabolism. Alternatively sitting time at work may be interrupted more often than sitting while viewing TV, and breaking up sitting time appears beneficial for cardio-metabolic risk factors.