A lot of physical activity can offset the association between sitting time and mortality risk
Sitting Time, Physical Activity, and Risk of Mortality in Adults
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Introduction and methods
The benefits of physical activity (PA) are well-established and include the prevention, management and treatment of CV disease. At the lowest end of the physical activity spectrum, we see sedentary behavior (SB), which is commonly defined as low energy expenditure, of <1.5 metabolic equivalents (MET) in a sitting or reclining posture during waking hours .
Several studies have studied the effect of sitting time on CVD, while taking into account PA [2,3]. A drawback of the studies to date is, however, that they did not take into account that an increase in any type of activity will mean a decrease of time spent on another activity.
Due to the lack of evidence on the health-enhancing potential of PA alternatives to sitting, guidelines on SB are currently nonspecific and not always evidence-based. This study aimed to address this evidence gap, by examining the joint associations of sitting and PA with all-cause mortality (ACM) and CVD mortality in a large population sample of middle-age and older Australian adults. The theoretical effects of replacing sitting with standing, PA and sleep on mortality were estimated as a secondary aim. Data from the 45 and Up Study  were used, a large prospective cohort of men and women of 45 years and older (n=267.119). They completed a questionnaire about their usual activities at baseline, from January 2006 through December 2009.
- Total weekly time for walking for recreation or exercise, moderate intensity PA (MPA), and vigorous PA (VPA) was assessed and categorized into inactive, insufficiently active at the lower Australian PA recommendations limit (1-149 minutes), sufficiently active at lower recommendations limit (150-299 min), sufficiently active at upper limit (300-419 min) and ≥420 min. To calculate total time of MVPA (moderate and vigorous PA), each minute of VPA counted as 2 min of MPA.
- Daily sitting time was categorized as follows: <4, 4 - <6, 6 – 8 and >8h. Sitting was treated as a piecewise variable with a breakpoint at 6 h (≤6 or >6 h/day).
- Sleeping was treated as a piecewise variable with a breakpoint at 7h.
The core analytic sample consisted of 149.077 participants. Median follow-up for ACM was 8.9 years (1.355.574 person-years [PY]), in which 8689 deaths occurred. Median follow-up for CVD mortality was 7.4 years (1.144.279 PY), with 1.644 deaths.
Joint and stratified associations of sitting and PA with mortality risk
- In the joint MVPA-sitting analyses, combinations of higher sitting time and lower MVPA were associated with ACM risk in the physically inactive and insufficiently active groups (up to HR: ~1.8). In those meeting the lower PA recommendation, only sitting for >8 h/day was associated with increased risk (HR: 1.27).
- A dose-response relation between higher sitting and lower MVPA and risk of CVD mortality was seen in the inactive and insufficiently active groups. In those meeting the lower PA recommendation, all sitting groups were at higher risk (e.g. HR: 1.45 for 4-6 hours, similar risk in higher categories).
Replacement effects of sitting time with physical activity, standing and sleep
- Each additional hour of daily sitting was associated with higher ACM risk in high-sitters (HR: 1.04, 95%CI: 1.02-1.06), but not in low-sitters (HR: 1.01, 95%CI: 0.99-1.02).
- Replacing sitting with standing was associated with a small reduction in ACM risk, only in low sitters (HR: 0.97, 95%CI: 0.96-0.99).
- Replacing sitting with walking and VPA was associated with a lower ACM risk, in high sitters (walking HR: 0.78, 95%CI: 0.70-0.87, VPA: 0.69, 95%CI: 0.56-0.88).
- Each additional hour of daily sitting was associated with higher CVD mortality risk in high-sitters (HR: 1.07, 95%CI: 1.03-1.12), but not in low-sitters (HR: 1.02, 95%CI: 0.99-1.04).
- Replacement of 1 hour of sitting with standing was associated with a lower CVD mortality risk in low-sitters only (HR: 0.94, 95%CI: 0.91-0.98).
- Replacing sitting with MPA and VPA was associated with lower CVD mortality risk in both sit groups, but more pronounced in high-sitters (MPA: HR: 0.80, 95%CI: 0.70-0.93, VPA: HR: 0.36, 95%CI: 0.17-0.74).
Via a comprehensive analytic approach that took into account effects of both PA and sitting, as well as replacement effects, this study found that higher sitting times were associated with higher risk of all-cause and CV death, mostly restricted to those not meeting PA recommendations. Meeting the lower PA recommendation (150-299 min/week), by walking and VPA, eliminated the association of sitting with ACM risk.
The sitting level seemed to modify the magnitude of the associations between intensity of PA and mortality risk: for instance, replacing sitting by standing was beneficial in low-sitters, but had no effect in those who sit >6 h/day. The largest replacement effects were seen for VPA, while those of MPA were less consistent.
Matthews  applauds the efforts of Stamatakis and colleagues to extend our understanding of the interplay among sitting, PA and mortality and to ‘probe for the level of PA required to minimize the risks linked to too much sitting.’ He finds the joint-analysis results ‘particularly important because they shed light on how 20 different sitting-MVPA combinations may influence longevity.’ At recommended levels of MVPA (150-299 min/week), overall risk of ACM decreased as compared to less active adults, but those who sat >8 h/day were still at increased risk. This excess risk in high-sitters was only eliminated at >300 min/week MVPA. This is just above the current US recommendation of 150-300 min/week, but much above what the majority of Americans report to achieve.
Matthews translates the results of Stamatakis to three opportunities of lowering risk of early mortality for those who do not exercise at all and sit >8 h/day:
- Increase PA to the recommended 150-299 min/week without changes in sitting time. Some residual risk may remain due to too much sitting.
- Reduce sitting time to <4 h/day, without increasing MVPA. Some residual risk may remain due to physical inactivity
- Increase MVPA to recommended levels and reduce sitting time to <8 h/day. The results suggest this gives greater benefits than the first two options.
The recently published PA guidelines for Americans also focus more on sitting less and moving more. This is increasingly important, ‘given that sedentary behaviors appear to be vastly outcompeting more healthy PA behavior during our discretionary time’.
Do you encourage patients to sit less and move more (when appropriate)?