Do periods of sedentary behavior, in particular sitting, increase the risk of mortality and age-related disease regardless of whether or not there are periods surrounding exercise? The epidemiological research community can take decades and dozens of studies to chew over questions like this. Most recently, evidence was presented to suggest that sitting for longer periods of time is an independent risk factor for mortality even for those who exercise. The study here presents evidence for a more nuanced conclusion, that exercise does compensate for periods of time spent sitting.
This sort of contradictory data is very much par for the course in this area of study: ignore any single set of results, and look for consensus across as many studies as possible. Meanwhile consider whether or not the arrow of causation might point from health and mortality risk to behavior such as sitting and activity; are less active people exhibiting higher mortality because unhealthy people tend to be less active, for all the obvious reasons, for example?
For less active adults, the amount of time spent sitting may be associated with an increased risk of death; however, increasing physical activity to recommended levels may eliminate this association in some. Recent studies have determined that high levels of sedentary behavior are associated with adverse health outcomes. However, the link between sedentary behavior, mortality, and heart disease are not always well understood.
In this study, researchers aimed to determine the association between sedentary behavior and physical activity on mortality and to estimate the effects of replacing sitting with standing, physical activity and sleep. Participants included 149,077 Australian men and women aged 45 years and older who were asked to complete a questionnaire that determined how many hours per day an individual spent sitting, standing and sleeping. They also were questioned about the total time spent walking or participating in moderate or vigorous physical activity.
After a median follow up time of 8.9 years for all-cause mortality and 7.4 years for cardiovascular disease mortality, higher sitting times (more than six hours) were associated with higher all-cause and cardiovascular disease mortality risks, but mostly in those did not meet physical activity recommendations. Meeting even the lowest requirements for physical activity eliminated the association with all-cause mortality risk, with the exception of those who sat the most (more than 8 hours a day). Compared to those who were highly active and sat for less than four hours per day, the risk remained substantially elevated even among physically inactive participants who sat for 4 hours per day only.
While replacing sitting with standing was associated with risk reduction in low sitters, replacing sitting with physical activity was more consistently associated with risk reduction in high sitters. The researchers found that moderate physical activity only reduced cardiovascular disease death risk among high sitters. The largest replacement effects were seen for vigorous physical activity, but this level of activity may not be possible for all adults.