Clinical Context

The average adult spends 50% to 60% of his or her time in sedentary behavior, according to the authors of the current study. Although television viewing is considered to be a classic form of sedentary behavior, it is not a strong correlate for total sedentary time, particularly among men. Moreover, persons with low levels of sedentary time are not necessarily highly physically active; most nonsedentary time is replaced with light physical activity, such as standing and light ambulation, instead of more vigorous pursuits.

Higher rates of sedentary behavior have previously been associated with a higher risk for diabetes, but the association between sedentary behavior and other clinical outcomes is less clear. The current meta-analysis by Wilmot and colleagues addresses this issue.

Study Synopsis and Perspective

The more time people spend sitting, the greater their risks of diabetes, cardiovascular events, and death, a new meta-analysis has shown [1]. This is the first research to systematically quantify the strength of association between sedentary behavior–beyond just TV viewing–and health outcomes and shows a particularly consistent relationship for diabetes, say Dr Emma G Wilmot (University of Leicester, UK)and colleagues in their paper in the November 2012 issue of Diabetologia.

Wilmot says that a number of important messages have emerged from the research. ”People don’t think about sitting as being dangerous, and it’s quite a change, having to think, ’how can I reduce my sitting?’ rather than just ’how much exercise can I do?’ We’ve traditionally been focused on making sure we meet the physical-activity guidelines of 30 minutes per day, but with that approach we’ve overlooked what we do with the other 23 and a half hours in the day. If you sit for the rest of the day, that is going to have an impact on health, and that’s essentially what our meta-analysis shows,” she told heartwire .

She stresses, however, that this does not mean that exercise is not important. ”That’s obviously not the case. There’s a wealth of data showing that physical activity is important, but if people are spending a large percentage of their time sitting, they need to start thinking about how they can reduce this.”

And this message applies across the world, says Wilmot, who says she has had journalists calling her from as far afield as Canada, Chile, India, Russia, South Africa, and the United States.

She and her colleagues add that much more research is needed to figure out how best to quantify and standardize measures of sedentary behavior and to formulate guidelines. ”At the moment, we don’t have enough of an evidence base to be able to give very specific recommendations about how much to reduce sitting time by. We need intervention studies to give us some guidance on what approach we should take.” She and her colleagues are now running a study in 200 young people at increased risk of diabetes, which they expect to report next year and which they hope will add to this evidence base.

Greatest vs Least Sedentary Time Doubles Risk of Diabetes

Wilmot and colleagues say the hazards of high levels of sitting were first highlighted in the 1950s, when a twofold increase in the risk of a myocardial infarction was identified in London bus drivers compared with active bus conductors. But since then, the ”potentially important distinction” between sedentary (sitting) and light-intensity physical activity has been ”largely overlooked” in research, they observe.

”The opportunities for sedentary behavior in modern society, such as watching television, sitting in a car, or using the computer, are ubiquitous,” they add, stating that estimates have put the time the average adult spends in sedentary pursuits at around 50% to 60% of their day.

For their review, the researchers searched for terms related to sedentary time and health outcomes. They combined the results of 18 studies including a total of 794 577 participants. The data were adjusted for baseline event rate and pooled using a random-effects model.

The greatest sedentary time compared with the lowest was associated with a doubling of diabetes (relative risk [RR] 2.12), around a 2.5-fold increase in the risk of cardiovascular events (RR 2.47), a 90% rise in risk of cardiovascular death (hazard ratio [HR] 1.90) and a 49% higher risk of all-cause mortality (HR 1.49). Based on the pooled effects, all of these findings were significant.

Further statistical analysis showed that the predictive effects were significant only for diabetes, which means the reproducibility of the diabetes finding was greater, suggesting this is the ”most robust” result, Wilmot noted.

”People don’t realize that doing just small amounts of activity–it doesn’t even need to be a proper walk–are important,” she says. ”If you are having a chat with a friend at your desk or the phone rings, stand up and chat. Just these small changes could make a big difference.”

Specific Reasons Why Sitting Ups Risk of Diabetes

Wilmot explained to heartwire that there appear to be specific reasons why sitting too long can be particularly deleterious in terms of diabetes. ”Sitting seems to have an immediate effect on how our bodies metabolize glucose. When we sit, our muscles are not used, and we quickly become more insulin resistant.” Studies have shown that people who sit after eating have 24% higher glucose levels than people who walk very slowly after a meal, she says.

It is also known that there are some individuals who are genetically predisposed to the adverse effects of sitting, including those who are susceptible to diabetes, ”so it might be especially important for these people to avoid prolonged sitting,” she observes. The exact metabolic pathways involved are not known, ”but what we do know is that when rats have their hind legs immobilized, there is a reduction in lipoprotein lipase, a key regulator of metabolic health.”

Further studies in this area are required, she says, and future diabetes-prevention programs should consider promoting reduced sedentary behavior–including environmental restructuring to promote less sitting–alongside more traditional lifestyle behaviors such as increased physical activity and dietary change.

Also needed is research on how best to quantify sitting using devices called accelerators–which can calculate how long people sit for–as well as work on how to standardize measures of sedentary time. This will include looking at the feasibility of reducing sitting time too, by employing simple concepts such as standing or ”walking” desks with treadmills or gadgets that people wear on their waist and that vibrate when the user has been sitting continuously for 40 minutes.

Wilmot and colleagues report no conflicts of interest.

References

1. Wimot EG, Edwardson CL, Achana FA, et al. Sedentary time in adults and the association with diabetes, cardiovascular disease and death: Systematic review and meta-analysis. Diabetologia 2012; 55:2895-2905. Abstract

Clinical Implications

• The average adult spends 50% to 60% of his or her time in sedentary behavior. Television viewing does not correlate well with total sedentary time, particularly among men. Time not spent in sedentary behavior is more likely to be spent in light physical activity vs more vigorous exercise.
• The current meta-analysis by Wilmot and colleagues finds a significant association between high levels of sedentary behavior and an increased risk for diabetes, with weaker associations between sedentary behavior and both cardiovascular events and mortality outcomes.