As Travis Saunders has explained, evidence is accumulating about the unhealthy effects of excessive sedentary time. This isn't just because sitting burns fewer calories than walking or standing, but because sedentary behavior is associated with changes in triglyceride uptake, HDL cholesterol, and insulin resistance. And bouts of intense exercise every morning or evening can't completely offset the effects of spending several hours sitting at a desk or behind the wheel.
Given that a large segment of our population works at sedentary jobs, this is disturbing news. But a recent story by NPR's Patti Neighmond offered some hope, with a mention of an Australian study that suggests brief activity breaks can reduce the risk of metabolic problems from extensive sedentary time. (The online version of the story links to the study's PubMed page, which is a helpful touch.)
The study, by researchers from the Cancer Prevention Research Centre of the University of Queensland, Brisbane's School of Population Health, involved 168 adult participants without diabetes who were recruited from five sites of the Australian Diabetes, Obesity and Lifestyle Study. Researchers measured participants' waist circumference, blood pressure, glucose levels, fasting serum triglycerides, and HDL cholesterol levels, and then instructed participants to wear accelerometers (which measure physical activity) during all non-waking hours for seven consecutive days.
The accelerometer data let researchers see how much time participants spent being sedentary and engaging in physical activity, as well as how intense their activity was. Breaks in sedentary time of at least one minute were noted; these could include light-intensity activities like "standing from a seated position or walking a step." Researchers divided participants into quartiles based on the number of breaks they took. On average, the participants' waking hours broke down as 57% sedentary, 39% in light-intensity activity, and 4% in moderate-to-vigorous-intensity activity.
The researchers then used forced-entry linear regression models to examine the association of breaks with the metabolic risk variables of waist circumference, BMI, serum triglycerides, HDL cholesterol, blood pressure, and plasma glucose. They adjusted their models for potential confounders, including the intensity of breaks, total sedentary time, and time spent in moderate-to-vigorous-intensity activity, as well as a range of other demographic and health factors (smoking, alcohol use, diet quality, etc.). The results showed that breaks in sedentary time were associated with better scores on several measures of metabolic health (emphasis added):
Overall, independent of total sedentary time, the total number of breaks in sedentary time was associated with significantly lower waist circumference, BMI, triglycerides, and 2-h plasma glucose (Table 2). The associations with triglycerides (Î² = â0.13, 95%CI â0.29 to 0.03, P = 0.097) and 2-h plasma glucose (Î² = â0.14, 95%CI â0.29 to 0.02, P = 0.082) were attenuated when waist circumference was included in the model. ... Compared to those in the lowest quartile of breaks in sedentary time, those in the highest quartile had, on average, a 5.95 cm lower waist circumference (P = 0.025) and a 0.88 mmol/L lower 2-h plasma glucose (P = 0.019).
The journal Diabetes Care published the study results, and the full text is available online for free.
The authors note that "this is an observational study using cross-sectional data, and further investigations are required to determine possible causal associations." They also point out that since they were only identifying breaks using the one-minute cutoff, they could not determine whether breaks of even shorter duration or less intensity might also be associated with beneficial metabolic effects.
I'd love to see a study that gets at the causality angle with a workplace population. For instance, take a population of workers doing similar repetitive tasks at computers all day, and randomly assign them to a control group or one of several intervention groups. The intervention groups would receive prompts on their computers that would guide them through a series of movements, with different groups experiencing the prompts at different intervals and engaging in breaks of different durations. Metabolic measurements like the ones in the Australian study would be made before and after the intervention, which ideally would last for a few months. Comparisons between, say, the group that takes 90-second breaks every 20 minutes and the group that takes 30-second breaks every 60 minutes would help determine how much the length and duration of breaks matters, while the randomization and pre- and post-testing could get at the issue of causality.
A quick PubMed search suggests that many workplace stretching programs have been studied as interventions to reduce musculoskeletal disorders (some of them specifically study stretch-prompting software), but the effect of exercise breaks on metabolic health seems to be relatively new territory for research. (Maybe more research is underway and we'll see more publications soon?) The Mayo Clinic's James Levine -- who was featured in a New York Times magazine piece last month -- is one researcher who's been carefully tracking research subjects' every movement and combining that data with information on calorie intake, weight, and body fat. As I wrote in a previous post, findings by Levine and his colleagues add to the evidence that the relationship between physical activity, obesity, and metabolic effects is not as straightforward as we might have assumed.
More research will be helpful to learn more about the metabolic effects of sedentary time and how best we can prevent or limit those effects. In the meantime, though, those of us who spend long hours sitting (or lounging on the couch) should feel encouraged to take more breaks - even if that just means standing up and walking around for a minute or two. If such breaks aren't feasible at work (e.g., if you're a long-distance trucker), fitting them in during web-surfing or TV-watching leisure time would still be a good idea. If you've been sitting down to read this, wouldn't now be a good time to get up and stretch?
Healy, G., Dunstan, D., Salmon, J., Cerin, E., Shaw, J., Zimmet, P., & Owen, N. (2008). Breaks in Sedentary Time: Beneficial associations with metabolic risk Diabetes Care, 31 (4), 661-666 DOI: 10.2337/dc07-2046
Interestingly, my colleagues at Queen's University have just published a study in children looking at this same question and found both total sedentary time and breaks in sedentary time to have little or no association with metabolic risk. It was using a large, nationally representative sample (NHANES 2003-2006) as well. Very interesting, and not quite certain how all of these findings will eventually fit together.
Thanks for the link, Travis - that's a fascinating study, and its 2,500 subjects makes it far larger than the others I've read. As the authors note, there's a possibility that children's physiological differences are responsible for the difference between the findings of this study and the studies on adults. They do also point out that their way of measuring breaks was different from the Australian study, which didn't specify that breaks had to occur after 30+ minutes of sedentary time ... but since their methodology was otherwise very similar, I'd be surprised if that accounted for very much of the difference in findings.
The relationship they found between hours of TV time -- but not computer time -- and cardio-metabolic risks scores was also interesting.