It shouldn't come as a surprise that sitting for long periods of time is bad for us. But new research suggests it isn't just the total amount of time we spend sitting each day that we need to worry about, it's also the length of time between bouts of physical activity. While still incomplete, these results suggest a sensible life hack that could help certain individuals stave off some of the effects of prolonged sitting.
According to the new study, excessive bouts of sitting are a major factor for all-cause mortality (that is, dying of any cause), whether the time spent sitting down was accumulated in one lengthy session or during prolonged bouts across the day. Discouragingly, this held true for people regardless of how much they exercised, or other health factors. On a more positive note, the researchers found that people who get up at least once every 30 minutes were less likely to die over the course of the study, hinting at a possible countermeasure to this modern health hazard.
But as other experts we spoke to made clear, these findings may not be generalisable across the entire population, and much more work needs to be done to determine why physical inactivity is so harmful to our health, and the kinds of things we can do to offset these risks.
What's clear is that unlike our physically-active forebears, many modern humans now spend an inordinate amount of time sitting each day -- and that isn't necessarily voluntary. Desk work is now ubiquitous, and the modern world provides other trappings as well, including long commutes in a vehicle and binge-watching TV. To date, studies have linked sedentary behaviour -- defined as any waking activity done while sitting or reclined -- to organ damage, muscle degeneration, back pain and many other conditions. Perhaps most alarmingly, sedentary behaviour has also been linked to mortality; the more you sit, the more likely you are to meet an untimely end.
The trouble with these prior studies, however, is that virtually all measures of physical inactivity were self-reported by the participants, which often leads to inaccuracies in the data. What's more, studies typically fail to measure the various ways in which people are inactive throughout the day, and how these differences may have influenced risk of death. The new study, headed by Keith Diaz from Columbia University Medical Center and published today in the Annals of Internal Medicine, corrects both of these shortcomings, suggesting that long periods of inactivity may be just as harmful as a person's daily total.
For the study, Diaz and his team analysed data collected by hip-mounted activity monitors to measure physical inactivity. Nearly 8000 African American and Caucasian participants in the United States over the age of 45 were monitored over a period of seven days (this data was collected as part of the Reasons for Geographic and Racial Differences in Stroke study). In this sample pool, the participants were sedentary for an average of 12.3 hours over the course of a 16-hour day, while the mean duration of sedentary bout was 11.4 minutes.
When the researchers followed-up four years later, 340 participants, or four per cent, had died. The researchers found that total accumulated sedentary time and longer bouts of sedentary time were associated with higher risk for all-cause mortality -- even after adjusting for age, sex, race, BMI or exercise habits. Participants who kept their bouts of physical inactivity to less than 30 minutes had the lowest risk for death. But for every 30 minutes of sedentary time, the researchers found that the likelihood of death among the study participants increased by about 19 per cent.
"In short, we found that long sitting bouts increases a person's risk of death," Diaz told Gizmodo. "We furthermore studied how long is too long for sitting bouts. We found that those individuals who frequently kept their sitting bouts to less than 30 minutes had a lower risk of death. So if you are a person who sits for long periods at work or at home, we think these findings suggest that taking movement breaks every 30 minutes could reduce your risk of death."
As to why sedentary behaviour is so apparently unhealthy, that's still an open question. Diaz speculates that it may have something to do with our skeletal muscles, which require fuel to operate and take in glucose from our blood. When we're inactive for extended periods, our body is continually exposed to higher blood glucose levels, which can lead to host of health problems. But this is still just a hypothesis.
"This paper is significant in that it has used rigorous scientific methodology to examine the associations and patterns of sedentary time in relationship to all-cause mortality," said Carol Ewing Garber, a behavioural sciences professor at Columbia University who wasn't involved in the new study. "This study has extended previous findings that show more definitively that both prolonged sedentary time and breaking up sedentary time are both important correlates of all cause mortality."
But Garber also pointed out a bunch of shortcomings and limitations to the research.
"First of all, this is an epidemiological study that shows associations or relationships between sedentary time, sedentary breaks and mortality," she told Gizmodo. "The study does not show causality between the variables. Also the results of this study cannot apply to an individual -- that is, if an individual alters their sedentary time and pattern of sedentary time, this does not mean that their personal risk is changed. We need more data from randomised control trials to provide further support for causality between changing behaviours and health-related outcomes to make more specific individual recommendations."
And as Garber also pointed out, this study is a secondary analysis of data collected for a different purpose, namely to examine racial, ethnic and geographical variation in stroke. And importantly, the participants were less healthy as a whole compared with the general population, as shown by higher rates of cardiovascular disease risk factors and cardiovascular diseases.
"While these factors were adjusted for statistically, readers should note that for this reason, the findings may not be generalizable to other populations comprised of different nationalities, racial and ethnic identities or healthier populations," said Garber. "With all studies, we need replication of the findings in different populations to solidify our conclusions concerning the veracity of the findings."
Sadly, that means that Diaz's prescribed "30-minute-rule" may not apply to everyone. As Garber points out, we're all different, and these conclusions aren't specific to individuals.
Diaz admits that only middle-aged African American and Caucasian individuals participated in the study, and that the findings "can't necessarily be generalised to younger adults or other race/ethnicities". Nonetheless, he says his team has "no reason to suspect that sedentary time [acts] differently in other populations," he told Gizmodo.
Vadim Zipunnikov, an Assistant Professor at the Department of Biostatistics at Johns Hopkins Bloomberg School of Public Health, says the study is important in that it's the first to explore the joint effect of excessive total sedentary time and prolonged sedentary bouts on mortality in a national sample pool, but like Garber, he has some reservations.
"The device that has been used in the study [did not allow the researchers] to estimate posture (sitting vs. standing vs. lying), therefore, estimated sedentary time may overestimate sitting time," Zipunnikov told Gizmodo. And, he added, "mean bout duration of sedentary bout may not be enough to describe all patterns of sedentary time accumulation."
While more work clearly needs to be done in this area, a growing body of literature points to a sedentary lifestyle being harmful to our health. But that doesn't mean we can't adjust our behaviours.
"Continue to strive to meet physical activity recommendations of 150 minutes of moderate intensity physical activity per week," Deborah Rohm Young, Director of Behavioural Research at Kaiser Permanente Southern California, told Gizmodo. "Whenever possible break up sedentary time with movement. For example, when watching TV, get up and move during commercials. Set an alarm at work to break up sitting when you can. Stay tuned to future research and recommendations that will come out when the studies are more confirmatory."