In the past few years, a number of epidemiological studies have suggested a correlation between more time spent sitting and a greater risk of mortality and age-related disease. Intriguingly, the claim is that this correlation persists even in people who do in fact exercise sufficiently. This should be considered in the context that statistical studies of human health and activities are notoriously challenging, given the degree to which data must be massaged into shape. Seemingly small differences in choice of metric can produce opposing outcomes: for example, differences in assessing the trajectory of weight over a lifetime, as well as whether to measure body mass index or waist circumference, have at times in the recent past produced quite distorted views of the degree to which excess fat tissue is harmful.
So the matter is far from settled as to whether lengthy periods of sedentary behavior - such as sitting - are a cause of harm even in people who meet guidelines for activity and exercise. There is no good mechanism, beyond that this might reflect an association with any number of other behaviors or conditions or choices or states of life that are themselves correlated with shorter life expectancy. Sitting time correlates with vascular calcification, for example, but that doesn't say anything about why this might be the case in people who do exercise in between their seated periods. The most obvious suggestion is that this is a question of overall activity levels in life, and length of time spent sitting is just a good proxy for overall activity levels. If the past fifty years are any guide, researchers will keep turning out this sort of study to argue subtle points of statistical interpretation well into the era of functional rejuvenation therapies - by which point the whole exercise becomes somewhat irrelevant.
A new study finds that it isn't just the amount of time spent sitting, but also the way in which sitting time is accumulated during the day that can affect risk of early death. Adults who sit for one to two hours at a time without moving have a higher mortality rate than adults who accrue the same amount of sedentary time in shorter bouts. "We tend to think of sedentary behavior as just the sheer volume of how much we sit around each day. But previous studies have suggested that sedentary patterns - whether an individual accrues sedentary time through several short stretches or fewer long stretches of time - may have an impact on health."
The researchers used hip-mounted activity monitors to objectively measure inactivity during waking time over a period of seven days in 7,985 black and white adults over age 45. (The participants were taking part in the REGARDS study, a national investigation of racial and regional disparities in stroke.) On average, sedentary behavior accounted for 77 percent of the participants' waking hours, equivalent to more than 12 hours per day. Over a median follow-up period of four years, 340 of the participants died. Mortality risk was calculated for those with various amounts of total sedentary time and various sedentary patterns. Those with the greatest amount of sedentary time - more than 13 hours per day - and who frequently had sedentary bouts of at least 60 to 90 consecutive minutes had a nearly two-fold increase in death risk compared with those who had the least total sedentary time and the shortest sedentary bouts.
Excessive sedentary time is ubiquitous in Western societies. Previous studies have relied on self-reporting to evaluate the total volume of sedentary time as a prognostic risk factor for mortality and have not examined whether the manner in which sedentary time is accrued (in short or long bouts) carries prognostic relevance. Sedentary time was measured using a hip-mounted accelerometer. Prolonged, uninterrupted sedentariness was expressed as mean sedentary bout length. Hazard ratios (HRs) were calculated comparing quartiles 2 through 4 to quartile 1 for each exposure (quartile cut points: 689.7, 746.5, and 799.4 min/d for total sedentary time; 7.7, 9.6, and 12.4 min/bout for sedentary bout duration) in models that included moderate to vigorous physical activity.
Over a median follow-up of 4.0 years, 340 participants died. In multivariable-adjusted models, greater total sedentary time (HR, 1.22; HR, 1.61; and HR, 2.63) and longer sedentary bout duration (HR, 1.03; HR, 1.22; and HR, 1.96) were both associated with a higher risk for all-cause mortality. Evaluation of their joint association showed that participants classified as high for both sedentary characteristics (high sedentary time [≥12.5 h/d] and high bout duration [≥10 min/bout]) had the greatest risk for death.
These findings highlight the importance of the total volume of sedentary time and its accumulation in prolonged bouts as important health risk behaviors. Meta-analyses have shown that total sedentary time is associated with cardiovascular disease and mortality, independent of moderate physical exercise. However, these findings are largely based on self-reported sedentary time, data that may underestimate the magnitude of the relationship between sedentariness and health risk. Use of accelerometers reduces potential biases and measurement error inherent in self-reported data. To our knowledge, this is the largest study to date with objective measures of sedentary behavior and prospective health outcomes. The magnitude of the association between total sedentary time and all-cause mortality (2.6-fold greater risk for quartile 4 vs. quartile 1) is notably higher than that reported in meta-analyses (HR 1.22).
A key finding of our study, which we believe is the first to report, is that patterns of sedentary time accumulation are associated with mortality. Previous cross-sectional studies have reported associations between the total number of breaks in sedentary time per day (the reciprocal to mean sedentary bout length) and cardiometabolic biomarkers. These findings led to the "prolonger" versus "breaker" hypothesis, which postulates that it is not only the amount of sedentary time that is important to cardiometabolic health, but also the manner in which it is accumulated.