Prolonged sitting time is a common feature of contemporary society.21
However, compared with other modifiable health behaviours, such as diet, smoking, and lack of exercise, the potential health risks of this ubiquitous behaviour are relatively unknown. This is the first study in a large, representative, multi-ethnic, population-based sample to examine the associations of objectively derived total sedentary time and breaks in sedentary time with cardio-metabolic and inflammatory risk biomarkers. Independent of exercise time and other potential confounders, total sedentary time was detrimentally associated with several biomarkers, whereas breaks, independent of sedentary time, were beneficially associated with waist circumference, C-reactive protein, and fasting plasma glucose. These findings complement and build upon previous results from smaller and/or less-diverse populations and highlight the importance of considering prolonged sedentary time as a distinct health risk behaviour that warrants explicit advice in future public health guidelines.5
In particular, the findings are likely to have implications for settings where prolonged sitting is widespread, such as office workplaces.
Our findings overall, and among non-Hispanic white participants, were consistent with those observed among Australian and European adults;11–15
namely, the strongest associations with sedentary time were observed for triglycerides and markers of insulin resistance, rather than for blood pressure. This is consistent with plausible physiological mechanisms: fewer skeletal muscle contractions may result in reduced lipoprotein lipase activity and clearance of plasma triglycerides, reduced clearance of an oral glucose load from plasma, and less glucose-stimulated insulin secretion.21
The magnitude of the differences between the top and bottom quartiles were clinically meaningful for triglycerides and insulin resistance suggesting that, in theory, population wide reductions in sedentary time (of a magnitude of 1–2 h per day) could have a substantial impact on CVD prevention.22
This study also showed, for the first time, that sedentary time was detrimentally associated with C-reactive protein, while breaks were beneficially associated. Given that C-reactive protein is an inflammatory marker associated with increased risk of several major disease, including coronary heart disease and vascular mortality,4
inflammation may be an adjunct pathway (along with reduced muscular contractions) though which prolonged sedentary time may impact on CVD risk. Furthermore, as the correlation between sedentary time and light-intensity time was almost perfectly inverse, findings could also reflect the benefits of light-intensity activity.11,12
A key contribution of this study is the confirmation and extension of our previous findings15
indicating that patterns of sedentary time accumulation are important in addition to amount of sedentary time. This was particularly pertinent for waist circumference where those in the top quartile of breaks had, on average, a 4.1 cm lower waist circumference than those in the lowest quartile. Of importance to note is that a break could be as short as 1min and not necessarily entail ‘exercise’, suggesting that regular breaks from sedentary time are probably feasible in many contexts. However, the measure of breaks is relatively unsophisticated: it does not differentiate between breaks of a long and/or high intensity, and those of a short, low intensity. More detailed examination of sedentary time patterns, as well as laboratory experimental studies and real-world intervention trials examining the effects of reducing and/or breaking up sedentary time, are needed.
A unique element of this study was the examination of the relationship of objectively derived sedentary time with cardio-metabolic biomarkers by race/ethnicity. The patterning of findings by race/ethnicity has some coherence with studies showing racial/ethnic differences in the relationship of waist circumference with visceral adiposity,23
the compensatory responses to insulin resistance,24
lipoprotein responsiveness to exercise,25
and the relationship between triglycerides and insulin resistance.26
For waist circumference in particular, the racial/ethnic differences were quite pronounced—with no evidence of a detrimental association of sedentary time at all with Mexican Americans and non-Hispanic blacks. Regardless of whether the racial/ethnic differences are biological, or due to unmeasured confounding factors, it is clear that this field of research needs to expand beyond predominantly white populations, to explore this heterogeneity and avoid potentially inappropriate generalization of findings.
A strength of our study is the objective measurement of exposure variables. Compared with self-report, objective measures are more precise, less biased, and reduce the potential for differential measurement errors. However, some error could still be present. First, estimates depend heavily on wear time, which was estimated rather than directly measured. Second, the uniaxial accelerometer predominantly captures ambulatory activities and cannot distinguish between different postures or variations in walking conditions. Thus, though the sedentary cutpoint (<100 cpm) provides a useful estimate of sitting time,8
some standing still time may also be included as sedentary time.
Although we controlled for confounding using several well-measured relevant variables, including measures of health status, residual confounding is possible. For example, adjustments were not made for occupational characteristics (unavailable for the 2005/06 survey). This is unlikely to be an important issue, as a sensitivity analysis of the 2003/04 data (not reported) adjusted for employment status and work type, did not attenuate any of the associations statistically or in terms of effect size; most interactions were also unaffected. There may have been some selection bias as we excluded a large proportion of participants, predominantly for lacking sufficient accelerometer data to acquire habitual estimates of sedentary time; however, this bias is likely to be minimal, particularly in view of the reweighting for accelerometer non-response. Of importance to note is that these associations were cross-sectional. Thus, reverse causation is a possibility, and causality cannot be determined.
In summary, these population-based findings provide further evidence on the deleterious associations of sedentary time with cardio-metabolic health in adults, and provide novel evidence on the relationship of sedentary time with the inflammatory biomarker C-reactive protein. Furthermore, we found significant beneficial associations of breaking up sedentary time with cardio-metabolic health—particularly waist circumference—independent of overall sedentary time. In general, these associations were consistent across sex, age, and race/ethnicity subgroups. Prolonged sedentary time is likely to increase with future technological and social innovations,21
and it is important to consider a whole of day approach to physical activity promotion. Reducing and regularly breaking up sedentary time may be an important adjunct health message, alongside the well-established recommendation for regular participation in exercise. While further evidence of a causal nature is required from longitudinal and intervention studies, less sitting time would be unlikely to do harm, and would, at the very least, contribute to increase overall levels of energy expenditure.