Too Much Sitting: The Population Health Science of Sedentary Behavior

Neville Owen; Geneviève N. Healy; Charles E. Matthews; David W. Dunstan

Disclosures

Exerc Sport Sci Rev. 2010;3(3):105-109. 

In This Article

Objective Assessment of Sedentary Time: New Findings

Advances in the Objective Measurement of Sedentary Behavior

These Australian studies previously summarized have all relied on self-reported TV time or overall sitting time. However, advances in measurement technology now provide significantly enhanced scientific traction, which is helping to deal with the methodological limitation of measurement error related to the use of self-report items. Before summarizing findings from our objective measurement studies with AusDiab study participants, it is helpful to consider the new perspectives that emerge when accelerometer data on sedentary time and physical activity are examined. Accelerometers (as distinct from pedometers that count and display the number of steps taken) are small electronic devices worn on the hip that provide an objective record of the volume, intensity, and frequency of activity between and within days, which may be downloaded to computer databases and used to derive scientifically meaningful activity variables. Accelerometers have been used as part of the National Health and Nutrition Examination Survey (NHANES), gathering data from large population-based samples of adult residents of the United States. Findings reported to date suggest that compared with what has been assumed to be the case from self-report surveys, levels of participation in moderate- to vigorous-intensity physical activities are extremely low,[44] and that some 60% or more of these adults' waking hours are spent sedentary.[29]

Sedentary Behavior during Adults' Waking Hours

To illustrate the overall patterns of activity in adults' daily lives, Figure 1 shows a cluster heat map.[50] This is a graphic representation from Genevieve Healy, showing accelerometer data for one individual during 1 wk, in the manner originally presented by Foulis et al.[15] The values taken by the accelerometer counts within each minute are represented as colors in the two-dimensional map. The dark blue shading shows accelerometer counts that are less than the currently used, but still debated, cutoff of 100 counts per min for sedentary time, and which are taken to be indicative predominantly of sitting (a caveat, however, is that some of the minutes shown as sedentary will include standing quite still). The pale blue through yellow colorings indicate light-intensity to moderate-intensity physical activities. The yellow through red colorings indicate moderate- to vigorous-intensity physical activity. From an energy expenditure perspective, the dark blue translates to very low levels of energy expenditure, with the red reflecting high energy expenditure levels. What is striking in Figure 1 is the extent to which this person spends his or her time either in light-intensity activities (pale blue to white) and being sedentary (dark blue). Although we would not contend that this is a totally precise and unambiguous representation of sitting time and light-, moderate-, and vigorous-intensity activities, it nevertheless is an informative perspective.

Figure 1.

Being physically active, but also highly sedentary: 1 wk of accelerometer count data showing, on average, 31 min·d−1 moderate- to vigorous-intensity activity time (>1951 counts per minute) and 71% of waking hours sedentary (<100 counts per minute).

Figure 1 illustrates one of our key messages about the role of sedentary time in the physical activity and health equation: it is possible to achieve a level of activity consistent with the public health guidelines for health-related physical activity (30 min of moderate-intensity physical activity on most days of the week) but to spend most of waking hours involved in sedentary behaviors. In this case, we see that the accumulated moderate- to vigorous-intensity physical activity time is 31 min; however, this person spends 71% of his or her waking hours in sedentary time. Thus, it is possible for individuals to be physically active, yet highly sedentary — the Active Couch Potato phenomenon identified in the AusDiab TV time studies.[24]

The main scientific caveat for this perspective is that these data show "activity," which we infer is reflective of "behavior." However, there are scientific devils in the details of these objective movement data. Debate remains about what are the most appropriate activity count cut points to identify sedentary and light-intensity activity time; also, different cut points may be appropriate for adults of different ages, race/ethnicity, and adiposity status.

Objectively Assessed Sedentary Time: Key Studies

As well as demonstrating remarkably low levels of physical activity and high levels of sedentary time within contemporary human environments,[29,44] objective measures also have demonstrated the adverse impact of prolonged sedentary time on cardiometabolic biomarkers of risk. At least three studies in Europe and Australia have examined the associations of objectively measured sedentary time with continuous cardiometabolic biomarkers: the ProActive trial conducted in the United Kingdom (UK), the European RISC study, and the AusDiab study.[2,13,14,23,25] For those in the UK ProActive trial (258 participants aged 30–50 yr with a family history of type 2 diabetes), sedentary time was detrimentally associated with insulin in the cross-sectional analysis[14] but was of borderline statistical significance (P = 0.07) in the 1-yr prospective analysis.[13] Detrimental cross-sectional associations of sedentary time with insulin also were observed in participants of the European RISC study (801 healthy participants aged 30–60 yr), although the associations were attenuated after adjustment for total activity.[2] In the AusDiab accelerometer study sample (169 participants aged 30–87 yr, general population), we observed detrimental associations of sedentary time with waist circumference, triglyceride levels, and 2-h plasma glucose.[22,24] It is important to point out that the participants in all of these studies were primarily white adults of European descent.[2,13,14,22,24] A key next step for this research is to examine whether the associations are consistent across different racial/ethnic groups, which is becoming feasible with the public availability of large multiethnic population-based data sets, particularly NHANES.[29,44]

Objectively Assessed Sedentary Behavior: AusDiab Findings

We used accelerometers to assess sedentary time in a subsample of the AusDiab study participants. Sedentary time was defined as accelerometer counts less than 100 per minute (previously described) and was associated with a larger waist circumference and more adverse 2-h plasma glucose and triglyceride profiles, as well as a clustered metabolic risk score.[22,24] The associations of sedentary time with these biomarkers (with the exception of triglyceride levels) remained significant after adjustment for time spent in moderate- to vigorous-intensity physical activities.[22,24]

As logically would be expected, sedentary time and light-intensity activity time were highly negatively correlated (r = −0.96); more time spent in light-intensity activity is associated with less time spent sedentary. This suggests that it may be a feasible approach to promote light-intensity activities as a way of ameliorating the deleterious health consequences of sedentary time. Our evidence suggests that having a positive light-intensity activity/sedentary time balance (i.e., spending more time in light-intensity activity than sedentary time) is desirable because light-intensity activity has an inverse linear relationship with a number of cardiometabolic biomarkers.[22,24]

Breaks in Sedentary Time: AusDiab Findings

One of the intriguing findings from our accelerometer measurement studies is that breaks in sedentary time (as distinct from the overall volume of time spent being sedentary) were shown to have beneficial associations with metabolic biomarkers.[21] Sedentary time was considered to be interrupted if accelerometer counts rose up to or more than 100 counts per min.[21] This can include behaviors that result in a transition from sitting to a standing position or from standing still to beginning to walk. Figure 2 is based on data from two of our AusDiab accelerometer study participants, showing a simple contrast between adults who have the same total volume of sedentary time, but who break up that time in contrasting patterns. The person whose data are shown in the right-hand panel of Figure 2 (the "Breaker") interrupts his or her sedentary time far more frequently than the person whose data are shown on the left panel (the "Prolonger").

Figure 2.

Breaks in sedentary time: same amount of sedentary time, but different ways of accumulation. CPM, counts per minute. (Reprinted from Dunstan DW, Healy GM, Sugiyama T, Owen N. Too much sitting and metabolic risk — has modern technology caught up with us? US Endocrinol. 2009; 5(1):29–33. Copyright © 2009 Touch Briefings. Used with permission.)

Independent of total sedentary time, moderate- to vigorous-intensity activity time, and mean intensity of activity, we found that having a higher number of breaks in sedentary time was beneficially associated with waist circumference, body mass index, triglycerides, and 2-h plasma glucose.[21] Figure 3 shows objectively measured waist circumference across quartiles of breaks in sedentary time. Those in the bottom quarter of the "breaks" distribution had, on average, a 6-cm larger waist circumference than did those in the top quarter of that distribution.[21]

Figure 3.

Associations of breaks in sedentary time with waist circumference (based on data from Healy et al.[21]).

These findings on breaks in sedentary time provide intriguing preliminary evidence on the likely metabolic health benefits of regular interruptions to sitting time, which we would argue are additional to the benefits that ought to accrue from reducing overall sedentary time. Interestingly, in a recent study,[5] patterns of sedentary time accumulation (but not total sedentary time) were shown to differ among four groups of adults with various activity patterns (healthy group with active occupation, healthy group with sedentary occupation, group with chronic back pain, group with chronic fatigue syndrome). As we will go on to propose, although we believe that these are strongly indicative findings, there is the need to determine whether these associations can be confirmed in experimental manipulations of sitting time in the laboratory. Intervention studies where sedentary time is reduced or broken up in naturalistic settings such as the domestic environment or the workplace would also be needed.

Sedentary Behavior and Mortality

The significance of the evidence on the adverse cardiometabolic health consequences of prolonged sitting time is underscored by findings from a mortality follow-up of participants in the Canada Fitness Surveys. Canadians who reported spending most of their day sitting had significantly poorer long-term mortality outcomes than did those who reported that they spent less time sitting. These relationships with mortality were consistent across all levels of a self-report measure of overall sitting time. Participants estimated the broad fractions of their waking hours that were spent sitting. Importantly, the sitting time-mortality relationships were apparent even among those who were physically active and the relationships were stronger among those who were overweight or obese.[25] In a follow-up of AusDiab study participants during 6.5 yr, high levels of TV time were significantly associated with increased all-cause and cardiovascular disease mortality rates.[9] Each 1-h increment in TV time was found to be associated with an 11% and an 18% increased risk of all-cause and cardiovascular disease mortality rates, respectively. Furthermore, relative to those watching less TV (<2 h·d−1), there was a 46% increased risk of all-cause mortality and an 80% increased risk of cardiovascular disease mortality in those watching TV 4 h·d−1 or more. These increased risks were independent of traditional risk factors such as smoking, blood pressure, cholesterol level, and diet, as well as leisure time physical activity and waist circumference. A recent study from the United States[47] examined sedentary behaviors in relation to cardiovascular mortality outcomes based on 21 yr of follow-up of 7744 men. Those who reported spending more than 10 h·wk−1 sitting in automobiles (compared with <4 h·wk−1) and more than 23 h of combined television time and automobile time (compared with <11 h·wk−1) had an 82% and 64% greater risk of dying from cardiovascular disease, respectively. TV time alone was not a significant predictor.[47]

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