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

Research Directions

Looking Back through a Sedentary Behavior Lens

Emerging findings on sedentary behavior suggest a different perspective through which findings of earlier physical activity and health research studies may be reexamined (we thank William L. Haskell, Ph.D., FACSM for stimulating these observations). For example, physical activity epidemiology studies that have assessed physical activity comprehensively often have included measures of sitting time, which has been used mainly to derive overall daily energy expenditure estimates. We would predict (perhaps boldly) that if such studies were to be revisited, with further analyses being conducted using sitting time as a distinct exposure variable, that strong evidence would be found for deleterious effects on subsequent health outcomes, independent of those related to physical inactivity.

Other potentially fruitful areas in which the relevance of existing evidence could be reexamined are the NASA zero-gravity studies. Comparing findings of those studies (which relate to the metabolic consequences of extreme muscular unloading) with those of the recent findings from inactivity physiology[16,17] may yield further insights relating to the underlying biology of prolonged sedentary time.

Research on physical activity and health had its roots in early occupational epidemiological studies that assessed workers in jobs that primarily involve sitting as the comparison groups against which the protective benefits of physically active work were highlighted.[4,17,18] In the perspective of the new evidence that we have highlighted, conducting further occupational epidemiological studies using new objective measurement capabilities and examining a range of cardiometabolic and inflammatory biomarkers as intermediate outcomes could yield valuable insights.

Sedentary Behavior Research Strategy

Our population health research program on sedentary behavior is guided by the behavioral epidemiology framework.[34,36] Figure 4 shows six research phases. As we previously demonstrated, evidence within the first phase (examining the relationships of sedentary behavior to cardiometabolic biomarkers and health outcomes) has strengthened rapidly during the past 10 yr.

Figure 4.

Behavioral epidemiology framework: phases of evidence for a population health science of sedentary behavior.

Prolonged periods of sitting in people's lives need to be measured precisely (phase 2). Their contextual determinants — that is, behavior settings[32,35] — need to be identified in domestic, workplace, transportation, and recreation contexts (phase 3). We have argued previously for a research focus on the distinct environmental determinants of sedentary behaviors, in contexts where they can be amenable to intervention.[31,32,37,41] The feasibility and efficacy of such interventions need to be tested rigorously (phase 4). Importantly, public health policy responses need to be informed by evidence from all of these phases. Compared with the challenges for physical activity and public health,[19] sedentary behavior may be less of a "moving target" in this context and may be shown to be a tractable public health objective.[4]

The Population Health Science of Sedentary Behavior: Research Opportunities

Different sedentary behaviors and their interactions with physical activity need to be examined in a range of contexts. For example, we have demonstrated that leisure time Internet and computer use is related to overweight and obesity in Australian adults,[45] and that habitual active transport reduces the impact of TV time on body mass index.[40] Having identified these relationships, our program is now broadening the evidence base through research with other populations. New studies include work with the large population-based data set from the NHANES from the United States, examining potential racial and ethnic differences in the relationships of total sedentary time and breaks in sedentary time with cardiometabolic biomarkers. We have demonstrated significant associations of TV time with excess body weight among high school students in regional mainland China.[52] In the context of the rapid economic development and increasing urbanization among the populations of many developing countries, documenting the health consequences of reductions in physical activity and increases in sedentary time will be crucial for informing preventive health measures.[38]

Studies with high-risk groups also are required. For example, we examined accelerometer-derived physical activity, sedentary time, and obesity in breast cancer survivors, showing physical activity to be protective, but there was no deleterious relationship for sedentary time.[28] Significant prospective relationships of TV time with weight gain during 3 yr were identified in a large population-based cohort of Australian colorectal cancer survivors.[49] More such etiologic research is needed to examine potential relationships between too much sitting and the development of other diseases that have been linked to metabolic risk factors.

For the second phase of the behavioral epidemiology framework (measurement; Fig. 4), there is the need to identify the reliability and validity of self-report instruments.[6] Population-based descriptive epidemiological studies using high-quality measures are needed. For example, we have shown that Australian adults with lower levels of educational attainment and those living in rural areas are more likely to be in the highest TV time categories.[7] We also have demonstrated that for Australian women, being in the higher categories of TV time can be associated with a broader pattern of leisure time sedentary behavior and with being less likely to meet physical activity recommendations.[39] Using American Cancer Society data from a large population-based study, we have identified clusters of adults in the 4 h or more category of TV time who are less educated, obese, and snacking while watching TV.[26]

Studies have begun to identify the environmental correlates of sedentary behavior, and initial findings seem puzzling. Among urban Australians, lower levels of objectively assessed neighborhood walkability (poorly connected streets, low levels of residential density, and limited access to destinations) were found to be associated with higher TV time in women.[41] However, a recent study in the city of Ghent, Belgium, showed higher levels of walkability to be associated with higher amounts of accelerometer-assessed sedentary time.[46] These apparently contradictory outcomes require further research. Such findings have potential implications for the emerging area of research on built environment/obesity relationships, within which sedentary behavior is likely to have a significant role.[30]

Research on sedentary behaviors also needs to be extended beyond the promising initial studies on TV time to understand the potential health consequences of other common sedentary behaviors. Evidence on the metabolic correlates of prolonged sitting in motor vehicles would be particularly informative in the light of recent evidence on relationships with premature mortality.[47] The social and environmental attributes associated with high levels of time spent sitting in automobiles also need to be identified.

The highest priority for the sedentary behavior research agenda is to gather new evidence from prospective studies, human experimental work, and intervention trials. There is the particular need to build on the promising findings on relationships of sedentary time — overall sitting time, TV time, and time sitting in automobiles — with premature mortality.[9,25,47] Controlled experimental studies with humans also should be particularly informative. For example, we are currently conducting a laboratory study experimentally manipulating different "sedentary break" conditions and examining associated changes in cardiometabolic biomarkers (focusing on levels of triglycerides, glucose, and insulin).

Field studies also are needed on the feasibility and acceptability of reducing and breaking up occupational, transit, and domestic sedentary time. For example, in a weight control intervention trial for adults with type 2 diabetes, we are testing the impact of a sedentary behavior reduction intervention module and examining behavioral and biomarker changes associated with reducing and breaking up sedentary time. There are multiple research opportunities that can be explored through integrating sedentary behavior change intervention into physical activity trials. When accelerometer data are gathered from such studies, sedentary time measures can be derived,[21,22,24] and unique hypotheses may readily be tested. It is imperative that the field now moves to obtain such evidence through intervention trials, which will take the science beyond the inherent logical limitations of cross-sectional evidence.

Eleven Research Questions for a Science of Sedentary Behavior

  1. Can further prospective studies examining incident disease outcomes confirm the initial sedentary behavior/mortality findings?

  2. Can sedentary behavior/disease relationships be identified through reanalyses of established prospective epidemiological data sets by treating sitting time as a distinct exposure variable?

  3. What are the most valid and reliable self-report and objective measures of sitting time for epidemiological, genetic, behavioral, and population health studies?

  4. Are the TV time-biomarker relationships for women pointing to important biological and/or behavioral sex differences?

  5. What amounts and intensities of activity might be protective in the context of prolonged sitting time?

  6. What genetic variations might underlie predispositions to sit and greater susceptibility to the adverse metabolic correlates?

  7. What is the feasibility of reducing and/or breaking up prolonged sitting time for different groups (older, younger) in different settings (workplace, domestic, transit)?

  8. If intervention trials show significant changes in sitting time, are there improvements in the relevant biomarkers?

  9. What are the environmental determinants of prolonged sitting time in different contexts (neighborhood, workplace, at home)?

  10. What can be learned from the sitting time and sedentary time indices in built-environment/physical activity studies?

  11. Can evidence on behavioral, adiposity, and other biomarker changes be gathered from "natural experiments" (e.g., the introduction of height-adjustable workstations or new community transportation infrastructure)?

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