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Too Much Sitting:
The Population-Health Science of Sedentary Behavior
The physical, economic and social environments in which modern humans sit or move within the contexts of their daily lives have been changing rapidly, and particularly so since the middle of the last century. These changes — in transportation, communications, workplace and domestic-entertainment technologies — have been associated with significantly-reduced demands for physical activity. However, these reductions in the environmental demands for being physically active are associated with another class of health-related behaviors.
Sedentary behaviors (typically in the contexts of TV viewing, computer and game-console use, workplace sitting, and time spent in automobiles) have emerged as a new focus for research on physical activity and health .
Put simply, the perspective that we propose is that too much sitting is distinct from too little exercise.
Research findings on sedentary behavior and health have proliferated in the 10 years following publication of our first Exercise and Sport Sciences Reviews paper on this topic (32). As we will demonstrate, initial findings on the metabolic correlates of prolonged TV viewing time (TV time) have since been confirmed by recent objective-measurement studies, which also show that breaking up sedentary time can be beneficial. Furthermore, we describe recent studies from Canada, Australia, and the United States, which show prospective relationships of sedentary behaviors with premature mortality. Importantly, adults can meet public-health guidelines on physical activity, but if they sit for prolonged periods of time, their metabolic health is compromised. This is a new and challenging area for exercise science, behavioral science, and population-health research.
However, many scientific questions remain to be answered before it can be concluded with a high degree of certainty that these adverse health consequences are uniquely due to too much sitting, or if what has been observed so far can be accounted for by too little light, moderate, and/or vigorous activity.
The updated recommendation for adults on Physical Activity and Public Health from the American College of Sports Medicine and the American Heart Association (ACSM/AHA) “clearly states that the recommended amount of aerobic activity (whether of moderate- or vigorous-intensity) is in addition to routine activities of daily living which are of light intensity, such as self care, casual walking or grocery shopping, or less than 10 min of duration such as walking to the parking lot or taking out the trash” ((20) p. 1426). Logically, doing such daily activities differently could involve reductions in sitting time, but sitting per se is not addressed specifically in the recommendations. In this context, the key question to be asked about the strength of the evidence on sedentary behavior and health that we present in this paper is: Would one expect to see a statement on reducing sitting timeincluded in future physical activity recommendations?
Sedentary behaviors (from the Latin sedere, “to sit”) include sitting during commuting, in the workplace and the domestic environment, and during leisure time. Sedentary behaviors such TV viewing, computer use, or sitting in an automobile typically are in the energy-expenditure range of 1.0 to 1.5 METs (multiples of the basal metabolic rate)(1). Thus, sedentary behaviors are those that involve sitting and low levels of energy expenditure. In contrast, moderate-to-vigorous physical activity such as bicycling, swimming, walking, or running may be done in a variety of body positions, but require an energy expenditure of 3 to 8 METs (1). In this perspective, light intensity activity behaviors are those done while standing, but that requires expenditure of no more than 2.9 METS.
Addressing research on the health consequences of sedentary behavior requires some initial clarification of terminology. We refer to sedentary behaviors (different activities, for different purposes in different contexts; see above). We refer also to sitting time, a generic descriptor covering what these sedentary behaviors primarily involve. As we demonstrate below, adults spend the majority of their waking hours either sitting, or in light intensity activity (predominantly standing with some gentle ambulation).
Time in sedentary behaviors is significant, if only because it displaces time spent in higher intensity physical activity — contributing to a reduction in overall physical activity energy expenditure. For example, displacement of two hours per day of light intensity activity (2.5 METS) by sedentary behaviors (1.5 METS) would be predicted to reduce physical activity energy expenditure by about two MET-hrs/d, or approximately the level of expenditure associated with walking for 30 min per day (0.5 hrs * 3.5 METs = 1.75 MET-hrs).
Research on physical activity and health has concentrated largely on quantifying the amount of time spent in activities involving levels of energy expenditure of 3 METs or more, characterizing those with no participation at this level as “sedentary” (33). However, this definition neglects the substantial contribution that light intensity (1.9 to 2.9 METs) activities make to overall daily energy expenditure (8), and also the potential health benefits of participating in these light-intensity activities, rather than sitting. Furthermore, although individuals can be both sedentary and physically inactive, there is also the potential for high sedentary time and being physically active to co-exist (the Active Couch Potato phenomenon, which we explain below). An example would be an office worker who jogs or bikes to and from work, but who then sits all day at a desk and spends several hours watching TV in the evening.
Common behaviors in which humans now spend so much time — TV viewing, computer use and electronic games, sitting in automobiles — involve prolonged periods of these low levels of metabolic energy expenditure. It is our contention that sedentary behavior is not simply the absence of moderate-to-vigorous physical activity, but rather is a unique set of behaviors, with unique environmental determinants and a range of potentially-unique health consequences (43). Our population-health research perspective is on the distinct role of sedentary behavior, as it may influence obesity and other metabolic precursors of major chronic diseases (type 2 diabetes, cardiovascular disease, and breast and colon cancer).
Sedentary Behavior and Health:A Unique Underlying Biology?
Physiologically, distinct effects are observed between prolonged sedentary time and too little physical activity (17). There are broad consistencies between the patterns of findings from epidemiologic studies on the cardio-metabolic correlates of prolonged sitting that we will describe, and recent evidence on biological mechanisms — “inactivity physiology” — identified in animal models. It seems likely that there is a unique physiology of sedentary time, within which biological processes that are distinct from traditionally-understood exercise physiology are operating. The groundbreaking work of Hamilton and colleagues (3, 16) provides a compelling body of evidence that the chronic, unbroken periods of muscular unloading associated with prolonged sedentary time may have deleterious biological consequences. Physiologically, it has been suggested that the loss of local contractile stimulation induced through sitting leads to both the suppression of skeletal muscle lipoprotein lipase (LPL) activity (which is necessary for triglyceride uptake and HDL-cholesterol production) and reduced glucose uptake (3, 16). A detailed account of findings and implications from Hamilton’s studies has been provided in recent reviews (17, 18).
Hamilton’s findings suggest that standing, which involves isometric contraction of the anti-gravity (postural) muscles and only low levels of energy expenditure, elicits EMG and skeletal muscle LPL changes. However, in the past, this form of standing would be construed as a “sedentary behaviour” because of the limited amount of bodily movement and energy expenditure entailed. This highlights the need for an evolution of the definitions used for sedentary behavior research. Within this perspective, standing would not be a sedentary activity and our approach (subject to revision as further findings accumulate) is to equate “sedentary” with ”sitting.”
THE METABOLIC HEALTH CONSEQUENCES OF TOO MUCH SITTING
TV Viewing Time: The AusDiab Studies
AusDiab (the Australian Diabetes, Obesity and Lifestyle study) conducted initially in 1999/2000, of a common leisure-time sedentary behavior — TV viewing time — with biomarkers of cardio-metabolic risk. AusDiab recruited a large, population-based sample of some 11,000 adults from all Australian states and the Northern Territory. Some of our first AusDiab findings were that among adults without known diabetes, self-reported TV viewing time was positively associated with undiagnosed abnormal glucose metabolism (12) and the metabolic syndrome (11). The strongest relationships were observed in the highest TV time category (four hours or more per day). When TV time was considered as a continuous measure (10), a detrimental, dose-response association was observed in women between TV viewing time and 2-h plasma glucose and fasting insulin. Importantly, all of these associations persisted after adjustment for sustained moderate-to-vigorous intensity leisure time physical activity and waist circumference. Some of these cross sectional relationships have been replicated recently in prospective analyses: increases in TV viewing over five-years predicted significant adverse changes in waist circumference for men and women and in diastolic blood pressure and a clustered cardio-metabolic risk score for women. These associations were independent of baseline television viewing time, baseline physical activity and physical activity change, and other potential confounders.
Being Sedentary and Meeting Physical Activity Guidelines: The Active Couch Potato
We further examined relationships of TV time with continuous metabolic risk in men and women who reported at least 150 min a week of moderate-to-vigorous intensity physical activity — the generally-accepted public health guidelines for health-enhancing physical activity (20). Among these healthy, physically-active adults, significant detrimental dose-response associations of TV time were observed with waist circumference, systolic blood pressure, and 2-h plasma glucose in both men and women, as well as fasting plasma glucose, triglycerides, and HDL-cholesterol in women only (23). This observation — the Active Couch Potato phenomenon — is important. The particular metabolic consequences of time spent watching TV are adverse, even among those considered to be sufficiently physically active to reduce their chronic disease risk. This finding reinforces the potential importance of the deleterious health consequences of prolonged sitting time, which may be independent of the protective effect of regular moderate-intensity physical activity.
TV Viewing Time: Associations with Biomarkers for Men and for Women
One of the striking findings in the AusDiab TV-time studies was that the associations with cardio-metabolic biomarkers were stronger for women than for men (10-12, 23). We subsequently examined the associations of both TV time and self-reported overall sitting time with these biomarkers in the 2004/2005 AusDiab sample (42). The TV time relationships for women were replicated, but for self-reported overall sitting time (which is inclusive of the TV time component), the associations were similar for men and women. So, the question remains as to whether there is a particular relationship of TV time with metabolic health for women. There are some testable hypotheses that can be put forward in this context: Are there dietary or TV time-related snacking differences between men and women? Are women (who have a lower average skeletal muscle mass and a higher average fat mass than men) metabolically more susceptible to the adverse influences of prolonged sitting, following the typically-large evening meal?
Although some of our most striking initial findings on the adverse health consequences of sedentary behavior have been for TV time, there should be caution in treating this common leisure-time sedentary behavior as a marker for overall sedentary time. We have modest evidence (39) that for women, TV time is positively correlated with other leisure time sedentary behaviors and with being less likely to meet physical activity and health guidelines. However, these findings need to be replicated in other populations and with other measures. Furthermore, TV viewing is associated with other health-related behaviors (51) and those in the highest TV time categories are more likely to eat in front of the TV set (26). It is thus plausible that TV time will influence energy balance in two main ways. Most people sit to watch TV and it has a lower energy cost than the alternative activities that it replaces. Also, high levels of TV time are likely to increase energy intake because of prompts from frequent commercials about food and beverages, and unlike for many other activities, the hands are free to eat during TV time (51). It is thus a reasonable hypothesis that this latter factor may partially explain why higher levels of TV time are associated with higher waist circumferences and with adverse blood-glucose and lipid profiles.
We must emphasize that TV time is one of a number of sedentary behaviors that characterise how adults go about their daily lives, and there is potential measurement error associated with using the self-report measures that are common to most TV-time studies. However, based on our recent systematic review (6), we have some confidence that the TV-time measures that we have used are reasonably reliable and valid.
OBJECTIVE ASSESSMENT OF SEDENTARY TIME: NEW FINDINGS
Advances in the Objective Measurement of Sedentary Behavior
These Australian studies summarised above 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. Prior to summarising 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 which count and display number of steps taken) are small electronic devices worn on the hip, which 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 employed 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 to what has been assumed to be the case from self-report surveys, levels of participation in moderate-to-vigorous physical activity are extremely low (44), and that some 60% or more of these adults’ waking hours are spent sedentary (29).
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 have also demonstrated the adverse impact of prolonged sedentary time on cardio-metabolic biomarkers of risk. At least three studies in Europe and Australia have examined the associations of objectively-measured sedentary time with continuous cardio-metabolic 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 one-year prospective analysis (13). Detrimental cross-sectional associations of sedentary time with insulin were also observed in participants of the European RISC study (801 participants aged 30-60 yr, healthy adults), though the associations were attenuated following 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, triglycerides, and 2-hr 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, multi-ethnic population-based datasets, particularly NHANES (29, 44).
Objectively-Assessed Sedentary Behavior: AusDiab Findings
We used accelerometers to assess sedentary time in a sub-sample of the AusDiab study participants. Sedentary time was defined as accelerometer counts below 100 per minute (see above), 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 triglycerides) remained significant, following adjustment for time spent in moderate-to-vigorous intensity physical activity (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/sedentary time balance (that is; spending more time in light-intensity than sedentary time) is desirable, since light-intensity activity has an inverse linear relationship with a number of cardio-metabolic biomarkers (22, 24).
Sedentary Behavior and Mortality
The significance of the evidence on the adverse cardio-metabolic 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 the majority 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 were stronger among those who were overweight or obese (25). In a follow-up of AusDiab study participants over 6.5 yr, high levels of TV time were significantly associated with increased all-cause and cardiovascular disease mortality (9). Each one hour increment in TV time was found to be associated with an 11% and an 18% increased risk of all-cause and cardiovascular disease mortality, respectively. Furthermore, relative to those watching less TV (< 2 hours/day), there was a 46% increased risk of all-cause and an 80% increased risk of cardiovascular disease mortality in those watching four or more hours of TV per day, independent of traditional risk factors such as smoking, blood pressure, cholesterol 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 a week sitting in automobiles (compared to less than four hours a week), and more than 23 h of combined television time and automobile time (compared to less than 11 hours a week) 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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