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Sedentary Behaviors and Adiposity in Young People: Causality and Conceptual Model

Biddle, Stuart, J.H.1; Pearson, Natalie2; Salmon, Jo3

Exercise and Sport Sciences Reviews: January 2018 - Volume 46 - Issue 1 - p 18–25
doi: 10.1249/JES.0000000000000135
Articles
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Video Abstract

Research on sedentary behavior and adiposity in youth dates back to the 1980s. Sedentary behaviors, usually screen time, can be associated with adiposity. Although the association usually is small but significant, the field is complex, and results are dependent on what sedentary behaviors are assessed and may be mediated and moderated by other behaviors.

Sedentary behaviors can be associated with adiposity, but the field is complex. Results depend on the type of sedentary and other behaviors being assessed.

1USQ PALs – Physically Active Lifestyles Research Group, Institute for Resilient Regions, University of Southern Queensland, Springfield, Australia; 2School of Sport, Exercise & Health Sciences, Loughborough University, UK; and 3Institute for Physical Activity and Nutrition (IPAN), Deakin University, Geelong, Australia

Address for correspondence: Stuart Biddle, Institute for Resilient Regions, University of Southern Queensland, Education City, Sinnathamby Boulevard, Springfield Central, QLD 4300, Australia (E-mail: stuart.biddle@usq.edu.au).

Accepted for publication: August 10, 2017.

Associate Editor: Ryan E. Rhodes, Ph.D.

Key Points

  • Sedentary behavior — sitting time — has long been thought to be a risk factor for pediatric obesity, especially through television and other screen viewing, with claims made for clear and causal links.
  • A closer look at the literature reveals a complex picture of statistically significant but small associations for screen time and adiposity in youth, but small or no associations for total sedentary time assessed with accelerometers.
  • Current evidence does not support a causal association.
  • Results concerning obesity may depend on a variety of mediating, moderating, and confounding factors including light and moderate-to-vigorous physical activity, diet, and sleep.
  • Reducing sedentary behavior in youth probably is sensible, but we propose that the field is more complex than sometimes recognized.
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Figure

Editor's note: Go online to view the Video Abstract in the Supplemental Digital Content: seehttp://links.lww.com/ESSR/A40.

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INTRODUCTION

Over the past decade or so, there has been a substantial increase in research focused on what has been termed sedentary behaviors. Unlike the common use of the term sedentary to mean physically inactive, behavioral scientists have been more precise and adopted the term to reflect seated or reclining postures that have low energy expenditure and are performed during waking hours (1,2). This reflects, in practice, time spent sitting, and it sets itself apart from lack of movement or exercise.

The analysis by Morris et al. (3) of seated London bus drivers and active ticket collectors more than 60 yr ago could be seen as the first study concerning the health effects of sitting. However, the outcomes of that study focused on the ticket collectors and the health benefits of a physically active occupation. This meant that the health impacts of sitting were ignored, and this continued for several decades. However, in the 1980s, studies emerged on the health effects of leisure-time sitting in the form of television (TV) viewing. Research on sedentary behavior, either alone or alongside physical activity, then developed at pace from the early 2000s with a focus on all phases of the behavioral epidemiology framework: measurement, health outcomes, correlates, interventions, and translation, with both young people and adults (e.g., (4,5)).

A great deal of the literature has focused on health outcomes associated with different amounts of exposure to sedentary behaviors. Initially, research focused on health outcomes of TV viewing, then expanded to include screen time (TV viewing, computer use, and electronic games) and, with the advent of wearable technology, total sitting time across the day or in certain settings (e.g., at work). Additional sedentary behaviors, such as reading and other sedentary hobbies (e.g., board games, jigsaws, art, etc.), have been much less frequently investigated, although studies on sitting for transportation, particularly in cars, is expanding, mainly for adults (6).

The health outcomes investigated, usually from epidemiological studies, have included all-cause mortality, cardiovascular disease, cardiometabolic health (including diabetes and metabolic syndrome), and obesity. Emerging outcomes include cancer and psychological well-being (7). The most prolific area of coverage is that for weight status and obesity, and includes the early studies on TV viewing in children (8). Our research over the past two decades has led to the hypothesis that sedentary behaviors in young people can be positively associated with adiposity, but the association is small, complex, dependent on what sedentary behaviors are assessed, and may be mediated by other behaviors. Given the continued popular and scientific interest in weight status and adiposity and the volume of literature on this topic in the context of sedentary behavior, we focus on this area of research in the present article.

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EVIDENCE FOR AN ASSOCIATION BETWEEN SEDENTARY BEHAVIOR AND ADIPOSITY

Views expressed in the literature concerning whether an association exists between sedentary behaviors and adiposity have varied and seem to reflect how authors interpret the data. One of the first articles investigating health outcomes of the most prevalent leisure-time sedentary behavior — TV viewing — was published in 1985 by Dietz and Gortmaker (8). Drawing on data from a large national data set of more than 6500 children, they concluded that a small association did exist with adiposity and fulfilled criteria “necessary to establish a causal association” (p. 811). Data from New Zealand showed that hours of TV viewing in childhood were predictive of adult adiposity some 10 yr later (9). More recently, a very large international study of more than 77,000 children and 207,000 adolescents from 54 countries concluded that a positive association exists between TV viewing and body mass index (BMI) (10). The strength of association was small to moderate and stronger in girls than boys. For example, comparing adolescent boys watching less than 1 h·d−1 of TV with those watching 5 h or more, the latter had an increased BMI of only 0.16. But the difference in children was 0.36. Results showed great variability in data between countries for both TV viewing and BMI. Confounding by age may have influenced these results because BMI Z-scores were not analyzed (BMI Z-scores express the anthropometric value as a number of standard deviations, or Z-scores, below or above the reference mean or median value and accounts for age and sex).

National position and expert statements also have supported the view that sedentary behavior — mainly screen time — is a risk factor for greater adiposity. For example, more than 20 yr ago, the Australian College of Paediatrics (11) stated that “television has been implicated as a direct cause of obesity” (p. 7), and a Scientific Roundtable of the American College of Sports Medicine in 1996 concluded that obesity was directly related to the volume of TV viewing (12). The latter statement could be interpreted as obesity leading to more TV viewing — the reverse causality argument (see later). National guidelines recommending reductions in sedentary time to no more than 2 h·d−1 of recreational screen time now exist in many countries, although it should be noted that these are not focused just on obesity as a possible negative outcome of high screen time.

In contrast to the statements just highlighted, there are numerous studies and reviews that have expressed a more cautious view about how and whether sedentary behavior is associated with adiposity. We conducted the first meta-analysis concerning both TV and video/computer game use and associations with body fatness and showed small (r = 0.066), but significant, associations (13). We questioned whether such an association was practically meaningful, although this was challenged by others (14). Similar to the more cautious view we expressed in the meta-analysis of Marshall et al. (13), in our systematic review of the correlates of TV viewing in young people, we concluded that TV viewing was associated with body weight but not body fatness (4).

Studies using estimates of children’s total sedentary time also are inconclusive. In a large international sample of 9–11-year-old children assessed using accelerometers (15), sedentary time was shown to have a small association with obesity across the whole sample but was significant only in 5 of 12 countries. Moreover, this association was not independent of moderate-to-vigorous physical activity (MVPA). Similarly, a large multinational cohort study of accelerometer-assessed sedentary time showed no significant association with waist circumference in children and adolescents (16). In contrast, analysis of adolescents’ accelerometer data from the 2003/04 to 2005/06 U.S. National Health and Nutrition Examination Survey (NHANES) found that for every hour spent sedentary, BMI Z-scores decreased by 1.33 units. However, after adjusting for MVPA, this relation was no longer significant (17).

Given the inconsistent findings reported in the literature and, more importantly, the diverse interpretation of such data, we conducted a review of 29 systematic reviews concerning sedentary behavior and adiposity in youth (18). Specifically, we addressed observational and experimental studies through the assessment of both self-reported behaviors and wearable technology. A summary of conclusions from this analysis is presented in Table 1. Overall, it seems that evidence of associations between sedentary behavior and adiposity is most consistent for cross-sectional studies of TV viewing (screen time). The longitudinal and experimental evidence is inconsistent (ranges from no evidence to modest and strong evidence) and seems dependent on the outcome and sedentary behavior measures assessed.

TABLE 1

TABLE 1

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ANALYSIS OF CAUSALITY

Given the diversity of findings reported in our review of reviews (18), but also the conclusion that associations between sedentary behavior and adiposity in youth have been shown, a more robust analysis can be derived from assessing the nature of this relation against the Bradford Hill causality criteria (19). Hill proposed a number of criteria on which to judge whether an exposure is causally related to a health outcome. These include strength of association, consistency, specificity, temporality, coherence and biological plausibility, dose-response, and experimental evidence. The conclusions stated by Biddle et al. (18), using these criteria, are shown in Table 2. Discussion here will center on the key factors of strength of association, dose-response, experimental evidence, and coherence and biological plausibility.

TABLE 2

TABLE 2

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Strength of Association

From the first meta-analysis investigating TV viewing and body fatness in youth we published in 2004 (13), in which we reported a small but significant association (r = 0.066), evidence has shown consistent significant associations between sedentary behavior and markers of adiposity, although usually, such associations are small. Similar effects have been found in interventions. Prospective studies suggest an effect of almost zero for the relation between baseline TV viewing and BMI at follow-up when controlling for baseline BMI (20).

It seems that there is little dispute that associations and effects for screen time (but not total sedentary behavior assessed with wearable technology) on adiposity in youth are significant but small. So to what extent are such values clinically or practically meaningful? This has been an area of some dispute. Is the glass “half full” or “half empty”? Key issues in this debate, and which may reflect a “glass half-full” approach, concern a) small effects in large populations b) the measurement of sedentary behavior c) the lack of intervention fidelity and d) the tracking of sedentary behavior and consequences for health in adulthood.

All young people engage in sedentary behavior, and nearly all watch some TV or engage in some form of recreational screen time. This means that small effects on adiposity across a large population may have significant public health effects. Moreover, as argued by Hancox and Poulton (14), the association between TV viewing and adiposity may be attenuated by restriction in the range of values for TV viewing. They argue that very few young people do not watch any TV, hence associations are calculated from restricted values (i.e., from more than zero). Although this might lead to an underestimation of the true strength of association between sedentary behavior and adiposity, trend data suggest declines in the percentage of youth in the United States watching more than 3 h of TV a day (from 43% in 1999 to 35% in 2007) (21), which also may make it difficult for longitudinal studies to show associations. This is against a backdrop of overall increases in media exposure from 37 h·wk−1 in the early 1960s to as much as 75 h·wk−1 in 2009 (22), the latter figure likely inflated because of multitasking. But much of this increase will be attributable to electronic media. These changes in exposures to screen use make it difficult for studies to accurately determine the strength of association between sedentary behavior and adiposity in young people.

Measurement issues also may influence the inconsistency of associations, including the inability to differentiate sitting from standing using some wearable technology (23) and the difficulty of recalling long bouts of sitting and breaks in sitting. Discrete behaviors, such as TV viewing, may be more easily and accurately recalled and hence, allow for more consistent associations to be detected with less measurement error.

The weak effects of interventions may partly be related to poor intervention fidelity. In other words, the interventions may not have been delivered as intended (see section on “Experimental Evidence”).

Sedentary behavior has been found to track into adulthood. The strength of this is moderate and slightly larger for TV viewing than other measures. It is broadly comparable to the tracking of physical activity (24). Moreover, there is some evidence for increased risk of obesity in adulthood from sedentary behavior in childhood and adolescence (25). It is plausible, therefore, that the small associations found for sedentary behavior and adiposity in young people may have implications for health in adulthood. We have shown cross-sectional associations between TV viewing time and inflammatory and endothelial biomarkers (after adjusting for waist circumference, diet, and MVPA) in 8–9 year olds (26). The associations were modest (for every hour per week of TV viewing, 4.4% and 0.6% greater C-reactive protein and soluble vascular adhesion molecule 1, respectively). However, the tracking of low-grade inflammation from childhood to adulthood and relations between markers of inflammation and endothelial function with atherosclerotic lesions (27) support the findings from the review of reviews of Biddle et al. (18) that elevated biomarkers early in life may be indicative of cardiometabolic risk later in life.

These arguments suggest that it may not be appropriate to dismiss the small associations as clinically or practically irrelevant, although further work is needed on this. In conclusion, evidence for strength of association between TV viewing and adiposity is consistent but weak (i.e., associations are usually small), whereas associations between accelerometer-measured sedentary time and adiposity are inconsistent, though often null, although fewer studies exist here.

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Dose-Response

A dose-response relation — what Hill (19) referred to as a biological gradient — between sedentary behavior and adiposity in youth does seem to exist (see 18 and Table 2). However, this has not been tested extensively, and estimates vary. One meta-analysis of 10 cross-sectional studies showed a pooled odds ratio of 1.13 for obesity risk per hour of TV viewing (28). The graph provided in the review article suggested a linear relation. But dose-response curves can take many shapes, and it is plausible that obesity will be related to sedentary behavior only at higher levels of exposure. Given that studies vary in the way they assess and categorize sedentary time, this is not easy to test with precision.

It often is recommended by government health agencies that young people take part in less than 2 h of recreational screen time daily for a variety of physical and mental health benefits, not just obesity prevention and management (11). The first data to test for dose-response in this field were from the 1980s and showed that children watching TV less than 2 h·d−1 had the lowest prevalence of obesity, but a clear dose-response curve was not evident. On the other hand, a dose-response curve was seen more clearly for adolescents (8). The uneven distribution of TV viewing prevalence may attenuate effects on obesity or restrict the possibility of a clear dose-response curve being shown. Moreover, the use of screens is changing rapidly, with declines in TV viewing and increases in the use of other screens (21,22), not all of which will be engaged in through sitting. In conclusion, there does seem to be a moderate dose-response effect for TV viewing and adiposity in youth. However, caution needs to be expressed because true dose-response effects can be tested with only longitudinal data. Our conclusions also are drawn from data that include cross-sectional designs. In such studies, conclusions can be made only about the degree to which screen time and adiposity are graded in their relation.

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Experimental Evidence

The review by De Mattia et al. (29) highlights the inconsistent interpretation and reporting in relation to the impact of experimental studies to reduce sedentary behavior on children’s and adolescents’ weight status. In their abstract, they stated that interventions “reduced sedentary behavior and improved weight indices. An emphasis on decreasing sedentary behaviors is an effective intervention to decrease sedentary behaviors and control weight in children and adolescents” (p. 69). This reflects the glass half-full argument. Yet in their discussion they conclude that “the magnitude of weight parameters is modest and is difficult to interpret” (p. 79) — glass half empty. One reason they say this is that maturational factors often are not accounted for.

Not surprisingly, more substantial decreases in BMI may be achieved through reductions in sedentary behavior among obese children. A recent meta-analysis (30) showed a small, but significant, change in BMI from interventions involving sedentary behavior reduction (−0.158 BMI), which was higher in overweight and obese populations (−0.493 BMI) (32). A key issue in interpreting such findings is to consider what active behaviors are engaged in to substitute for reductions in sitting time and to determine whether diet and sleep also were affected. This is discussed in more detail later. Moreover, interventions have rarely targeted just sedentary time, making interpretation of intervention effects difficult.

Although sedentary behavior reduction may assist weight control, the evidence currently is weak. One reason for this conclusion is that the success of interventions in changing sedentary behavior in young people has been modest (5,53). Our meta-analysis of 17 studies showed a small but significant effect in changing behavior (Hedges' g = −0.192), and subsequent analyses have yielded similar results (31). However, it is noteworthy that in the meta-analysis by Kamath et al. (33), sedentary behavior interventions for young people showed a small but significant effect size (ES = −0.29), compared with physical activity (ES = 0.12) and healthy dietary change (ES = 0.00) interventions. However, it is not possible to conclude whether interventions were largely delivered as intended or whether intervention fidelity was weak. More process evaluations of interventions are required.

Our review of reviews suggested that the experimental evidence is still weak in showing effects for reductions in adiposity (18), but this could be due to only modest effects from interventions for actual behavior change. Obese young people may see stronger effects for adiposity from reductions in sedentary behavior (30).

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Coherence and Biological Plausibility

Sedentary behaviors, by definition, involve low energy expenditure, therefore, it is entirely plausible, and coherent with current knowledge, that they be associated with markers of adiposity. However, evidence also exists showing that sedentary behaviors need to be understood in the context of other potentially coexisting behaviors. The main behaviors of interest are physical activity, diet, and sleep.

Physical activity occurs on a movement (intensity) continuum ranging from sleep and sedentary behavior, to light, moderate, and vigorous physical activity. Two important implications stem from this. First, any change to sedentary behavior must be reflected in a change in at least one of the other behaviors or intensities across a 24-h period (34). Second, any reporting of associations between sedentary behavior and health outcomes (e.g., adiposity) must account for other relevant confounding or potentially mediating behaviors (e.g., dietary intake).

It has been shown that MVPA is only weakly inversely associated with sedentary behavior (35), suggesting that the two behaviors can coexist. However, given that any reduction in sitting time, during waking hours, will result in an increase in movement, it is light-intensity physical activity (LIPA) that is most likely to increase rather than MVPA. Some of this substitution effect from sedentary behavior will be into low LIPA, such as standing, which in adults has not been shown to change energy expenditure much in the short term (36). Although low LIPA has been found to be beneficially associated with cardiometabolic biomarkers in U.S. youth (37), few studies have explored relations with adiposity in younger age groups. A systematic review of the impact of height-adjustable desks on children’s sedentary behavior and physical activity found two studies that reported small increases in energy expenditure, and four of the six studies that examined changes in stepping reported small to moderate effects (38). Evidence is needed to further explore the longer term health effects of height-adjustable desks and to identify what intensities of activity are substituting for reductions in sitting time.

If sitting time is reduced and replaced with more high LIPA (e.g., incidental movement, light walking), and certainly, with moderate physical activity, then energy expenditure will increase. However, profiling of participants shows that sedentary time can coexist with MVPA (39). This means that some individuals will have high sedentary behavior and high MVPA, whereas others could have high sedentary behavior and low MVPA. Evidence exists for minimal deleterious health effects for high sedentary time among children who also are physically active at a high level (40).

The second important behavior to take into account is diet. Like physical activity, dietary intake is a modifiable health behavior that is independently associated with health outcomes such as adiposity. Dietary intake encompasses a diverse array of foods and food items that make categorizing diet as a whole extremely difficult. In terms of sedentary behavior, and its relation to diet, researchers tend to focus on elements of a less healthy diet such as lower fruit and vegetable consumption; higher consumption of energy-dense snacks, drinks, and fast foods; and higher total energy intake (41). Our own research has shown that sedentary behavior, in particular, screen time, is associated with a higher consumption of energy-dense snack foods and sugar-sweetened drinks and lower consumption of fruit and vegetables in young people (42).

Some of the plausible explanations for such an association in young people include that during time spent sitting in front of the TV and computers, young people are exposed to numerous advertisements (most often for junk foods) that can influence the type of food desired, requested, and consumed (43). Furthermore, screen viewing behaviors may cause distraction resulting in a lack of awareness of actual food consumption or overlooking food cues, which may lead to overconsumption and increased energy intake (44). Early research suggests that young people may associate TV viewing with eating from a young age if, for example, parents place their children in front of the TV with a snack or a meal while they do household chores (45).

The evidence for an association between sedentary behavior and unhealthy diet suggests that dietary intake may play a role in the relation between sedentary behavior and weight-related health outcomes. However, the mediating role of dietary intake in the associations between sedentary behaviors and adiposity rarely has been examined in young people. We recently reviewed 21 studies exploring whether the associations between various sedentary behaviors and cardiometabolic risk markers are independent of dietary intake in adolescents. Results suggested that significant positive associations exist between TV viewing, screen time and self-reported overall sedentary behavior with markers of adiposity, independent of dietary intake (46). However, only one study explored whether dietary intake played a mediating role. Recent analyses of the NHANES adolescent data found no evidence for dietary intake mediating the relation between TV viewing and BMI Z-scores (47). There was, however, a partial mediation of sugar-sweetened beverages (8.7%) and fruit and vegetables (4.1%) between TV viewing and metabolic syndrome (incorporating waist circumference, blood pressure, blood glucose and insulin, and serum lipids). Limitations of many of these studies included the inconsistent dietary categories explored, and none of the studies included measures of dietary intake during participation in the sedentary behavior.

A cross-sectional study of more than 1000 Canadian and U.S. 10 year olds showed that having a TV in their bedroom was associated with greater TV use and adiposity (48). However, this was not mediated by diet or sleep. This indicates that if diet is important in the relation between screen viewing and adiposity, it may not necessarily be so for all screen locations. Moreover, studies exploring the mediating role of dietary intake concurrent with sedentary behavior are needed.

The third important potential mediator in a relation between sedentary behavior and adiposity is sleep. Adolescents have shown a decline in sleep duration in recent decades, and short sleep duration has been associated with weight gain (49). In a recent review (50), young people with high physical activity, high sleep, and low sedentary behavior had healthier profiles, including less adiposity. It is thought that screen time disrupts sleep and may be associated with increased consumption of food late in the evening. Sleep disruption could be associated with higher fatigue and, thereby, less physical activity and more screen time late at night with associated exposure to light. The combined effects of high levels of sedentary behavior and reduced sleep, alongside physical activity and diet, require further investigation in the etiology of pediatric obesity. Evidence to date points to the need to facilitate 8–10 h of sleep per night for adolescents (49), and studies investigating sedentary behavior and adiposity that account for sleep hygiene are needed.

In addition to considering the role of physical activity, diet, and sleep to better understand the coherence of the relation between sedentary behavior and adiposity in youth, it also is necessary to recognize the likelihood of a bidirectional association. This so-called reverse causality argument suggests that in some cases, individuals who have greater adiposity will engage in higher levels of sedentary behavior. The reverse causality hypothesis rarely has been properly tested but is plausible. Although there are studies showing some direction of effect from longitudinal studies for sedentary behavior to precede adiposity (9), a great deal more work is required on the direction of association. Reverse causality may explain why stronger cross-sectional than longitudinal associations often are found. For now, with young people, we need to assume that a bidirectional effect is both plausible and, for some, likely.

In conclusion, although the mediating or confounding roles that physical activity, dietary intake, sleep and reverse causality play in the relation between sedentary behavior and adiposity in youth is not entirely clear, there is moderate evidence in support of coherence and biological plausibility for the association between sedentary behaviors and adiposity in youth.

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TOWARD A CONCEPTUAL MODEL

The link between sedentary behavior and adiposity in young people has been studied extensively for more than 30 yr. Much of this research has centered on screen time, and often, TV viewing time. There is clearly diverse opinion concerning the nature and extent of any association. As intimated earlier, it reflects a debate between the glass half-full argument (there is a small but meaningful association) and glass half-empty argument (the association is small or close to zero and not practically or clinically significant). It also could be argued that neither position is wholly correct and rather, a more appropriate conclusion is that “it’s complex” (51). One only has to view the “spaghetti diagram” depicting the multitude of influences on obesity displayed in the U.K. Foresight Report (52) to realize that we are not dealing with a simple issue. As Rutter (51) argues, obesity is complex rather than complicated: “Research within the biomedical paradigm tends to focus on specific topics such as dietary behavior and physical activity, psychological drivers, or genetic influences; the wider issue of obesity is then constructed from these elements. Obesity is, thus, treated as a complicated issue, not a complex one” (p. 746). Perhaps, we have fallen into the trap of taking only a biomedical view of sedentary behavior. We can look at sedentary behaviors in relative isolation, yet this ignores the complexity of a) a multitude of different sedentary behaviors; b) many other behaviors coexisting with sedentary behaviors; and c) multiple biological, genetic, social, cultural, psychological, and environmental influences on obesity across the domains of physical activity and diet. Sleep and other factors also are implicated. Moreover, single sedentary behaviors, such as TV viewing, although being important in their own right, may not be good markers of total sedentary time (53).

The complexity partly can be summarized by the factors identified in Table 3. As argued in this article, sedentary behaviors may be associated with adiposity, but this could be confounded by levels of LIPA, MVPA, dietary patterns, and sleep. Associations also may be bidirectional. Moreover, drivers of sedentary behavior may be somewhat context dependent (home, school, travel), and each context may differ in the degree to which sitting is a personal choice and has environmental and social constraints. In addition, a number of other potential moderators, mediators, and confounders could exist such as maturational status in adolescence and socioeconomic status (SES). The latter has been linked to both obesity and sedentary behavior (4).

TABLE 3

TABLE 3

In addition to Table 3, we have provided a simplified conceptual model in the Figure. Children and adolescents engage in multiple sedentary behaviors. The areas that have received the most attention include the following: i) self-reported screen time (including TV viewing) and ii) total sedentary time, usually assessed with wearable technology. From our assessment of review-level data, total sedentary time assessed with accelerometers is largely uncorrelated with markers of adiposity in youth (18).

Figure 1

Figure 1

Screen time — the most studied cluster of sedentary behaviors — has shown some variability in its association with adiposity, as discussed. Associations vary from strong to near zero. However, our analysis from a review of reviews (18) suggests that there is insufficient evidence to conclude that this association is causal. As shown in the Figure, screen time associations with adiposity may be mediated by coexisting behaviors of LIPA, MVPA, diet, and sleep, although work is required to further examine these relations. For example, although evidence on the link between screen time and diet has been shown, the link with adiposity is less clear (46). Moreover, these factors may be moderated, or even confounded, by maturational status in adolescence, SES, other coexisting behaviors, and the context that different sedentary behaviors take place in. The bidirectional nature of the association between sedentary behavior and adiposity (reverse causality) also is important to consider, as discussed. Contextual differences are shown in Table 3.

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CONCLUSIONS AND SUMMARY

Evidence for a relation between screen time, including TV viewing, and adiposity in children and adolescents often is statistically significant but small in magnitude. Studies assessing total sedentary time with wearable technology (accelerometers) tend to show smaller, and sometimes, no effects. Arguments about the practical significance of these findings reflect the difference between glass half-full and glass half-empty perceptions. Whatever the interpretation concerning adiposity, there may be important public health benefits for reducing recreational screen time in youth for many reasons, many beyond just obesity, and guidelines that suggest reductions are sensible.

This area of research is complex. Many interpretations of the study data fail to recognize this, and future research needs to account for likely mediators, moderators, and confounders, as well as the bidirectional nature of the relation. We have tried to show this in the Figure. The diagram is not a definitive statement summarizing the evidence. It provides a schematic heuristic and an overview of possibilities. Moreover, rapid changes to screen technologies make it difficult to capture sedentary behavior exposures of young generations. This is a “moveable feast” and is a challenge for researchers.

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Acknowledgment

Stuart Biddle thanks Dr. Simon Marshall and Dr. Trish Gorely for their important contributions to his research outputs and wider thinking on the subject of sedentary behavior in young people. He also thanks colleagues Jason Bennie, Tracy Kolbe-Alexander, Fernando Peruyero de León, George Thomas, and Ineke Vergeer for their intellectual input to the Figure. Sadly, former colleague and friend, Len Almond, died during the preparation of this manuscript. The social, professional, and intellectual support from Len, over many years, is gratefully acknowledged.

Jo Salmon thanks Professor Neville Owen and Professor David Dunstan for their contributions to her research outputs and conceptualization of sedentary behavior in young people.

Stuart Biddle was supported in his prior research in this field by the British Heart Foundation, funders of the National Prevention Research Initiative (U.K.), U.K. Department of Health, Medical Research Council (U.K.), and National Institute for Health Research (U.K.).

Natalie Pearson’s prior research in this field and salary were supported by the British Heart Foundation.

Jo Salmon was supported by a National Health and Medical Research Council of Australia Principal Research Fellowship (APP1026216).

Stuart Biddle has received funding since 2014 for consultancy work from Fitness First and Unilever. None of these are currently active. Funding was received in 2016 for consultancy work for Halpern PR Limited. In-kind support through the provision of a sit-to-stand desk was provided by Ergotron from 2012 to 2014. Advice has been requested by and offered to Active Working, Get Britain Standing, and Bluearth, none with funding.

The husband of Jo Salmon is the director of a company that manufactures sit-to-stand desks (Sit Less Pty Ltd) specifically designed for use in a range of settings including primary and secondary schools.

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Keywords:

causality; obesity; screen time; sitting time; television viewing

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