Systematic review of sedentary behaviour and health indicators in school-aged children and youth

  • Mark S Tremblay1Email author,

    Affiliated with

    • Allana G LeBlanc1,

      Affiliated with

      • Michelle E Kho2,

        Affiliated with

        • Travis J Saunders1,

          Affiliated with

          • Richard Larouche1,

            Affiliated with

            • Rachel C Colley1,

              Affiliated with

              • Gary Goldfield1 and

                Affiliated with

                • Sarah Connor Gorber3

                  Affiliated with

                  International Journal of Behavioral Nutrition and Physical Activity20118:98

                  DOI: 10.1186/1479-5868-8-98

                  Received: 8 April 2011

                  Accepted: 21 September 2011

                  Published: 21 September 2011

                  Abstract

                  Accumulating evidence suggests that, independent of physical activity levels, sedentary behaviours are associated with increased risk of cardio-metabolic disease, all-cause mortality, and a variety of physiological and psychological problems. Therefore, the purpose of this systematic review is to determine the relationship between sedentary behaviour and health indicators in school-aged children and youth aged 5-17 years. Online databases (MEDLINE, EMBASE and PsycINFO), personal libraries and government documents were searched for relevant studies examining time spent engaging in sedentary behaviours and six specific health indicators (body composition, fitness, metabolic syndrome and cardiovascular disease, self-esteem, pro-social behaviour and academic achievement). 232 studies including 983,840 participants met inclusion criteria and were included in the review. Television (TV) watching was the most common measure of sedentary behaviour and body composition was the most common outcome measure. Qualitative analysis of all studies revealed a dose-response relation between increased sedentary behaviour and unfavourable health outcomes. Watching TV for more than 2 hours per day was associated with unfavourable body composition, decreased fitness, lowered scores for self-esteem and pro-social behaviour and decreased academic achievement. Meta-analysis was completed for randomized controlled studies that aimed to reduce sedentary time and reported change in body mass index (BMI) as their primary outcome. In this regard, a meta-analysis revealed an overall significant effect of -0.81 (95% CI of -1.44 to -0.17, p = 0.01) indicating an overall decrease in mean BMI associated with the interventions. There is a large body of evidence from all study designs which suggests that decreasing any type of sedentary time is associated with lower health risk in youth aged 5-17 years. In particular, the evidence suggests that daily TV viewing in excess of 2 hours is associated with reduced physical and psychosocial health, and that lowering sedentary time leads to reductions in BMI.

                  Keywords

                  Inactivity sitting TV body composition fitness metabolic syndrome, cardiovascular disease self-esteem pro-social behaviour, academic achievement

                  Introduction

                  Engaging in regular physical activity is widely accepted as an effective preventative measure for a variety of health risk factors across all age, gender, ethnic and socioeconomic subgroups [16]. However, across all age groups, levels of physical activity remain low [712] and obesity rates continue to rise [10, 11, 13, 14]; collectively threatening the persistent increase in life expectancy enjoyed over the past century and efforts to counteract the inactivity and obesity crisis [15].

                  This inactivity crisis is especially important in the pediatric population as recent data from the Canadian Health Measures Survey [8] suggest that only 7% of children and youth aged 6-19 years participate in at least 60 minutes of moderate- to vigorous-intensity physical activity per day, thus meeting the current physical activity guidelines from Canada [16], the U.S. [6], the U.K [17], Australia [18] and the World Health Organization (WHO) [5]. However, even for those children and youth who meet current guidelines, there remains 23 hours per day for school, sleep, work, and discretionary time. Several sources report that children and youth spend the majority of their discretionary time engaging in sedentary pursuits (e.g. watching television (TV) or playing video games) [8, 1928]. Canadian children and youth are spending an average of 8.6 hours per day, or 62% of their waking hours being sedentary [8]. Similar trends are being reported in the U.S. where children and youth spend an average of 6-8 hours per day being sedentary [2228]. Accumulating evidence shows that, independent of physical activity levels, sedentary behaviours are associated with increased risk of cardio-metabolic disease, all-cause mortality, and a variety of physiological and psychological problems [2931]. Therefore, to maximize health benefits, approaches to resolve the inactivity crisis should attempt to both increase deliberate physical activity and decrease sedentary behaviours, especially in the pediatric population. However, to date, public health efforts have focused primarily on physical activity and have paid little attention to the mounting evidence to support sedentary behaviour as a distinct behaviour related to poor health.

                  A recent scoping review identified review articles, meta-analyses, and grey literature that examined the relationship between sedentary behaviour and health [32]. The large majority of this information reported on the relationship between screen time and body composition and did not include other indicators of health [2325]. Furthermore, none of these reviews followed the rigorous process of a systematic review and are therefore not able to be used to inform the development of clinical practice guidelines. As a result, to our knowledge, there are no systematic, evidence-based sedentary behaviour guidelines for any age group, anywhere in the world. Guidelines that do exist are largely based on expert opinion or narrative literature reviews [33, 34].

                  Therefore, the purpose of this systematic review was to gather, catalog, assess and evaluate the available evidence examining sedentary behaviours in relation to selected health outcomes in children and youth 5-17 years of age and present a summary of the best available evidence. Specifically, the review presents available evidence for minimal and optimal thresholds for daily sedentary time in children and youth, and when possible, how thresholds differ across health outcome or demographic status (i.e. age, gender). The information gathered in this review can serve to guide future research and inform the development of evidence-based clinical practice guideline recommendations for safe and healthy amounts of daily sedentary behaviour in the pediatric population.

                  Methods

                  Study Inclusion Criteria

                  The review sought to identify all studies that examined the relationship between sedentary behaviour and a specific health outcome in children and youth (aged 5-17 years). All study designs were eligible (e.g. cross sectional, retrospective, prospective, case control, randomized controlled trial (RCT), longitudinal). Longitudinal studies were included if the data presented in the article was consistent with the age limits that were set (i.e. if the study looked at participants at age 10 and then again at age 30, only baseline measurements from age 10 were used).

                  Studies were included only if there was a specific measure of sedentary behaviour. Eligible exposures of sedentary behaviours included those obtained via direct (e.g., measurements of sitting, or low activity measured by accelerometer) and self-reported (e.g., questionnaires asking about TV watching, video gaming, non-school computer use, and screen time - composite measures of TV, video games, computers) methods. Sedentary behaviour was often measured as a composite measure of all time engaging in sedentary behaviours including screen time outside of school hours. Six health indicators were chosen based on the literature, expert input, and a desire to have relevant measures from a range of holistic health indicators (i.e. not only physical health, but also emotional, mental and intellectual health). The six eligible indicators in this review were:
                  1. 1.

                    Body composition (overweight/obesity measured by body mass index (BMI), waist circumference, skin folds, bio-impedance analysis (BIA), dual-energy x-ray absorptiometry (DXA or DEXA));

                     
                  2. 2.

                    Fitness (physical fitness, physical conditioning, musculoskeletal fitness, cardiovascular fitness);

                     
                  3. 3.

                    Metabolic syndrome (MS) and cardiovascular disease (CVD) risk factors (unfavourable lipid levels, blood pressure, markers for insulin resistance or type 2 diabetes);

                     
                  4. 4.

                    Self-esteem (self-concept, self-esteem, self efficacy);

                     
                  5. 5.

                    Behavioural conduct/pro-social behaviour (child behaviour disorders, child development disorder, pro-social behaviour, behavioural conduct, aggression);

                     
                  6. 6.

                    Academic achievement (school performance, grade-point average).

                     
                  No Language or date limits were imposed in the search. The following definitions were used to help guide the systematic review [31]:
                  • Sedentary: A distinct class of behaviours (e.g. sitting, watching TV, playing video games) characterized by little physical movement and low energy expenditure (≤ 1.5 METs).

                  • Sedentarism: Engagement in sedentary behaviours characterized by minimal movement, low energy expenditure, and rest.

                  • Physically active: Meeting established physical activity guidelines (e.g. see Tremblay et al. 2011 for Canadian Physical Activity Guidelines [16]).

                  • Physical inactivity: The absence of physical activity, usually reflected as the proportion of time not engaged in physical activity of a pre-determined intensity and therefore not meeting established physical activity guidelines.

                  Study Exclusion Criteria

                  As the volume of literature on sedentary behaviour was anticipated to be very high, to control the feasibility of this project, the following sample size limits were set a priori: population based studies (observational, cross sectional, cohort, and retrospective studies) were required to have a minimum sample size of 300 participants; RCTs, and intervention studies were required to have at least 30 participants. Studies of 'active gaming' (e.g., Nintendo Wii™, Microsoft Kinect™, Sony's Playstation Move™, video arcades, etc.) were excluded. Finally, studies that defined sedentary behaviour as 'failing to meet physical activity guidelines' were excluded from the review.

                  Search strategy

                  The following electronic bibliographic databases were searched using a comprehensive search strategy to identify relevant studies: Ovid MEDLINE(R) (1950 to February Week 2 2010), Ovid EMBASE (1980 to 2010 Week 07), and Ovid psycINFO (1806 to February Week 3 2010). The search strategy was created by a single researcher (JM) and run by a second researcher (AL). The search strategies can be found in Additional file 1. The search was limited to studies looking at 'school-aged' children and youth (mean age of 5-17 years). Articles were extracted as text files from the OVID interface and imported in to Reference Manager Software (Thompson Reuters, San Francisco, CA). Duplicate articles were first removed using Reference Manager Software, and any remaining duplicates were removed manually. All articles were given a unique reference identification number in the database.

                  Titles and abstracts of potentially relevant articles were screened by two reviewers (AL and one of GG, MT, RC, RL or TS) and full text copies were obtained for all articles meeting initial screening by at least one reviewer. Two independent reviewers examined all full text articles (AL and one of GG, MT, RC, RL or TS) and any discrepancies were resolved by discussion and consensus between the two reviewers. If the reviewers were unable to reach consensus, a third reviewer was asked to look at the article in question. Consensus was obtained for all included articles.

                  Twelve key content experts were contacted and asked to identify the most influential papers from their personal libraries examining sedentary behaviour and health in the pediatric age group. Government documents from the U.S [6], the U.K. [17], and Australia [18] were used for reference and to help guide the review process.

                  Data extraction

                  Standardized data extraction tables were created; data extraction was completed by one reviewer (AL) and checked by another (one of GG, RC, RL, or TS) for accuracy. Information was extracted regarding study characteristics (i.e. year, study design, country, number of participants, age), type of sedentary behaviour, measure of sedentary behaviour (i.e. direct, or indirect), and health outcome. Reviewers were not blinded to the authors or journals when extracting data.

                  Risk of bias assessment

                  The Downs and Black checklist was used to asses study quality [35]. This 27 point checklist assesses the quality of reporting (e.g. "Are the main findings of the study clearly described"); external validity (e.g. "Were the subjects asked to participate representative of the entire population from which they were recruited"); internal validity (e.g. "Were subjects randomized to intervention groups"); and power (e.g. "Was there sufficient power such that the difference being due to chance is less than 5%"). The maximum score a study can receive is 32, with higher scores indicating better quality. Inter-rater reliability was calculated using Cohen's kappa.

                  Quality of evidence was determined by the study design and by Downs and Black score. Level of evidence was used to explain the quality of available studies and the confidence of the findings [36]. RCTs were considered to have the highest level of evidence while anecdotal reports were considered to have the lowest evidence. See Table 1 for more details. When possible, studies were examined for differences among age and gender subgroups.
                  Table 1

                  Criteria for assigning level of evidence to a recommendation

                  Level of evidence

                  Criteria

                  Level 1

                  - Randomized control trials without important limitations

                  Level 2

                  - Randomized control trials with important limitations

                  - Observational studies (non-randomized clinical trials or cohort studies) with overwhelming evidence

                  Level 3

                  - Other observational studies (prospective cohort studies, case-control studies, case series)

                  Level 4

                  - Inadequate or no data in population of interest

                  - Anecdotal evidence or clinical experience

                  Adapted from: Lau DC et al. 2007 [36]

                  Analysis

                  A meta-analysis was performed with the data that were sufficiently homogeneous in terms of statistical, clinical, and methodological characteristics using Review Manager Software 5.0 (The Cochrane Collaboration, Copenhagen Denmark). Pooled estimates for the meta-analysis and their 95% confidence intervals were obtained using the random effects estimator of DerSimonian-Laird [37]. Studies were weighted by the inverse of their variance. Cochrane's Q was used to test for heterogeneity among studies and the I2 (squared) index [10] was used to determine the degree of heterogeneity [38]. Funnel plots were used to assess publication bias (data not shown). Qualitative syntheses were conducted for remaining studies.

                  Results

                  Description of studies

                  After de-duplication, the preliminary search of electronic databases, reference lists, and grey literature identified 5,291 potentially relevant articles (Figure 1). Of these, 3,299 were identified in MEDLINE, 1,016 in EMBASE, 912 in psycINFO, and 64 through key informants, government documents, and bibliographies. After a preliminary review of titles and abstracts, 828 articles were included for detailed assessment of the full text article. Of these, 232 met the criteria for study inclusion (8 RCTs, 10 intervention studies, 37 longitudinal studies and 177 cross sectional studies). Individual study characteristics can be seen in Table 2. Reasons for excluding studies included: ineligible population (e.g. ineligible age or sample size) (n = 161), ineligible exposure (e.g. diet, physical activity) (n = 145), ineligible measure of sedentary behaviour (i.e. not meeting physical activity guidelines) (n = 19), ineligible outcome (n = 60), ineligible analysis (e.g. analysis focused on content of screen time versus duration of screen time, analysis focused on active video gaming) (n = 60), and 'other' (n = 216) (e.g. commentary article or methodological paper). Some studies were excluded for multiple reasons. Some articles (n = 9) could not be retrieved due to missing or incorrect reference information.
                  http://static-content.springer.com/image/art%3A10.1186%2F1479-5868-8-98/MediaObjects/12966_2011_Article_492_Fig1_HTML.jpg
                  Figure 1

                  Flow of information through the different phases of the review.

                  Table 2

                  Summary of characteristics of included studies

                        

                  nanalyzed

                      

                  First Author

                  Year

                  Country

                  Grade

                  Age Range

                  Mean age

                  Total

                  Boys

                  Girls

                  Units of sedentary behaviour

                  Exposure

                  Outcome

                  RANDOMIZED CONTROLLED TRIALS

                             

                  Epstein LH [265]

                  1995

                  US

                   

                  8-12

                  10.1

                  61

                    

                  hour

                  week

                  TV

                  BC

                  Epstein LH [50]

                  2008

                  US

                   

                  4-7

                  6

                  70

                  37

                  33

                  hour

                  day

                  TV

                  BC

                  Goldfield GS [264]

                  2006

                  Canada

                   

                  8-12

                  10.4

                  30

                  13

                  17

                  min

                  day

                  TV

                  BC

                  Gortmaker SL [57]

                  1995

                  US

                    

                  11.7

                  1295

                  668

                  627

                  hour

                  day

                  TV

                  BC

                  Hughes AR [262]

                  1991

                  Scotland

                   

                  5-11

                  8.8

                  134

                  59

                  74

                  hour

                  day

                  SB

                  BC

                  Robinson TN [58]

                  1999

                  US

                     

                  192

                    

                  hour

                  week

                  TV, GAMES

                  BC

                  Robinson TN [221]

                  2003

                  US

                   

                  8-10

                  9.5

                  61

                  0

                  61

                  hour

                  week

                  TV

                  BC, SE

                  Shelton D [263]

                  2007

                  Australia

                   

                  3-10

                  7.5

                  43

                  20

                  23

                  hour

                  day

                  TV

                  BC

                  INTERVENTION STUDIES

                            

                  Epstein LH [56]

                  2000

                  US

                   

                  8-12

                  10.5

                  76

                  24

                  52

                  hour

                  month

                  SB, ST

                  BC, FIT

                  Epstein LH [59]

                  2004

                  US

                   

                  8-12

                  9.8

                  60

                  23

                  39

                  times

                  week

                  TV

                  BC

                  Epstein LH [60]

                  2005

                  US

                   

                  8-16

                   

                  58

                  28

                  30

                  hour

                  day

                  SB, TV

                  BC

                  Gentile DA [61]

                  2009

                  US

                    

                  9.6

                  1323

                  685

                  674

                  hour

                  day

                  ST

                  BC

                  Goldfield GS [52]

                  2007

                  Canada

                   

                  8-12

                  10.4

                  30

                  13

                  17

                  hour

                  day

                  SB

                  BC, SE

                  Harrison M [62]

                  2003

                  Ireland

                    

                  10.2

                  312

                  177

                  135

                  min

                  day

                  TV, ST

                  BC

                  Ochoa MC [53]

                  2007

                  Spain

                   

                  6-18

                  11.6

                  370

                  196

                  174

                  hour

                  week

                  TV

                  BC

                  Salmon J [51]

                  2008

                  Australia

                   

                  1011

                  10.8

                  311

                  152

                  159

                  hour

                  day

                  TV

                  BC

                  Simon C [54]

                  2002

                  France

                    

                  11.7

                  954

                  468

                  486

                  hour

                  day

                  TV, COMP

                  BC, SE

                  Tanasescu M [55]

                  2000

                  Puerto Rico

                  7-10

                  9.2

                  53

                  22

                  31

                  hour

                  day

                  TV

                  BC

                  LONGITUDINAL STUDIES

                         

                  hour

                    

                  Aires L [83]

                  2010

                  Portugal

                  11-19

                   

                  345

                  147

                  198

                  hour

                  day

                  SCREEN

                  BC, FIT

                  Berkey CS [76]

                  2003

                  US

                   

                  10-15

                   

                  11887

                  5120

                  6767

                  hour

                  day

                  TV, GAMES

                  BC

                  Bhargava A [77]

                  2008

                  US

                     

                  7635

                    

                  min

                  day

                  TV

                  BC

                  Blair NJ [68]

                  2007

                  England

                    

                  5.5

                  591

                  287

                  304

                  hour

                  day

                  SB, TV

                  BC

                  Borradaile KE [86]

                  2008

                  US

                    

                  11.2

                  1092

                  501

                  591

                  hour

                  week

                  TV

                  BC

                  Burke V [71]

                  2006

                  Australia

                    

                  7.6/10.8

                  1569

                  630

                  648

                  hour

                  week

                  SCREEN

                  BC

                  Chen JL [78]

                  2007

                  Chinese

                   

                  7-8

                  7.52

                  307

                  147

                  160

                  hour

                  day

                  TV, GAMES

                  BC

                  Danner FW [66]

                  2008

                  US

                     

                  7334

                  3674

                  3660

                  hour

                  day

                  TV

                  BC

                  Dasgupta K [215]

                  2006

                  Canada

                    

                  12.7/15.1/17.0

                  662

                  319

                  343

                  hour

                  week

                  SB, TV

                  MS

                  Day RS [85]

                  2009

                  US

                   

                  8-14

                   

                  556

                  277

                  279

                  min

                  day

                  TV

                  BC

                  Dietz WH [181]

                  1985

                  US

                   

                  12-17

                   

                  2153

                    

                  hour

                  day

                  TV

                  BC

                  Elgar FJ [79]

                  2005

                  Wales

                    

                  11.7

                  654

                  293

                  361

                  hour

                  week

                  TV

                  BC

                  Elgar FJ [79]

                  2005

                  Wales

                    

                  15.3

                  392

                  181

                  211

                  hour

                  week

                  TV

                  BC

                  Ennemoser M [237]

                  2007

                  German

                   

                  6-8

                   

                  332

                    

                  min

                  day

                  TV

                  SE, AA

                  Fulton JE [84]

                  2009

                  US

                   

                  10-18

                   

                  472

                  245

                  227

                  min

                  day

                  TV

                  BC

                  Gable S [70]

                  2007

                  US

                     

                  8000

                    

                  hour

                  day

                  TV

                  BC

                  Hancox RJ [88]

                  2004

                  New Zealand

                  5-15

                   

                  1013

                    

                  hour

                  day

                  TV

                  BC, MS

                  Hancox RJ [72]

                  2006

                  New Zealand

                  5-15

                   

                  603

                  372

                  339

                  hour

                  day

                  SCREEN

                  BC

                  Henderson VR [67]

                  2007

                  US

                   

                  11-19

                   

                  2379

                  0

                  2379

                  hour

                  day

                  TV, SCREEN

                  BC

                  Hesketh K [80]

                  1997

                  Australia

                   

                  5-10

                  7.6

                  1278

                  630

                  648

                  hour

                  day

                  SCREEN

                  BC

                  Hesketh K [80]

                  1997

                  Australia

                   

                  8-13

                  10.7

                  1278

                  630

                  648

                  hour

                  day

                  SCREEN

                  BC

                  Hesketh K [64]

                  2009

                  Australia

                   

                  5-10

                  7.7

                  1943

                  972

                  971

                  hour

                  day

                  TV, GAMES

                  BC

                  Hesketh K [64]

                  2009

                  Australia

                   

                  8-13

                   

                  1569

                  816

                  753

                  hour

                  day

                  TV, GAMES

                  BC

                  Jackson LA [223]

                  2009

                  US

                    

                  12

                  500

                  235

                  265

                  hour

                  day

                  COMP, SCREEP

                  SE

                  Jago R [82]

                  2005

                  US

                   

                  5-6

                  6.5

                  138

                  65

                  73

                  min

                  hr

                  SB, TV

                  BC

                  Janz KF [73]

                  2005

                  US

                    

                  5.6/8.6

                  378

                  176

                  202

                  hour

                  day

                  SCREEN

                  BC

                  Johnson JG [41]

                  2007

                  US

                        

                  hour

                  day

                  TV

                  AA

                  Kaur H [75]

                  2003

                  US

                   

                  12-17

                   

                  2223

                  1149

                  1074

                  hour

                  day

                  TV

                  BC

                  Lajunen HR [128]

                  2007

                  Finland

                   

                  15-19

                   

                  5184

                    

                  hour

                   

                  SB

                  BC

                  Lonner W [238]

                  1985

                  US

                   

                  9-19

                  14.2

                  367

                    

                  hour

                  day

                  TV

                  AA

                  Maffeis C [89]

                  1998

                  Italy

                    

                  8.7

                  298

                  148

                  150

                  min

                  day

                  SCREEN

                  BC

                  Mistry K [229]

                  2007

                  US

                        

                  hour

                  day

                  TV

                  PRO

                  Mitchell JA [49]

                  2009

                  UK

                   

                  11-12

                  11.8

                  5434

                  2590

                  2844

                  hour

                  day

                  SB

                  BC, FIT

                  Must A [87]

                  2007

                  US

                   

                  10-17

                   

                  156

                  0

                  156

                  hour

                  day

                  SB, SCREEN

                  BC

                  O'Brien M [69]

                  2007

                  US

                   

                  2-12

                   

                  653

                    

                  hour

                  week

                  TV

                  BC

                  Parsons TJ [74]

                  2005

                  England/Scotland/Wales

                  11/16

                  17733

                    

                  hour

                  day

                  TV

                  BC

                  Purslow LR [63]

                  2008

                  England

                   

                  8-9

                   

                  345

                  176

                  169

                  min

                  day

                  SB

                  BC

                  Timperio A [65]

                  2008

                  Australia

                   

                  10-12

                   

                  344

                  152

                  192

                  times

                  week

                  SB, SCREEN

                  BC

                  Treuth MS [29]

                  2007

                  US

                    

                  11.9

                  984

                  0

                  984

                  min

                  day

                  SB

                  BC

                  Treuth MS [27]

                  2009

                  US

                    

                  13.9

                  984

                  0

                  984

                  min

                  day

                  SB

                  BC

                  Wosje,K.S [205]

                  2009

                  US

                   

                  6.75-7.25

                   

                  214

                    

                  hour

                  day

                  SCREEN

                  FIT

                  CROSS SECTIONAL STUDIES

                            

                  Al SH [192]

                  2009

                  International

                   

                  12-18

                   

                  17715

                  8503

                  9212

                  hour

                  day

                  TV

                  BC

                  Albarwani S [207]

                  2009

                  Oman

                   

                  15-16

                   

                  529

                  245

                  284

                  hour

                  week

                  TV, COMP

                  FIT

                  Alves JG [191]

                  2009

                  Brazil

                   

                  7-10

                   

                  733

                  407

                  326

                  hour

                  day

                  TV

                  BC

                  Aman J [218]

                  2009

                  Sweden

                   

                  11-18

                  14.5

                  2093

                  1016

                  991

                  hour

                  week

                  TV, COMP

                  MS

                  Andersen LF [155]

                  2005

                  Norway

                   

                  8-14

                   

                  1432

                  702

                  730

                  hour

                  day

                  TV

                  BC

                  Andersen RE [142]

                  1998

                  US

                   

                  8-16

                   

                  4063

                  1985

                  2071

                  hour

                  day

                  TV

                  BC

                  Anderson SE [103]

                  2008

                  US

                   

                  4-12

                  8

                  2964

                  1509

                  1455

                  hour

                  day

                  TV

                  BC

                  Armstrong CA [213]

                  1998

                  US

                    

                  9.28

                  588

                  304

                  284

                  hour

                  day

                  TV

                  FIT

                  Asante PA [183]

                  2009

                  US

                   

                  3-13

                  8.5

                  324

                  182

                  142

                  hour

                  day

                  SCREEN

                  BC

                  Aucote HM [163]

                  2009

                  Australia

                  5-6

                   

                  11.09

                  393

                  198

                  195

                  hour

                  week

                  TV, GAMES

                  BC

                  Barlow SE [151]

                  2007

                  US

                   

                  6-17

                  12.1

                  52845

                    

                  hour

                  day

                  TV

                  BC

                  Basaldua N [109]

                  2008

                  Mexico

                   

                  6-12

                  8.9

                  551

                  278

                  273

                  hour

                  day

                  TV

                  BC

                  Bellisle F [123]

                  2007

                  France

                   

                  9-11

                   

                  1000

                  500

                  500

                  hour

                  day

                  TV

                  BC

                  Berkey CS [90]

                  2000

                  US

                    

                  Sep-14

                  10769

                  4620

                  6149

                  hour

                  day

                  TV

                  BC

                  Beyerlein A [105]

                  2008

                  Germany

                   

                  4.5-7.3

                   

                  4967

                  2585

                  2382

                  hour

                  day

                  TV

                  BC

                  Boone JE [164]

                  2007

                  US

                    

                  15.9

                  9155

                  4879

                  4276

                  hour

                  week

                  SCREEN

                  BC

                  Boone-Heinonen J [104]

                  2008

                  US

                   

                  11-21

                   

                  9251

                    

                  hour

                   

                  SB

                  BC

                  Boutelle KN [130]

                  2007

                  US

                   

                  16-18

                   

                  1726

                  890

                  836

                  hour

                  day

                  TV

                  BC

                  Brodersen NH [235]

                  2005

                  England

                    

                  11.8

                  4320

                  2578

                  1742

                  hour

                  week

                  SB

                  SE, PRO

                  Bukara-Radujkovic G [96]

                  2009

                  Bosnia

                   

                  11-12

                  11.5

                  1204

                  578

                  626

                  hour

                  day

                  TV, COMP

                  BC

                  Butte NF [119]

                  2007

                  US

                   

                  6-17

                  10.8

                  897

                  441

                  456

                  hour

                  day

                  SCREEN

                  BC

                  Caldas S [245]

                  1999

                  US

                   

                  4-19

                   

                  34542

                    

                  hour

                  day

                  TV

                  AA

                  Carvalhal MM [131]

                  2007

                  Portugal

                  10-11

                    

                  3365

                  1755

                  1610

                  hour

                  day

                  TV, COMP

                  BC

                  Chaput J [154]

                  2006

                  Canada

                   

                  5-10

                  6.6

                  422

                  211

                  211

                  hour

                  day

                  SCREEN

                  BC

                  Chen MY [78]

                  2007

                  Taiwan

                   

                  13-18

                  15.03

                  660

                  351

                  309

                  hour

                  day

                  TV, COMP

                  BC, SE, PRO

                  Chowhan J [232]

                  2007

                  Canada

                   

                  12-15

                   

                  2666

                    

                  hour

                  day

                  TV

                  PRO

                  Christoforidis A [95]

                  2009

                  Greece

                   

                  4-18

                  11.41

                  1549

                  735

                  814

                  hour

                  day

                  SCREEN

                  BC, FIT

                  Collins AE [149]

                  2008

                  Indonesia

                  12-15

                   

                  1758

                  815

                  916

                  hour

                  day

                  TV, COMP

                  BC

                  Colwell J [200]

                  2003

                  Japan

                   

                  12-13

                   

                  305

                  159

                  146

                  hour

                  day

                  SCREEN

                  BC, PRO

                  Cooper H [247]

                  1999

                  US

                  7-11

                    

                  424

                  225

                  199

                  hour

                  day

                  TV

                  AA

                  Crespo CJ [177]

                  2001

                  US

                   

                  8-16

                   

                  4069

                  1994

                  2075

                  hour

                  day

                  TV

                  BC

                  Da CR [157]

                  2003

                  Brazil

                   

                  7-10

                   

                  446

                  107

                  107

                  hour

                  day

                  TV

                  BC

                  Dasgupta K [215]

                  2007

                  Canada

                   

                  13-17

                   

                  1267

                    

                  hour

                  week

                  SCREEN

                  MS

                  Delva J [125]

                  2007

                  US

                     

                  11265

                  5274

                  5991

                  hour

                  week

                  TV

                  BC

                  Dietz WH [181]

                  1985

                  US

                   

                  12-17

                   

                  6671

                    

                  hour

                  day

                  TV

                  AA

                  Dietz WH [181]

                  1985

                  US

                   

                  6-11

                   

                  6965

                    

                  hour

                  day

                  TV

                  BC, AA

                  Dollman J [211]

                  2006

                  Australia

                  6

                  10-11

                   

                  843

                  439

                  404

                  min

                  Day

                  TV

                  FIT

                  Dumais SA [255]

                  2009

                  US

                   

                  10-12

                   

                  15850

                    

                  hour

                   

                  TV

                  AA

                  Dominick JR [225]

                  1984

                  US

                  10, 11

                  14-18

                   

                  250

                  110

                  140

                  hour

                  Day

                  TV, GAME

                  SE, PRO

                  Eisenmann JC [175]

                  2002

                  US

                   

                  14-18

                   

                  15143

                    

                  hour

                  day

                  TV

                  BC

                  Eisenmann JC [113]

                  2008

                  US'

                    

                  16.2

                  12464

                  6080

                  6384

                  hour

                  day

                  TV

                  BC

                  Ekelund U [134]

                  2006

                  Europe

                   

                  9-16

                   

                  1921

                  911

                  1010

                  hour

                  day

                  TV

                  BC, MS

                  Fetler M [249]

                  1984

                  US

                  6

                    

                  10603

                    

                  hour

                  day

                  SCREEN

                  AA

                  Forshee RA [201]

                  2004

                  US

                   

                  12-16

                  14

                  2216

                  1075

                  1141

                  hour

                  day

                  TV

                  BC

                  Forshee RA [188]

                  2009

                  US

                   

                  5-18

                   

                  1459

                  734

                  725

                  hour

                  week

                  SCREEN

                  BC

                  Gaddy GD [257]

                  1986

                  US

                     

                  5074

                    

                  hour

                  day

                  TV

                  AA

                  Giammattei J [140]

                  2003

                  US

                   

                  11-14

                  12.6

                  385

                  186

                  199

                  hour

                  day

                  TV

                  BC

                  Gibson S [156]

                  2004

                  England

                   

                  7-18

                   

                  1294

                  655

                  639

                  min

                  day

                  TV

                  BC

                  Gomez LF [150]

                  2007

                  Colombia

                   

                  5-12

                   

                  11137

                  5539

                  5598

                  hour

                  day

                  TV, GAMES

                  BC

                  Gordon-Larsen P [176]

                  2002

                  US

                   

                  11-19

                  15.9

                  12759

                  6290

                  6496

                  hour

                  week

                  TV, GAMES

                  BC

                  Gortmaker SL [143]

                  1996

                  US

                   

                  10-15

                  11.5

                  746

                  388

                  358

                  hour

                  day

                  TV

                  BC

                  Gortmaker SL [57]

                  1999

                  US

                   

                  6-11

                   

                  1745

                    

                  min

                  week

                  TV

                  SE, AA

                  Gortmaker SL [57]

                  1999

                  US

                   

                  12-17

                   

                  1745

                    

                  min

                  week

                  TV

                  SE, AA

                  Graf C [167]

                  2004

                  Germany

                    

                  6.8

                  344

                  177

                  167

                  hour

                  day

                  TV, COMP

                  BC

                  Grusser SM [40]

                  2005

                  Germany

                  6

                   

                  11.83

                  323

                  175

                  148

                  hour

                  day

                  TV

                  AA

                  Hardy LL [133]

                  2006

                  Australia

                   

                  11-15

                   

                  2750

                  1446

                  1304

                  hour

                  day

                  SCREEN

                  FIT

                  Hernandez B [178]

                  1999

                  Mexico

                   

                  9-16

                   

                  461

                  244

                  217

                  hour

                  day

                  TV

                  BC

                  Hirschler V [144]

                  2009

                  Argentina

                  7-11

                  8.9

                  330

                  168

                  162

                  hour

                  day

                  TV

                  BC

                  Holder MD [222]

                  2009

                  Canada

                   

                  8-12

                   

                  375

                  252

                  262

                  hour

                  day

                  SCREEN

                  SE

                  Hume C [190]

                  2009

                  Netherlands

                   

                  13

                  580

                  277

                  303

                  hour

                  day

                  SCREEN

                  BC

                  Islam-Zwart K [195]

                  2008

                  US

                     

                  480

                  198

                  282

                  hour

                  day

                  TV

                  BC

                  Jackson LA [223]

                  2009

                  US

                    

                  12.18

                  515

                  259

                  256

                  hour

                  day

                  GAMES, COMP

                  AA

                  Janssen I [166]

                  2004

                  Canada

                   

                  11-16

                   

                  5890

                  2812

                  3078

                  hour

                  day

                  TV, COMP

                  BC

                  Janz K [174]

                  2002

                  US

                   

                  4-6

                  5.3

                  462

                  216

                  246

                  hour

                  day

                  TV

                  BC

                  Jaruratanasirikul S [241]

                  2009

                  Thailand

                  7-12

                   

                  15.9

                  1492

                  562

                  929

                  hour

                   

                  GAMES

                  AA

                  Johnson CC [41]

                  2007

                  US

                    

                  12

                  1397

                  0

                  1397

                  hour

                  day

                  SB

                  SE

                  Katzmarzyk PT [197]

                  1998

                  Canada

                   

                  9-18

                   

                  784

                  423

                  361

                  min

                  day

                  TV

                  BC, FIT

                  Katzmarzyk PT [184]

                  1998

                  Canada

                     

                  640

                  356

                  284

                  hour

                  day

                  TV

                  BC, FIT

                  Kautiainen S [135]

                  2005

                  Finland

                   

                  14-18

                   

                  6515

                  2916

                  3599

                  hour

                  day

                  SCREEN

                  BC

                  Keith TZ [256]

                  1986

                  US

                  high school seniors

                   

                  28051

                    

                  hour

                  day

                  TV

                  AA

                  Klein-Platat C [165]

                  2005

                  France

                    

                  12

                  2714

                  1357

                  1357

                  hour

                  week

                  SB

                  BC

                  Kosti RI [196]

                  2007

                  Greece

                   

                  12-17

                   

                  2008

                  1021

                  987

                  hour

                  day

                  TV

                  BC

                  Kristjansson AL [243]

                  2009

                  Iceland

                   

                  14-15

                   

                  5810

                  2807

                  3004

                  hour

                  day

                  TV

                  AA

                  Kuntsche E [230]

                  2006

                  International

                  11-15

                   

                  31177

                    

                  hour

                  day

                  TV

                  PRO

                  Kuriyan R [117]

                  2007

                  India

                   

                  6-16

                   

                  598

                  324

                  274

                  hour

                  day

                  TV

                  BC

                  Lagiou A [160]

                  2008

                  Greece

                   

                  10-12

                   

                  633

                  316

                  317

                  hour

                  day

                  TV, GAMES

                  BC

                  Lajous M [92]

                  2009

                  Mexico

                   

                  11-18

                  13.9

                  9132

                  3519

                  5613

                  hour

                  day

                  TV

                  BC

                  Lajunen HR [128]

                  2007

                  Finland

                    

                  17.6

                  4098

                  1981

                  2117

                  hour

                  week

                  COMP

                  BC

                  Lasserre AM [116]

                  2007

                  Switzerland

                  10.1-14.9

                  12.3

                  5207

                  2621

                  2586

                  hour

                  day

                  TV

                  BC

                  Laurson KR [107]

                  2008

                  US

                   

                  7-12

                   

                  709

                  318

                  391

                  hour

                  week

                  SCREEN

                  BC

                  Lazarou C [217]

                  2009

                  Cyprus

                    

                  11.7

                  622

                  306

                  316

                  hour

                  day

                  TV

                  MS

                  Leatherdale ST [11]

                  2008

                  Canada

                   

                  14-19

                   

                  25416

                  12806

                  12610

                  hour

                  day

                  TV

                  BC, PRO

                  Lioret S [127]

                  2007

                  France

                   

                  3-14

                   

                  1016

                  528

                  488

                  hour

                  day

                  SB, TV, COMP

                  BC

                  Lobelo F [208]

                  2009

                  US

                   

                  14-18

                   

                  5210

                  0

                  5210

                  hour

                  day

                  SCREEN

                  FIT

                  Lowry R [173]

                  2002

                  US

                     

                  15349

                  7445

                  7828

                  hour

                  day

                  TV

                  BC

                  Lutfiyya MN [118]

                  2007

                  US

                   

                  5-17

                   

                  7972

                    

                  hour

                  day

                  TV

                  BC

                  Maffeis C [114]

                  2008

                  Italy

                   

                  8-10

                  9.3

                  1837

                  924

                  913

                  hour

                  day

                  TV

                  BC

                  Mark AE [220]

                  2008

                  US

                   

                  12-19

                  15.9

                  1803

                  1005

                  798

                  hour

                  day

                  TV

                  BC, MS

                  McMurray RG [187]

                  2000

                  US

                   

                  10-16

                  12.7

                  2389

                  1149

                  1240

                  hour

                  day

                  TV

                  BC

                  Mihas C [193]

                  2009

                  Greece

                   

                  12-17

                  14.4

                  2008

                  1021

                  987

                  hour

                  day

                  SCREEN

                  BC

                  Mikolajczyk RT [194]

                  2008

                  Germany

                   

                  11-17

                  13.5

                  4878

                  2433

                  2445

                  hour

                  low/high

                  SB

                  BC

                  Moraes SA [135]

                  2006

                  Mexico

                   

                  6-14

                  8.0/11.3

                  662

                  343

                  339

                  hour

                  week

                    

                  Morgenstern M [94]

                  2009

                  Germany/US

                  10-17

                  12.8

                  4810

                  2294

                  2516

                  hour

                  day

                  SCREEN

                  BC

                  Morgenstern M [94]

                  2009

                  Germany/US

                  12-16

                  14

                  4473

                  2239

                  2234

                  hour

                  day

                  SCREEN

                  BC

                  Mota J [199]

                  2006

                  Portugal

                   

                  14.6

                  450

                  220

                  230

                  hour

                  day

                  TV, COMP

                  BC

                  Muller MJ [179]

                  1999

                  Germany

                   

                  5-7

                   

                  1468

                  739

                  729

                  hour

                  day

                  TV

                  BC

                  Nagel G [193]

                  2009

                  Germany

                   

                  6-9

                  7.57

                  1079

                   

                  498

                  hour

                  day

                  TV, GAMES

                  BC

                  nastassea-Vlachou K [240]

                  1996

                  Greece

                   

                  6-13

                   

                  4690

                  2279

                  2411

                  hour

                  day

                  TV

                  AA

                  Nawal LM [148]

                  1998

                  US

                   

                  5-18

                   

                  62976

                    

                  hour

                  day

                  TV, COMP

                  BC

                  Nelson MC [233]

                  2006

                  US

                   

                  7-12

                   

                  11957

                  5979

                  5978

                  hour

                  day

                  SCREEN

                  PRO

                  Neumark-Sztainer D [224]

                  2004

                  US

                   

                  11-18

                  14.9

                  4746

                  2382

                  2364

                  hour

                  week

                  TV

                  SE, PRO

                  Nogueira JA [45]

                  2009

                  Brazil

                   

                  8.3-16.8

                  13

                  326

                  204

                  122

                  hour

                  day

                  SB

                  BC

                  Obarzanek E [180]

                  1994

                  US

                   

                  9-10

                  10.1

                  2379

                  0

                  2379

                  hour

                  week

                  TV

                  BC

                  Ohannessian CM [226]

                  2009

                  US

                   

                  14-16

                  14.99

                  328

                  138

                  190

                  hour

                  day

                  SCREEN

                  SE, PRO, AA

                  Ortega FB [122]

                  2007

                  Spain

                   

                  13-18.5

                  15.4

                  2859

                  1357

                  1502

                  hour

                  day

                  SB

                  BC

                  Overby NC [219]

                  2009

                  Norway

                   

                  6-19

                   

                  723

                  375

                  348

                  min

                  day

                  TV

                   

                  Ozmert E [42]

                  2002

                  Turkey

                     

                  689

                  343

                  346

                  hour

                  day

                  TV

                  PRO, AA

                  Padez C [99]

                  2009

                  Portugal

                   

                  7-9

                   

                  3390

                  1696

                  1694

                  hour

                  day

                  TV

                  BC

                  Page RM [234]

                  2001

                  Philippine

                   

                  15.1

                  3307

                  1267

                  1819

                  hour

                  week

                  TV

                  PRO

                  Pate RR [210]

                  2006

                  US

                   

                  12-19

                  15.4

                  3287

                  1686

                  1601

                  hour

                  day

                  TV

                  FIT

                  Patrick K [169]

                  2004

                  US

                   

                  11-15

                  12.7

                  878

                  407

                  471

                  min

                  day

                  TV

                  BC

                  Pratt C [101]

                  2008

                  US

                    

                  12

                  1458

                  223

                  1235

                  hour

                  day

                  SB

                  BC

                  Purath J [185]

                  1995

                  US

                  3-5

                    

                  365

                  189

                  176

                  hour

                  day

                  TV

                  BC, MS

                  Ramos E [126]

                  2007

                  Portugal

                  13

                   

                  2161

                  1045

                  1116

                  min

                  week

                  SB, TV, COMP

                  BC

                  Rapp K [138]

                  2005

                  Germany

                    

                  6.2

                  2140

                  1015

                  1125

                  hour

                  day

                  TV

                  BC

                  Ridley-Johnson R [252]

                  1983

                  US

                  5-8

                    

                  290

                    

                  hour

                  day

                  TV

                  AA

                  Roberts DF [250]

                  1984

                  US

                     

                  539

                    

                  hour

                  week

                  TV

                  AA

                  Robinson TN [58]

                  1999

                  US

                    

                  12.4

                  971

                  0

                  971

                  hour

                  day

                  TV

                  BC

                  Ruangdaraganon N [141]

                  2002

                  Thailand

                   

                  6-12

                  9.4

                  4197

                  2126

                  2035

                  hour

                  day

                  TV

                  BC

                  Russ SA [147]

                  2009

                  US

                   

                  6-17

                   

                  54863

                  28153

                  26710

                  hour

                  day

                  SCREEN

                  BC, SE

                  Sakamoto A [236]

                  1994

                  Japan

                  4-6

                    

                  307

                  165

                  142

                  times

                  week

                  GAMES

                  PRO

                  Sakamoto A [236]

                  1994

                  Japan

                  4-6

                    

                  537

                  287

                  250

                  hour

                  week

                  COMP, GAMES

                  PRO

                  Sakamoto A [236]

                  1994

                  Japan

                  4-5

                    

                  118

                  118

                  0

                  hour

                  week

                  COMP, GAMES

                  PRO

                  Salmon J [136]

                  2006

                  Australia

                   

                  5-12

                   

                  1560

                  743

                  817

                  hour

                  day

                  TV

                  BC

                  Sardinha LB [48]

                  2008

                  Portugal

                  9-10

                  9.8

                  308

                  161

                  147

                  hour

                  day

                  SB

                  MS

                  Scott LF [254]

                  1958

                  US

                  6-7

                    

                  407

                    

                  hour

                   

                  TV

                  AA

                  Sharif I [244]

                  2006

                  US

                   

                  10-14

                   

                  6522

                  3169

                  3353

                  hour

                  day

                  TV, GAMES

                  PRO, AA

                  Sharif I [260]

                  2010

                  US

                   

                  9-15

                  12

                  4508

                  2209

                  2299

                  hour

                  day

                  TV, GAMES

                  AA

                  Shejwal B [246]

                  2006

                  India

                    

                  16.05

                  654

                  368

                  286

                  hour

                  day

                  TV

                  AA

                  Shields M [162]

                  2006

                  US/Can

                   

                  2-17

                   

                  8661

                    

                  hour

                  day

                  SB, TV

                  BC

                  Shin N [239]

                  2004

                  US

                   

                  6-13

                  9

                  1203

                  605

                  598

                  min

                  day

                  TV

                  AA

                  Singh GK [106]

                  2003

                  US

                   

                  10-17

                   

                  46707

                  24072

                  22635

                  hour

                  day

                  TV

                  BC

                  Singh GK [106]

                  2003

                  US

                   

                  10-17

                   

                  46707

                  24072

                  22635

                  hour

                  day

                  TV

                  BC

                  Skoric MM [258]

                  2009

                  Singapore

                  8-12

                  10

                  333

                  180

                  153

                  hour

                   

                  TV, GAMES

                  AA

                  Smith BJ [161]

                  2007

                  Fiji

                   

                  11-16

                   

                  443

                  200

                  245

                  hour

                  day

                  TV

                  BC

                  Spinks AB [124]

                  2007

                  Australia

                   

                  5-12

                   

                  518

                  282

                  236

                  min

                  week

                  SB, SCREEN

                  BC

                  Steffen LM [98]

                  2009

                  US

                   

                  8-11

                   

                  526

                  256

                  270

                  hour

                  day

                  TV

                  BC

                  Stettler N [168]

                  2004

                  Switzerland

                   

                  8

                  872

                  410

                  462

                  hour

                  day

                  TV, GAMES

                  BC

                  Sugiyama T [47]

                  2007

                  US

                   

                  12-19

                  15.9

                  4508

                  2295

                  2213

                  hour

                  day

                  SB

                  MS

                  Sun Y [91]

                  2009

                  Japan

                   

                  12-13

                  .

                  5753

                  2842

                  2911

                  hour

                  day

                  TV

                  BC

                  Taylor WC [158]

                  2002

                  US

                   

                  6-15

                  11.1

                  509

                  231

                  278

                  kcal

                  day

                  SB

                  BC

                  te Velde SJ [129]

                  2007

                  International

                  9-14

                  11.4

                  12538

                  6256

                  6282

                  hour

                  day

                  TV, COMP

                  BC

                  Thompson AM [189]

                  2009

                  Canada

                  3, 7, 11

                    

                  1777

                  795

                  982

                  min

                  day

                  TV

                  BC

                  Toschke AM [112]

                  2008

                  Germany

                   

                  5-6

                   

                  4884

                    

                  hour

                  day

                  TV

                  BC

                  Toschke AM [121]

                  2007

                  Germany

                   

                  5-6

                   

                  5472

                    

                  hour

                  day

                  TV

                  BC

                  Trang NHHD [146]

                  2009

                  Australia

                   

                  11-16

                   

                  2660

                  1332

                  1328

                  hour

                  day

                  SCREEN

                  BC

                  Tremblay MS [172]

                  2003

                  Canada

                   

                  7-11

                   

                  7261

                    

                  hour

                  day

                  TV

                  BC

                  Treuth MS [27]

                  2009

                  US

                   

                  11-12

                  11.9

                  1579

                  0

                  1579

                  hour

                  day

                  SB

                  BC

                  Tsai H [153]

                  2007

                  Taiwan

                   

                  11-12

                   

                  2218

                  1146

                  1072

                  hour

                  day

                  TV

                  BC

                  Tsai H [145]

                  2009

                  Taiwan

                   

                  11-12

                   

                  1329

                  615

                  672

                  hour

                  day

                  SB, TV

                  BC

                  Tucker LA [212]

                  1987

                  US

                    

                  15.7

                  406

                  406

                  0

                  hour

                  day

                  TV

                  FIT, SE, PRO

                  Tucker LA [206]

                  1986

                  US

                    

                  15.7

                  379

                  379

                  0

                  hour

                  day

                  TV

                  FIT

                  Tucker LA [214]

                  1996

                  US

                   

                  9-10

                  9.8

                  262

                  162

                  100

                  hour

                  day

                  TV

                  FIT

                  Ussher MH [231]

                  1007

                  England

                   

                  13-16

                   

                  2623

                    

                  hour

                  day

                  TV

                  PRO, AA

                  Utter J [171]

                  2003

                  US

                    

                  14.9

                  4480

                  2240

                  2240

                  hour

                  day

                  SCREEN

                  BC

                  Utter J [152]

                  2007

                  New Zealand

                  5-14

                   

                  1743

                  959

                  784

                  hour

                  day

                  TV, COMP

                  BC

                  Vader AM [97]

                  2009

                  US

                    

                  11, 7

                  11594

                  6162

                  5432

                  hour

                  day

                  TV

                  BC

                  van Schie EG [261]

                  1997

                  Netherlands

                  10-14

                  11.5

                  346

                  171

                  175

                  hour

                  day

                  SCREEN

                  PRO, AA

                  van Zutphen M [159]

                  2007

                  Australia

                   

                  4-12

                  8

                  1926

                  939

                  987

                  min

                  day

                  TV

                  BC

                  Vandewater EA [170]

                  2004

                  US

                   

                  1-12

                  6

                  2831

                  1444

                  1387

                  hour

                  day

                  SB, SCREEN

                  BC

                  Vaughan C [198]

                  2007

                  Australia

                   

                  11-18

                  14

                  443

                  189

                  254

                  hour

                  day

                  SCREEN

                  BC

                  Vicente-Rodriguez G [110]

                  2008

                  Spain

                   

                  13-18.5

                   

                  1960

                  1012

                  948

                  hour

                  day

                  TV, GAMES

                  BC

                  Violante R [137]

                  2005

                  Mexico

                   

                  6-14

                   

                  8624

                  258

                  4366

                  hour

                  day

                  TV

                  BC

                  Wake M [186]

                  2003

                  Australia

                   

                  5-13

                  9.1

                  2862

                  1445

                  1417

                  hour

                  week

                  SCREEN

                  BC

                  Walberg HJ [251]

                  1984

                  US

                  2-6

                   

                  13

                  2890

                  1445

                  1445

                  hour

                  day

                  TV

                  AA

                  Walberg HJ [253]

                  1982

                  US

                    

                  17

                  2001

                  1031

                  970

                  hour

                  day

                  TV

                  AA

                  Waller CE [202]

                  2003

                  China

                   

                  6-11

                  9

                  880

                    

                  hour

                  week

                  TV

                  BC

                  Wang Y [120]

                  2007

                  US

                    

                  11.9

                  498

                  218

                  280

                  hour

                  day

                  SCREEN

                  BC

                  Welch WW [248]

                  1986

                  Australia

                  3-4

                  9

                  9

                  1960

                      

                  TV

                  AA

                  Wells JC [108]

                  2008

                  Brazil

                   

                  10-12

                   

                  4452

                  2193

                  2258

                  hour

                  day

                  TV

                  BC, MS

                  Whitt-Glover MC [24]

                  2009

                  US

                   

                  6-19

                   

                  749

                  351

                  398

                  min

                  day

                  SB

                  BC

                  Wiggins J [227]

                  1987

                  US

                  4-12

                    

                  483

                  252

                  231

                  min

                  day

                  TV

                  SE, AA

                  Wolf AM [203]

                  1998

                  US

                   

                  11-14

                   

                  552

                  0

                  552

                  hour

                  day

                  TV

                  BC

                  Wong SL [100]

                  2009

                  Canada

                    

                  15.5

                  25060

                  12806

                  12254

                  hour

                  day

                  SB, SCREEN

                  BC

                  Zabinski MF [132]

                  2007

                  US

                   

                  11-15

                   

                  878

                  425

                  453

                  hour

                  day

                  SB

                  BC

                  SB, sedentary behaviour; TV, television viewing; COMP, computer time; GAME, video game playing; SCREEN, composite measure of 2 or more screen activities (i.e. television viewing, computer time, or video game playing); BC, body composition; MS, measures of metabolic syndrome and/or cardiovascular disease (e.g. insulin resistance, blood pressure); SE, self-esteem; PRO, pro-social behaviour; AA, academic achievement.

                  Table 2 provides a summary of all studies included in the review. The majority of the studies included in this systematic review were cross sectional (n = 177). In total, data from 983,840 participants were included in this review. Studies ranged from 30 participants in intervention studies and RCTs, to 62,876 participants in cross sectional observational investigations. Articles were published over a 51 year period from 1958 to 2009, and included participants ranging from 2-19 years of age. Although the scope of the review focused on those 5-17 years of age, studies that had a range below 5 years or over 17 years were not excluded as long as the mean age was between 5-17 years. Included studies involved participants from 39 countries; there were a greater number of articles reporting on female-only data than those reporting on male-only data. Translators were contracted to read non-English articles and complete any necessary data extraction for studies that met inclusion criteria (n = 8).

                  Of the 232 studies, 170 studies reported data on body composition, 15 on fitness, 11 on MS and CVD, 14 on self-esteem, 18 on pro-social behaviour, and 35 on academic achievement. The majority of studies (n = 223) used indirect measures to assess sedentary behaviour (i.e. parent-, teacher-, or self-report questionnaires). There were 14 studies [24, 27, 28, 3949] that directly measured sedentary behaviour with accelerometers and one that directly measured television viewing through a monitoring device [50]. The direction of the association between increased sedentary behaviour and health outcomes were similar between direct and indirect measures. Meta-analysis was conducted for RCTs examining change in body mass index.

                  Risk of bias assessment

                  Risk of bias assessment was completed for all included studies (Additional file 2). The mean Downs and Black score was 20.7 (range = 16-26). The studies were then split into groups and labeled as 'high quality' (score 23-26, n = 36), 'moderate quality' (score 19-22, n = 169), and 'lower quality' (score 16-18, n = 27). Quality of study did not affect the outcome of the study; in other words, both lower quality and high quality studies showed a positive relationship between increased time spent sedentary and health risk. Inter-reviewer assessment using the Downs and Black tool was very high (kappa = 0.98).

                  Data Synthesis

                  Body composition

                  Of the 232 studies included in this review, 170 examined body composition, with the majority of these focusing on the relationship between overweight and obesity and time spent watching TV (Table 3). Body composition was measured in a variety of ways including body mass index (BMI), sum of skin folds, percent body fat and various composite measures (e.g. BMI + sum of skin folds). Of the 8 RCTs, 7 showed that decreases in sedentary time lead to reductions in body weight (see meta-analysis below for details). Intervention studies reported desirable changes in body weight, BMI, and weight status among children and youth who successfully decreased their sedentary time [5160]. Three intervention studies [6163] reported that although sedentary behaviour decreased, there was no change in weight status (measured through BMI and skinfold thickness); however, these studies had relatively short follow-up periods (~1 year) and no control group leading the authors hypothesized that a longer follow up period was needed to detect a significant change in body composition. While nine-teen longitudinal studies reported that children who watched greater amounts of TV at baseline saw steeper increases in BMI, body weight and fat mass over time [6482], nine longitudinal studies reported no significant relationship between time spent sedentary and weight status or fat mass [6163, 8389]. Of the 119 cross sectional studies, 94 reported that increased sedentary time was associated with one or more of increased fat mass, increased BMI, increased weight status and increased risk for being overweight [28, 90182]. Risk for obesity increased in a dose response manner with increased time spent engaging in sedentary behaviours [92, 106, 110, 128, 156, 178]. Twenty-five cross sectional studies reported no significant relationship between sedentary time and weight status [24, 85, 137, 183204]. One study [131] reported an effect in boys but not girls and one showed an effect in girls but not boys [139]. One study showed that among boys, being underweight was associated with more screen time [111]. The level of evidence reporting on the relationship between sedentary behaviour and body composition was of moderate quality and was classified as Level 2 with a mean Downs and Black score of 20.6 (standard deviation: ± 1.9).
                  Table 3

                  Summary table of results showing relation between sedentary behaviour and measures of body composition

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  8

                  1886

                  Reductions in sedentary behaviour are directly related to improved body composition.

                  Intervention

                  10

                  3547

                  TV watching and overweight/obesity were related in a dose-response manner (i.e. those who watched more TV were more likely to be overweight/obese).

                  Longitudinal

                  33

                  85753

                  TV watching and overweight/obesity were related in a dose-response manner (i.e. those who watched more TV were more likely to be overweight/obese).

                  Cross sectional

                  119

                  691759

                  > 2 hrs of sedentary behaviour related to increased risk of being overweight or obese.

                  Total of all studies

                  170

                  782884

                  Meta-analysis was performed on randomized controlled studies that looked at change in BMI. They found an effect of -0.89 kg/m2 (95% CI of -1.67 to -0.11, p = 0.03) decrease in mean BMI in the intervention group.

                  > 2 hrs of sedentary behaviour per day is associated with an increased risk for overweight/obesity. This risk increases in a dose-response manner.

                  Each additional hour of TV viewing increased risk for obesity. > 2 hrs/day significantly increased risk for overweight/obesity.

                  Mean Downs and Black score = 20.9 (± 1.9), Level 2 evidence.

                  Fitness

                  Fifteen studies assessed the relationship between time spent engaging in sedentary behaviour and fitness (Table 4). Increased time spent being sedentary was associated with decreased scores for overall physical fitness, VO2 max, cardiorespiratory fitness, and musculoskeletal fitness. An intervention reported that targeting decreased sedentary behaviour lead to increases in aerobic fitness [56]. This study (n = 13 boys and 26 girls, mean age = 10.5 years) showed that an intervention to decrease targeted sedentary behaviours (watching TV, playing computer games, talking on the telephone, or playing board games) led to increases in both physical activity and non-targeted sedentary behaviours. Longitudinal evidence was conflicting. One longitudinal study showed that > 2 hours per day of TV and computer use was associated with decreased musculoskeletal fitness [205]; while the second longitudinal study found no association between increased screen time and decreased fitness. Eight of 12 cross sectional studies showed that greater than 2 hours of screen time per day was associated with decreased VO2max, lower cardiorespiratory fitness, and lower aerobic fitness [95, 206212]. Two studies showed weak relationships between television watching and fitness [197, 213]. Two studies showed no consistent association between television viewing and aerobic and musculoskeletal fitness [184, 214]. The level of evidence related to fitness was classified as Level 3 with a mean Downs and Black score of 20.9 (standard deviation: ± 2.1), indicating moderate quality of reporting.
                  Table 4

                  Summary table of results showing relation between sedentary behaviour and fitness

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  0

                    

                  Intervention

                  1

                  76

                  Reductions in sedentary behaviour lead to increased fitness.

                  Longitudinal

                  2

                  561

                  One study showed no association whereas one study showed higher musculoskeletal fitness in those watching < 2 hrs of TV per day.

                  Cross sectional

                  12

                  17227

                  > 2 hrs of screen time per day is associated with better VO2max scores, better musculoskeletal and cardiorespiratory fitness scores.

                  Total of all studies

                  15

                  17864

                  Those watching less than 2 hours of TV a day showed higher results for fitness testing and more favourable bone health.

                  Mean Downs and Black score = 20.6 (± 2.1), Level 3 evidence.

                  Metabolic syndrome and risk for cardiovascular disease

                  Eleven studies assessed the relationship between time spent engaging in sedentary behaviour and risk factors for MS and CVD (Table 5). All of the studies reported that increased sedentary time was associated with increased risk for MS or CVD. However, the results of these studies should be viewed with caution as the proportion of children and youth who have measurable health risk factors for MS or CVD is quite low. Longitudinal studies found that those watching more than 2 hours of television per day had higher serum cholesterol levels [88] and were more likely to have high blood pressure [215] than their peers who watched less TV. Cross sectional studies reported that high levels of screen time and self-reported sedentary behaviour were associated with increased risk for high systolic and diastolic blood pressure [47, 108, 216, 217], higher HbA1 c [218], fasting insulin [134, 216], insulin resistance [48, 219], and MS [220]. These risk factors increase in a dose response manner with increased screen time [216, 220]. One cross sectional study reported a significant relationship between watching TV and increased cholesterol in adolescents, but not in younger children [185]. The level of evidence for MS and CVD risk factors was classified as Level 3 with a mean Downs and Black score of 21.7 (standard deviation: ± 2.1), indicating moderate quality of reporting.
                  Table 5

                  Summary table of results showing relation between sedentary behaviour and markers for metabolic syndrome and cardiovascular disease

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  0

                    

                  Longitudinal

                  2

                  1675

                  > 2 hr of TV per day is associated with higher serum cholesterol levels. > 1.2 hrs of TV per day is associated with increased systolic blood pressure.

                  Cross sectional

                  9

                  17339

                  > 2 of screen time per day is associated with higher blood pressure and increased risk for metabolic syndrome.

                  Intervention

                  0

                    

                  Total of all studies

                  11

                  19014

                  Increased screen time is associated with increased risk for markers of metabolic syndrome and cardiovascular disease. Risk increases in a dose-response manner.

                  Mean Downs and Black score = 21.7 (± 2.0), Level 3 evidence.

                  Self esteem

                  Fourteen studies assessed the relationship between time spent engaging in sedentary behaviour and self-esteem (Table 6). One RCT aimed to increase physical activity and decrease TV viewing [221], leading to a trend in improvements in self-esteem (P = 0.26) and concerns with body shape (p = 0.03). Intervention studies that targeted changes in sedentary behaviour produced inverse changes in physical self-worth and self-esteem [52, 54]. Cross sectional studies showed that increased screen time was associated with higher depressive symptoms, low self-esteem, and decreased perceptions of self-worth [44, 115, 147, 212, 221223]. There was evidence for a dose-response relationship as each additional hour of screen time seemed to increase the risk for lower self-esteem [147]. Two studies [224, 225] reported that increased TV viewing was associated with decreased self-esteem in boys but not girls, and increased aggression in girls but not boys. Two studies showed no significant relationship [226, 227]. One study [228] showed a significant relationship between increased TV viewing and decreased self-esteem in adolescents but not in young children. The level of evidence for studies examining self-esteem was classified as Level 3 with a mean Downs and Black score of 21.0 (standard deviation: ± 2.4) indicating moderate quality of reporting.
                  Table 6

                  Summary table of results showing relation between sedentary behaviour and self-esteem

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  1

                  61

                  Girls who decreased sedentary behaviour had lower body dissatisfaction and showed a trend towards improved self-esteem.

                  Intervention

                  2

                  984

                  Decreases in sedentary behaviour lead to improved self worth and self-esteem.

                  Longitudinal

                  0

                    

                  Cross sectional

                  11

                  71068

                  Those with higher reported sedentary behaviour had poorer scores on self worth. This association seems to increase in a dose-response manner

                  Total of all studies

                  14

                  72113

                  Each additional hour of TV viewing was associated with decreases in self-worth and self-concept.

                  Mean Downs and Black score = 21.0 (± 2.4), Level 3 evidence.

                  Pro-social behaviour

                  Eighteen studies assessed the relationship between time spent engaging in sedentary behaviour and pro-social behaviour (Table 7). The one longitudinal study examining the relationship between sedentary behaviour and pro-social behaviour found that sustained TV exposure (i.e. ≥ 2 hours per day) was a significant risk factor for behavioural problems [229]. Cross sectional studies reported similar findings. Those who watched less TV were more emotionally stable, sensitive, imaginative, outgoing, self-controlled, intelligent, moralistic, college bound, and less likely to be aggressive or to engage in risky behaviour [42, 115, 230235]. Two studies found a significant relationship between increased computer use and behaviour problems in boys [111, 236] but not girls. One study showed that increased TV viewing was associated with aggression in girls but not boys [225]. The level of evidence for studies reporting on pro-social behaviour was classified as Level 3 with a mean Downs and Black score of 19.9 (standard deviation: ± 1.3) indicating moderate quality of reporting.
                  Table 7

                  Summary table of results showing relation between sedentary behaviour and pro-social behaviour

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  0

                    

                  Longitudinal

                  1

                  2707

                  Watching > 2 hrs of TV per day is a risk factor for social behaviour problems

                  Intervention

                  0

                    

                  Cross sectional

                  17

                  91934

                  Individuals watching > 3 hrs of TV per day are more likely to exhibit poor social behaviours and be more aggressive. Limited evidence to suggest this relationship is stronger in boys.

                  Total of all studies

                  18

                  94391

                  > 2 hrs of TV per day is associated with poor pro-social behaviour.

                  Those watching less than 3 hrs of TV per day scored more positively in aspects of pro-social behaviour

                  Mean Downs and Black score = 19.9 (± 1.34), Level 3 evidence.

                  Academic achievement

                  Thirty five studies assessed the relation between time spent engaging in sedentary behaviour and academic achievement (Table 8). Academic achievement was measured in a variety of ways but included measures of I.Q., school grades, grade point average (GPA), performance on standardized tests, and self-report questionnaires (e.g. students rated their own level of academic achievement). The longitudinal studies included in this review found that children who watched higher amounts of TV had greater difficulties with attention as teenagers [41], showed lower progression for reading level [237], and performed worse on cognitive tests [238] than those watching less than one hour of television per day. The majority of cross sectional studies (75%) reported that children and youth who watched higher levels of TV tended to spend less time doing homework, studying, and reading for leisure which may lead to a decrease in academic achievement [42, 181, 239255]. This association increased in a dose response manner [181, 244, 248]. Ten of the cross sectional studies found no significant relationship [57, 226, 227, 238, 256261]. One study [228] found that this relationship was significant in adolescents but not younger children. The evidence for academic achievement was classified as Level 3 with a mean Downs and Black score of 19.2 (standard deviation: ± 2.1) indicating moderate quality of reporting.
                  Table 8

                  Summary table of results showing relation between sedentary behaviour and academic achievement

                  Type of Study

                  Number of Studies

                  Number of participants

                  Narrative recommendation and main findings

                  RCT

                  0

                    

                  Longitudinal

                  3

                  3530

                  Watching > 1 hr of TV per day is associated with attention difficulties.

                  Intervention

                  0

                    

                  Cross sectional

                  32

                  157637

                  > 2 hrs of screen time per day resulted in lower academic achievement.

                  Intervention

                  0

                    

                  Total of all studies

                  35

                  161167

                  > 2 hrs of screen time per day is negatively associated with academic achievement.

                  Dose-response relation between time spent playing video games, watching TV and using the computer (for non-academic purposes). > 3 hrs/day associated with poor school performance and lower I.Q. scores.

                  Mean Downs and Black score = 19.1 (± 2.1), Level 3 evidence.

                  Quantitative data synthesis

                  Data for each of the outcomes were assessed to determine if they were sufficiently homogeneous to make meta-analysis appropriate. The only outcome for which data were consistently collected and reported and for which the characteristics of the studies were similar enough to undertake a meta-analysis was body composition. However, this was only for the RCTs; the longitudinal, cross sectional and intervention studies that examined body composition had too many inconsistencies to allow for a quantitative synthesis of results.

                  Change in mean BMI before and after the intervention (at the longest point of follow-up for each study) was used as the point estimate for the meta-analysis of the RCT data. Of the 8 RCTs, only 6 had data that could be used to calculate the change in BMI after the intervention [50, 58, 221, 262264] (the other two reported on prevalence of overweight and obesity) [57, 265]. Of the remaining six studies, one [50] examined standardized estimates of BMI only and one [262] presented only median change in BMI and not a mean change. Study authors were contacted for missing information, but no additional data was made available and thus these studies were excluded from the meta-analysis. Meta-analysis of the 4 RCTs that remained revealed an overall significant effect of -0.89 kg/m2 (95% CI of -1.67 to -0.11, p = 0.03) indicating an overall decrease in mean BMI associated with the interventions (Figure 2). The Chi square test for heterogeneity was not significant but the I2 was 46% indicating that there was low to moderate heterogeneity in the data. The funnel plot showed no indication of publication bias (data not shown).
                  http://static-content.springer.com/image/art%3A10.1186%2F1479-5868-8-98/MediaObjects/12966_2011_Article_492_Fig2_HTML.jpg
                  Figure 2

                  Meta-analysis of randomized controlled studies examining decreases in sedentary behaviour and effect on body mass index.

                  Meta-analyses were not undertaken for other outcomes or study designs because there was substantial heterogeneity in the units of measures and type of reporting of sedentary behaviour, as well as the specific measures of each outcome. For example, when reporting on the relation between time spent watching TV and overweight and obesity, one study may report the relation between the frequency of TV watching and skin fold thickness, whereas another may examine the relation of daily volume of TV watching and BMI. Even for studies that examined the same outcome, for instance BMI, some would report the proportion overweight or obese, while others would report mean BMI. In addition, some studies reported on data for males or females only, while others reported only overall estimates and many were missing key information about participant characteristics or study design. As a result, we were unable to determine common point estimates and associated measures of errors for many of the studies. Due to the scope of the review, it was not feasible to contact every author for individual data to re-run the analyses. Developing reporting standards for primary studies examining the relationship between sedentary behaviour and health would help to ensure that appropriate data are available for future meta-analyses.

                  Discussion

                  Based on this systematic review of 232 studies, sedentary behaviour (assessed primarily through increased TV viewing) for more than 2 hours per day was associated with unfavourable body composition, decreased fitness, lowered scores for self-esteem and pro-social behaviour and decreased academic achievement in school-aged children and youth (5-17 years). This was true for all study designs, across all countries, using both direct and indirect measurements, and regardless of participant sample size. All studies examining risk factors for MS and CVD disease reported that increased sedentary time was associated with increased health risk; however, the included studies examined a wide range of risk factors, and thus there was insufficient evidence to draw conclusions on the relationship for metabolic risk as a whole.

                  High heterogeneity of the included studies limited meta-analysis to RCTs examining the relationship between television viewing and BMI. This revealed a trend to support the hypothesis that decreased time spent sedentary is associated with decreases in BMI. This result should be interpreted cautiously, given that it is only based on a small number of RCTs and that only half of the RCTs included in the review were included in the meta-analysis. Nonetheless, this meta-analysis of RCTs, which are considered to be the highest quality of research evidence, coupled with the qualitative syntheses of data from the other study designs, provides consistent evidence of the inverse relationship between sedentary behaviour and health outcomes, and that reducing sedentary behaviour can improve body composition. Furthermore, this finding was consistent with the results of observational studies and previous reviews [1921, 23, 25].

                  Studies included in this review used primarily indirect measures (i.e. parent, teacher, and self-report questionnaires) to assess time spent engaging in sedentary behaviour. Those studies that did use direct (i.e. accelerometer) measures found that children and youth are spending a large proportion of their day (up to 9 hours) being sedentary [24, 27, 29, 3947, 49, 178]. Therefore, for some children and youth, a viable approach to improving health may be to work towards a reduction of at least some of their sedentary behaviours either through smaller, micro-interventions (e.g. interrupting prolonged sedentary time), or lager macro-interventions (e.g. population-based interventions and public health initiatives). Decreasing sedentary time is important for all children and youth, but it may be may be especially important to promote gradual decreases in the most sedentary group as a stepping stone to meeting sedentary behaviour guidelines [266].

                  Strengths and limitations

                  Strengths of this review included a comprehensive search strategy, a-priori inclusion and exclusion criteria and analyses, and inclusion of non-English language articles. We included direct and indirect measures of sedentary behaviour and focused on 6 diverse health indicators in children and youth. Although efforts were made to include grey literature (e.g. by contacting key informants and reviewing government documents), we did not include conference proceedings and other types of grey literature because it was impractical and unfeasible to sift through all unpublished work, and also because of limitations in the quality of reporting in conference abstracts [267, 268]. We do not anticipate that additional, unpublished work would change the results.

                  Our study has limitations, including the types of outcome measurements and analyses reported in the primary studies and primary study quality. The scope of this review was large and included a great deal of health indicators and measurement tools. A more detailed meta-analysis would have allowed us to estimate the overall effect sizes for each outcome. However, due to the heterogeneity of the data, it was impossible to complete such analysis. Furthermore, some studies had missing information on participant characteristics making it impossible to determine if basic demographics act as a confounder for the relationship between sedentary behaviour and health. Many studies also grouped their variables into tertiles, or groups that also took into account physical activity level. Although it was still possible to ascertain information regarding the association between level of sedentary behaviour and health indicators, it made it very difficult to compare the information across studies. Similarly, very few studies measured time spent being sedentary directly (i.e. with direct observation or accelerometry). Previous work [269, 270] has shown significant differences between direct and indirect measures of physical activity; similar work needs to be completed with respect to sedentary behaviour to gain a better understanding of possible biases in previous studies. Indirect measurements of sedentary behaviour often lead to grouping for analyses. This may lead to bias in the results of the systematic review as many studies arbitrarily grouped their participants as ''high users" if they watched more than 2 hours of television per day. This could perhaps be falsely leading us to conclude that 2 hours is the critical cut-point or threshold. Further work using direct (i.e. accelerometer) measures of sedentary behaviour and screen time as continuous variables will help to clarify if a cut-point of 2 hours is in fact biased.

                  The final important limitation of this review was the type of primary studies that were available for analysis. Studies with small sample sizes were excluded; however we do not believe that this had a significant impact upon the strength or direction of associations observed in this review. The majority of studies (78.4%) included in this review were cross sectional, observational studies, using indirect (i.e. parent-, teacher, or self-report) measurements of sedentary behaviour. Cross sectional data make it impossible to infer causation and results should therefore be interpreted with caution. However, it should be noted that due to ethical considerations, it may be impossible to conduct a RCT on the effects of long periods of sedentary behaviours in children and youth. Due to the large and diverse sample sizes available in population-based cross sectional research, and given that this information demonstrates similar trends as those seen in RCTs and intervention studies, we believe that the evidence presented in this review provides important insights into the relationship between sedentary behaviour and health outcomes in school-aged children and youth.

                  Future work

                  The purpose of this review was to provide an evidence base to inform clinical practice sedentary behaviour guidelines for children and youth [266]. Future work is needed to translate this information into clinical practice guidelines and disseminate this information to health care providers and the general public. While this review was limited to children and youth, similar work is needed to inform sedentary guidelines for young children aged 0-5 years, adults, and older adults.

                  As the accessibility and popularity of multiple forms of screen-based technology increases among the pediatric population, future work needs to continue to focus on media engagement. Specifically, with increasing popularity for hand-held, portable devices, 'sedentary multitasking' is becoming increasingly common. Children and youth are able to watch television, talk on the phone, and use the computer at the same time. This is a relatively new phenomenon and we are currently unaware what, if any, are the health effects associated with this high level of 'multi-screen' time. This is also true for the effect of advancements in technology and their associated health effects. For example, 'active video gaming' (e.g., Nintendo Wii™, Microsoft Kinect™, Sony's Playstation Move™) is advertised as an effective mode of physical activity. Although it is true that some games can require sufficient energy expenditure for health benefits [271], the socio-cognitive and physiological aspects of remaining indoors for long periods are unknown. Furthermore, children and youth can learn quite quickly how to use minimal gestures (e.g., using wrist movement only) to play the game thereby substantially reducing energy expenditure.

                  Finally, as described above, the vast majority of the current evidence has been based on self-report questionnaires focused on TV viewing and body composition. It is now clear that these two variables are related. Future work needs to move beyond this relationship and focus on other modes of sedentarism (e.g., prolonged sitting, passive transport) and other associated health indicators. To do this, objective measures of the time, type and context of sedentary pursuits will be needed in combination with robust and standardized measures of health indicators.

                  Conclusions

                  Physical inactivity and sedentary behaviour are pervasive and persistent public health challenges to overcome. This review demonstrates that there is a need to advocate for increases in physical activity AND decreases in sedentary behaviour. It is believed that a multi-level, multi-sectoral approach is required for this to be successful [11]. Ultimately, resolving the problem of inactivity requires a sustained change in individual daily activity and sedentary patterns. From a public health perspective, a reduction in sedentary behaviour may be easier than increasing physical activity per se because there are fewer restrictions (i.e. no need to change clothing or use special equipment), and can be easily attained with minimal burden to a person's time or financial resources.

                  This systematic review summarizes the current evidence examining the relationship between sedentary behaviours and a series of health indicators. It was determined that increased sedentary time was associated with negative health outcomes in both boys and girls; this was true across all study designs with the majority of studies (85.8%) reporting similar relationships. The majority of current work has focused on television viewing and body composition and suggests that children and youth should watch less than 2 hours of TV per day during their discretionary time. Furthermore, children and youth should try to minimize the time they spend engaging in other sedentary pursuits throughout the day (e.g. playing video games, using the computer for non-school work or prolonged sitting). This work can be used to inform the development of evidence-based sedentary behaviour recommendations for children and youth.

                  List of Abbreviations

                  BMI: 

                  Body Mass Index

                  CVD: 

                  Cardiovascular disease

                  DXA or DEXA: 

                  Dual-energy x-ray absorptiometry

                  MS: 

                  Metabolic syndrome

                  RCT: 

                  Randomized controlled trial

                  TV: 

                  Television.

                  Declarations

                  Acknowledgements

                  The authors are grateful to Jessie McGowan and Margaret Sampson for their contributions to this project.

                  Michelle Kho is funded by a Fellowship Award and Bisby Prize from the Canadian Institutes of Health Research. Travis Saunders is supported by a Doctoral Research Award and Richard Larouche is supported by a Banting and Best Doctoral Award from the Canadian Institutes of Health Research. Partial funding for the completion of this review came from the Public Health Agency of Canada. The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

                  Authors’ Affiliations

                  (1)
                  Healthy Active Living and Obesity Research, Children's Hospital of Eastern Ontario Research Institute
                  (2)
                  Department of Physical Medicine and Rehabilitation, Johns Hopkins University
                  (3)
                  Office of the Task Force on Preventive Health Care, Public Health Agency of Canada

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                  This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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