Journal of Physical Activity and Health, 2015, 12, 20  -29 http://dx.doi.org/10.1123/jpah.2013-0019 © 2015 Human Kinetics, Inc.

Official Journal of ISPAH www.JPAH-Journal.com ORIGINAL RESEARCH

Combined Physical Activity/Sedentary Behavior Associations With Indices of Adiposity in 8- to 10-Year-Old Children Katya M. Herman, Jean-Philippe Chaput, Catherine M. Sabiston, Marie-Eve Mathieu, Angelo Tremblay, and Gilles Paradis Objective: Individuals may achieve high physical activity (PA) yet also be highly sedentary (SED). This study assessed adiposity in children classified by PA/SED groups. Methods: Participants were 520 8- to 10-year-old children with ≥ 1 obese parent. Moderate-to-vigorous PA (MVPA) and SED were measured by accelerometer, and screen-time was measured by self-report. Height, weight, waist circumference (WC), body fat percentage (BF%), and VO2peak were objectively measured; energy intake was measured by dietary recall. Elevated adiposity was defined as BMI ≥ 85th percentile, WC ≥ 90th percentile, BF% ≥ 85th percentile, or waist-to-height ratio (WHR) ≥ 0.5. Results: Up to 27% of boys and 15% of girls were active/SED. Adiposity was lowest for active/non-SED, highest for inactive/SED, and intermediate and similar for active/SED and inactive/non-SED. Using 60 min/d MVPA and 2 h/d screen-time cut-offs, prevalence ranges for elevated adiposity in the active/non-SED, active/SED, inactive/non-SED, and inactive/SED groups were 0% to 14%, 15% to 44%, 16% to 40%, and 32% to 51%, respectively. Corresponding odds and 95% confidence intervals of being overweight/obese for the latter groups were 3.8 (95% CI, 1.7–8.4), 3.8 (1.8–8.2), and 4.9 (2.3–10.3) versus active/non-SED. PA/SED-adiposity associations were mediated by fitness but not energy intake. Conclusions: Combined PA/SED levels are strongly associated with adiposity in children, but associations are mediated by fitness. Active children who accumulate >2 h/d of screen time and inactive children are equally likely to be overweight/obese. Keywords: obesity, inactivity, exercise, screen time, pediatrics Sedentary behavior (SED) is now viewed as a construct distinct from physical activity (PA), rather than merely representing the lower end of the PA continuum or insufficient moderate-to-vigorous PA (MVPA).1 SED is defined as any waking behavior characterized by an energy expenditure ≤ 1.5 metabolic equivalents of task (METs) while in a sitting or reclining posture.2–4 This includes sitting for long periods, use of motorized transportation, television viewing, and playing passive video games or playing on the computer.5 Thus, it is possible for someone to be simultaneously classified as both active and sedentary,1,3 the so-called “active couch potato.”1 This person might meet or exceed guidelines for recommended levels of MVPA, and yet this MVPA time represents only a small fraction of the day, leaving plenty of opportunity for SED. Indeed, time spent in MVPA has often been weakly associated with SED time in youth,6,7 and research has shown negative health consequences of SED distinct from those of inactivity and independent of the benefits of PA.1,3,8,9 Both objectively measured PA and SED time are inversely associated with adiposity and cardiometabolic health in children, although MVPA may be more strongly associated than total SED time.7,10–16 Different types of SED may be differently associated with health outcomes and are important to study in addition to total SED time; specifically, TV Herman ([email protected]) was with the Dept of Kinesiology and Physical Education, McGill University, Montreal, Quebec, Canada. Chaput is with the Children’s Hospital of Eastern Ontario, Ottawa, Ontario, Canada. Sabiston is with the Dept of Kinesiology and Physical Education, University of Toronto, Toronto, Ontario, Canada. Mathieu is with the Dept of Kinesiology, University of Montreal, Montreal, Quebec, Canada. Tremblay is with the Dept of Kinesiology, Université Laval, Quebec City, Quebec, Canada. Paradis is with the Dept of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, Quebec, Canada. 20

viewing and video game playing are inversely associated with cardiometabolic health.13,17 In Canada, only 9% of boys and 4% of girls currently accumulate the recommended ≥ 60 minutes of MVPA daily,18,19 and more than 8 h/d (or 62% of a child’s waking hours) are spent sedentary.18 At least 6 h/d are spent in front of a screen (television or computer),20 despite current guidelines recommending ≤ 2 h/d of recreational screen time.5 Concurrently, more than 1 in 4 Canadian children and youth are overweight or obese.21 Thus it is important to investigate the associations of PA and SED to adiposity in children, both as independent predictors and as profiles of high and low combinations of PA and SED. Although the existence of various PA/ SED profiles has been acknowledged,1,3 the associations between these profiles and adiposity in children have not been explored. Furthermore, these associations may be mediated by fitness and/or energy intake, because objectively measured PA and fitness have each shown independent associations with adiposity,22,23and SED has been associated with increased food intake.6,24 The objectives of this study were to identify combined PA/SED groups in a cohort of 8- to 10-year-old children at elevated risk of obesity (because of parental obesity) using both accelerometermeasured PA and SED and self-reported screen-time data, and to examine adiposity according to these groups. A second objective was to assess potential mediation of associations by cardiovascular fitness and/or energy intake.

Methods Study Population Data were from the baseline visit of the QUebec Adipose and Lifestyle InvesTigation in Youth (QUALITY) study; in-depth meth-

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Physical Activity, Sedentary Time, and Adiposity   21

odology regarding this study has been published elsewhere.25 The overall goal of this study was to investigate the development of childhood obesity and its metabolic and cardiovascular consequences. The cohort included 630 white children, ages 8 to 11 years old at baseline (2005–2008), and their 2 biological parents, at least one of whom was clinically obese (BMI ≥ 30 kg/m2, or waist circumference > 102 cm for men or > 88 cm for women). Exclusion criteria included pregnant or breastfeeding mother, family plans to move out of the province, child diagnosed with type 1 or 2 diabetes, child taking antihypertensive or steroid medications, or child following a severely calorically restricted diet. Children were recruited from primary schools within 75 km of the cities of Montreal, Quebec City, and Sherbrooke in the province of Quebec, Canada. Data collection included interviewer-administered questionnaires, 24-hour dietary recall, anthropometric measures, fitness testing, and a 7-day accelerometer monitoring period. The QUALITY study was approved by the ethics review boards of the Centre Hospitalier Universitaire Sainte-Justine and Laval University, and written informed consent and assent were obtained from the parents and children, respectively. The current analysis included 520 children (279 boys, 241 girls) with complete data for all study variables. Exclusion was due mainly to missing accelerometer data (n = 93); no significant differences existed for mean age, BMI, and parent education between children included and not included.

Measures Physical Activity and Sedentary Time.  MVPA and SED were

measured with the ActiGraph 7164 accelerometer (Pensacola, FL). Children were instructed to wear the accelerometer on a belt over their right hip during waking hours for 7 days, with removal for bathing or swimming. Movement counts were recorded in 1-minute epochs. Previously published guidelines were followed for identifying and removing invalid accelerometer data. 26,27 Accelerometer files containing implausible data were removed, as were those collected on documented days of illness or other activity limitation. A valid day required ≥ 10 hours of wear-time, and participants were required to have a minimum of 4 valid days out of 7. In young children, 4 days of measurement results in a correlation of r = .80 with a full week of activity monitoring.28 Nonwear time was defined as ≥ 60 minutes of consecutive zero counts, with allowance for 1 interruption of 1 to 2 minutes of counts > 0 but ≤ 100. Approximately 63% of children had 7 valid days (no significant differences in age, sex, or weight status), 98% of valid files included at least 1 weekend day (81% included 2 weekend days), and mean and median accelerometer wear time on valid days were both 13.6 hours. Moderate and vigorous PA were defined by the Actigraph pediatric cut-offs proposed by Evenson et al29 and validated for use in children,30 at ≥ 2296 counts per minute (cpm) and ≥ 4012 cpm, respectively. MVPA was the sum of moderate and vigorous PA minutes. SED was defined as time spent at ≤ 100 cpm. The mean time spent in MVPA and SED was calculated over all valid days for each child, representing mean daily MVPA and SED. MVPA was dichotomized at a mean of 60 min/d as an approximation of the Canadian PA guidelines for children ages 5 to 11, which recommend ≥ 60 minutes of MVPA daily.19 For an alternate definition, with an eye to the dose-response relationship between PA and health31 as well as the low numbers of girls in this study achieving 60 min/d MVPA, MVPA was divided into sex-specific tertiles and dichotomized, defining the top tertile as “active” (vs. lower 2 tertiles). SED was also defined by 2 methods: because no specific time guidelines

exist for overall SED, this variable was divided into sex-specific tertiles and dichotomized, testing both the top tertile (vs. lower 2) and the top 2 tertiles (vs. the lowest) defining “sedentary.” The use of the top 2 tertiles was included with the belief that if SED levels in the population are high, the top tertile alone might not fully identify the health risk associated with sedentariness. Screen time was assessed by questionnaire, adapted from the WHO Health Behaviour in School-Aged Children study.32 Children were asked: “How many hours of television (including video movies) do you usually watch in a single day? A) On weekdays…; B) On weekends….” They were similarly asked about using the computer (including internet) for fun or playing video/computer games (including Nintendo, Gameboy, etc). Total weekly hours were estimated, and mean daily hours were calculated. Television and computer/video game time were summed to give mean daily total screen time. Screen time was dichotomized according to the Canadian Sedentary Behavior Guidelines for children ages 5 to 11, which recommend ≤ 2 h/d of recreational screen time.5 Adiposity Outcomes.  Body mass index (BMI) was calculated from measured height and weight, as weight in kilograms divided by height in meters2. Childhood overweight and obesity were defined as ≥ 85th and 95th age- and sex-specific percentiles, respectively, according to the growth charts of the US Centers for Disease Control and Prevention.33 Waist circumference (WC) was measured halfway between the last floating rib and the iliac crest, at the end of a normal expiration. An elevated WC was defined at ≥ 90th age- and sex-specific percentile according to values derived by Fernandez et al34 for European-American children. Waist-to-height ratio (WHR) was calculated, and elevated WHR was defined as ≥ 0.5 on the basis of past literature evaluating metabolic risk in children.35,36 Body fat percentage (BF%) was measured by dual-energy X-ray absorptiometry (GE Prodigy Lunar, Madison, W), and elevated BF% was defined as ≥ 85th age- and sex-specific percentile according to curves developed by the US National Center for Health Statistics.37 Covariates.  Age, sex, Tanner stage (assessed by trained nurses;

range of 1–4), parental education (no parents university degree/ either or both parents university degree), and mother’s and father’s BMI (from measured height and weight) were included as covariates in multivariate analysis.

Mediators.  Cardiovascular fitness (aerobic capacity) was measured as peak oxygen consumption (VO2peak) with an incremental cycling test on an electromagnetic bicycle (Montreal site: Oxycon Pro, Jaeger, Wurzburg, Germany; Quebec City site: Cosmed, Quark B2, Rome, Italy). The cycling test was adapted from the McMaster protocol38 and continued until volitional exhaustion, with indirect calorimetry measurements taken continuously throughout the test. VO2peak relative to body weight (mL · min–1 · kg–1) was used as a continuous variable. Analysis was repeated for both the full sample and restricted to those children (n = 396) who achieved peak heart rate > 195 bpm or peak respiratory exchange ratio > 1.0.39 However, because neither mean VO2peak nor overall results differed appreciably from those of the full sample, the full sample was retained to maintain power for the primary research question. Energy intake was assessed by a dietician with 3 nonconsecutive 24-h dietary recalls (including 2 weekdays and 1 weekend day).25,40 Foods reported on the recalls were entered into the CANDAT software (Candat, London, ON, Canada) and converted to nutrients using the Canadian Nutrient File.41 Mean total energy intake measured in kilocalories per day was used as a continuous variable.

22  Herman et al

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Statistical Analysis Sample characteristics were described (means and proportions) by sex and by weight status. PA and SED were combined to make 4 unique groups: active/SED, active/non-SED, inactive/non-SED, and inactive/SED. Four definitions of these groups, based on PA and screen-time guidelines, as well as tertiles of MVPA and SED, were explored. The percentage of children in each group was identified. Differences in means for the 4 adiposity outcomes (BMI, WC, WHR, and BF%) by PA/SED group were assessed by KruskalWallis test with Bonferroni post hoc comparisons. Differences in proportions of children with elevated adiposity by PA/SED group were assessed using c2 tests and accompanying post hoc comparisons. Logistic regression was used to predict the odds of elevated adiposity, according to PA/SED group; the active/non-SED group was the reference group. Models controlled for age, sex, Tanner stage, parents’ education, and parents’ BMIs. To test mediation by fitness or energy intake on the PA/SEDadiposity relationship, we first assessed the associations between potential mediators (fitness, energy intake) and the 4 adiposity outcomes by t test for differences in means derived from elevated adiposity. Significant results prompted the addition of a potential mediator to the above-described multivariate logistic regression models, to observe whether odds ratios (ORs) were attenuated. To complete the 3-step mediation analysis,42 we used linear regression to assess the prediction of a potential mediator by the PA/SED groups (dummy coded), controlling for age, sex, Tanner stage, parents’ education, and parents’ BMIs.

Results Slightly more than 40% of the sample was overweight/obese, 23% had a WC ≥ 90th age-sex percentile, and just under 20% had a BF% ≥ 85th age-sex percentile (Table 1). On average, children spent approximately 6 h/d sedentary, and only 46% met the screen-time guidelines of ≤ 2 h/d; 46% of boys but only 15% of girls achieved a mean of ≥ 60 min/d of MVPA. Mean time spent in low, middle, and high MVPA tertiles was approximately 32, 57, and 90 min/d, respectively, for boys, and 22, 37, and 62 min/d, respectively, for girls. Mean time spent in low, middle, and high SED tertiles was 290, 360, and 435 min/d, respectively, and was similar for boys and girls. Adiposity outcome means increased from lowest to highest SED tertile and from highest to lowest MVPA tertile. Notably, WC was 8.0 cm and 6.6 cm greater in boys and girls, respectively, in the highest versus lowest SED tertiles; BF in boys was 6.1% and 8.5% higher for those in the highest SED tertile and the lowest MVPA tertile, respectively (data not shown). Combined PA/SED groups were defined by 4 methods, leading to some variation in proportions of children in each group (Figure 1). For example, the percentage of children classified simultaneously as active/SED varied from only 1.2% of girls (when defined as achieving ≥ 60 min/d MVPA and being in the highest SED tertile) to 27.1% of boys (when defined as achieving ≥ 60 min/d MVPA and exceeding SED guidelines of ≤ 2 h/d screen time). Generally, 10% to 30% of children were classified as active/non-SED, except that nearly 40% of boys achieved ≥ 60 min/d MVPA and ≤ 2 h/d screen time. Using the top tertiles of MVPA and SED to define active and sedentary, respectively, just under 30% of children were classified as inactive/SED; if we extended the definition of sedentary to include the top 2 SED tertiles, this percentage rose to more than 50%.

Mean BMI percentile, WC, BF%, and WHR were highest in the inactive/SED group and lowest in the active/non-SED group, with intermediate and similar values for the active/SED group and inactive/non-SED group (Figure 2). In fact, the active/SED children in several cases appeared to have a higher mean BMI and WC than those in the inactive/non-SED group, but these differences were not statistically significant. Differences in percentages followed a similar pattern (Table 2). Notably, no child achieving ≥ 60 min/d MVPA and meeting screen-time guidelines (≤ 2 h/d) had an elevated WC or BF%. In multivariate analysis, compared with active/non-SED children and adjusting for age, sex, Tanner stage, parents’ education, and parents’ BMIs (Table 3, models a), inactive children had significantly higher odds of elevated adiposity outcomes, regardless of SED. ORs were higher for the WC and BF% outcomes compared with BMI and WHR, and ORs were higher when defining SED according to screen time. Using the 60 min/d MVPA and 2 h/d screen-time categorization, active/SED children also had significantly increased odds of being overweight or obese (OR = 3.76) and having a WHR ≥ 0.5 (OR = 7.14). Mean VO2peak (mL·min–1·kg–1) was significantly lower (P < .001) in children with elevated adiposity as measured by BMI percentile, WC, BF%, or WHR (data not shown). However, no association was found between energy intake and adiposity. The addition of VO2peak to the multivariate logistic regression models (Table 3, models b) eliminated the majority of significant associations between PA/SED group and adiposity outcomes, and attenuated the remainder, indicating complete or partial mediation depending on the adiposity outcome. The odds of elevated WHR were attenuated but remained strong for all 3 groups, compared with the active/ non-SED, when using the 60 min/d MVPA and 2 h/d screen-time cut-offs. In the final step of mediation analysis, linear regression revealed that all PA/SED definitions were significantly associated with VO2peak, controlling for covariates, thus confirming potential mediation in the PA/SED-adiposity associations (data not shown).

Discussion Over the course of a day, there are plenty of hours for children to achieve adequate levels of MVPA and yet still spend many hours sedentary and in front of a screen. This study explored the categorization of 8- to 10-year-old children at elevated risk of obesity (because of parental obesity) into 4 combined PA/SED behavior groups based on both objective accelerometer and self-report screentime data, and the association between membership in these groups and indicators of adiposity. Results suggest that although MVPA may be more important to adiposity in children, both MVPA and SED play a combined role, and their association to adiposity is mediated at least in part by cardiovascular fitness. The prevalence of each combined PA/SED group differs on the basis of the cut-offs used to define active and SED. When using tertiles of accelerometer-measured PA and SED, the prevalence of girls and boys in each combined group was similar. However, when using the 60 min/d MVPA and 2 h/d screen-time cut-offs, prevalences differed; few girls achieved a mean of 60 min/d MVPA but boys were more likely to exceed screen-time guidelines, regardless of MVPA level. In fact, almost 60% of the “active” boys were also “sedentary” given their ≥ 2 h/d of screen time. At least one third of children were both inactive and SED, but a large number, especially girls, were inactive but non-SED, indicating activity throughout the day at a lighter intensity than MVPA. In the only

Physical Activity, Sedentary Time, and Adiposity   23

Table 1  Descriptive Characteristics of the Sample (N = 520)

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Characteristic Age Tanner stage > 1 BMI percentilea Weight status   Normal weight  Overweight  Obese WC (cm)   ≥ 90th percentileb Body fat   ≥ 85th percentilec Waist-to-height ratio   ≥ 0.5 Mother’s BMI Father’s BMI Parent weight status   2 OW/OB   2 OB Parent eduction: 1 or both university Family income   < $50,000  $50,000–99,999   > $100,000 SED (h/d) MVPA (min/d) MVPA ≥ 60 min/d TV (h/d) TV ≤ 2 h/d Computer, video (h/d) Total screen time (h/d) Screen time ≤ 2 h/d VO2peak (mL·min–1·kg–1) Energy intake (kcal/d)

Boys (n = 279)

Girls (n = 241)

NW (n = 307)

OW/OB (n = 213)

9.6 (0.9) 10.0% 68.8 (27.8)

9.6 (1.0) 35.3%*** 68.0 (29.5)

9.5 (1.0) 17.3% 50.1 (23.5)

9.7 (0.9) 28.2%** 94.8 (3.8)***

58.4% 20.4% 21.1% 67.6 (12.6) 21.1% 23.7% (10.8) 18.6% 0.48 (0.08) 30.5% 29.5 (6.6) 30.5 (5.3)

59.8% 16.6% 23.7% 67.5 (12.1) 26.1% 29.1% (10.1) 20.3% 0.49 (0.08) 36.9% 28.9 (6.2) 31.0 (5.8)

100%

59.5 (4.5) 0% 19.2% (6.7) 0% 0.44 (0.03) 1.3% 28.2 (5.9) 29.8 (5.0)

45.5% 54.5% 79.0 (10.8)*** 57.3%*** 36.2% (7.1)*** 47.4%*** 0.56 (0.06)*** 79.8%*** 30.7 (6.8)*** 32.1 (6.0)***

59.1% 21.0% 53.8%

58.9% 15.3% 53.6%

50.5% 12.9% 58.5%

71.6%*** 26.5%*** 46.7%**

18.0% 43.2% 38.8% 6.0 (1.2) 59.7 (27.0) 46.2% 2.0 (1.4) 62.3% 1.1 (1.1) 3.0 (2.0) 38.6% 42.6 (8.3) 1762.7 (405.3)

16.8% 48.3% 34.9% 6.1 (1.1) 40.3 (18.6)*** 15.4% *** 1.8 (1.5) 70.1% 0.6 (0.8)*** 2.4 (1.9)*** 56.4%*** 37.1 (6.7)*** 1570.5 (360.2)

15.1% 43.9% 41.0% 5.9 (1.1) 54.2 (27.7) 37.8% 1.6 (1.3) 72.2% 0.8 (1.0) 2.4 (1.8) 54.1% 44.1 (6.7) 1670.1 (388.1)

20.9% 47.9% 31.3%* 6.2 (1.2)** 45.7 (20.6)*** 23.5%*** 2.1 (1.5)*** 56.9%*** 1.0 (1.1) 3.1 (2.1)*** 36.5%*** 34.3 (6.1)*** 1679.5 (409.3)

Note. Values are means (SD) or percentages. * P < .05, ** P < .01, *** P < 001 for girls vs. boys, or overweight/obese vs. normal weight. a BMI percentile derived from CDC growth charts.33 b WC 90th percentile from Fernandez et al.34 c Body fat percentage 85th percentile from Ogden et al.37 Abbreviations: NW, normal weight; OW, overweight; OB, obese; BMI, body mass index; WC, waist circumference; SED, sedentary; MVPA, moderate-to-vigorous physical activity.

other study reporting combined PA/SED groups,43 3 groups each were created for self-reported screen time and self-reported MVPA, and participants in the middle groups of either (84% of participants) were dropped from analysis: although a larger percentage of youth (25%–42%) were identified as “active couch potatoes” compared with the current study, it is difficult to compare when measures differed and any participants not at the extremes of PA or SED were omitted from analysis.

Although it is a known possibility that someone can be both active and SED, research directly addressing this concept in relation to adiposity in children is sparse. In the few studies examining the independent associations of PA and SED with adiposity in children, adjusting for the other in analysis, MVPA has appeared to have a stronger association with adiposity independent of SED, whereas the association between SED and adiposity was weaker or nonexistent.7,14,15 Only one previous study explored combined PA/SED

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24  Herman et al

Figure 1 — Combined physical activity/sedentary categorization of 8- to 10-year-old children, by 4 definitions. Abbreviations: MVPA, moderate-tovigorous physical activity; SED, sedentary.

categories, using only the extreme tertiles of self-reported measures: low-MVPA/high-screen-time boys and girls and high-MVPA/highscreen-time girls were more likely to be overweight or obese than their high-MVPA/low-screen-time counterparts.43 In general, results of our study using combined PA/SED categories based on both objective accelerometer and screen-time measures showed that adiposity in children increases from active/non-SED to inactive/SED; children in the intermediate combined groups (active/ SED or inactive/non-SED) exhibited intermediate and similar levels of adiposity. This suggests that high levels of SED in active children may counteract some of the health benefits of MVPA and also highlights the importance of light-intensity activity in children who may not be achieving adequate levels of MVPA. Indeed, past research has demonstrated an association between light-intensity PA and fat mass in children of this age, independent of MVPA.44 In our study, mean differences in WC ranged from 2.4 to 5.3 cm between the active/SED and inactive/SED groups and from 3.2 to 5.5 cm between the active/non-SED and inactive/non-SED groups. These differences are similar to those recently reported in a study describing WC differences between high and low MVPA

tertiles for high-SED vs. low-SED children.15 Even more striking in our study, when comparing the active/non-SED with inactive/SED groups, mean differences in adiposity indices ranged from 7 to 12 cm WC, 8 to 13 kg/m2 BMI units, and 6% to 10% BF. Given that obesity tracks strongly from childhood to adulthood,45 these differences are of high clinical significance. Others have also recently suggested considerable health risks would arise in adulthood should these differences persist.15 For example, in adulthood, risk of allcause mortality increases by 17% and 13% for men and women, respectively, for each additional 5 cm in WC.46 In multivariate analyses in our study, only the inactive groups had significantly higher odds of elevated adiposity, regardless of whether they were SED or non-SED; the exception was when screen-time guidelines were used to define SED, in which case active/SED children and their inactive counterparts had similarly high odds of adiposity. Notably, using this definition, no children who were both active and met screen-time guidelines had elevated WC or BF%, and only a handful were overweight or obese or had an elevated WHR. Hence, although MVPA appears more important than SED to adiposity outcomes, screen time appears more impor-

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Physical Activity, Sedentary Time, and Adiposity   25

Figure 2 — Means of 4 adiposity outcomes according to combined physical activity/sedentary group (4 definitions), for 8- to 10-year-old children at elevated risk of obesity. Kruskal-Wallis tests: P < .001 for all. For all 4 adiposity outcomes and 4 PA/SED definitions, InA/S significantly higher adiposity vs. A/NS (P < .001); no significant differences between A/S and InA/NS. SED, sedentary A/NS, active/non-SED; A/S, active/SED; InA/NS, inactive/non-SED; InA/S, inactive/non-SED; MVPA, moderate-to-vigorous physical activity; BMI, body mass index.

tant than simply overall time spent SED. This is in agreement with previous research showing stronger associations of MVPA versus SED,7,14,15 and of screen time specifically versus total SED time,13 with various cardiometabolic health indicators including adiposity. Intervention studies have also pointed to the role of reducing screen time in obesity prevention and treatment.6 Although results are difficult to compare because of differences in measures and PA/SED group definitions, the current study corroborates a previous report that PA is not enough to counter the negative health consequences of high screen time.43 Indeed the failure of some past research to show a relation between PA and weight status in children47,48 may be due to lack of control for SED, specifically screen time.

Cardiovascular fitness appeared to fully or partially mediate the relationship between PA/SED and adiposity in this study. In children, fitness is associated with both PA49 and SED,50 as well as with obesity.51 It is possible that fitness gains made through PA are attenuated or negated by excessive SED time or behaviors and that the lack of fitness associated with inactivity is compounded by high levels of SED, especially given that the PA-fitness association may be stronger in more inactive children.49 Energy intake was not a factor in the PA/SED-adiposity association in this study, and indeed it did not differ between normal-weight and overweight/ obese children. Although it is intuitive to believe that the obesity epidemic has been fueled by an excess in energy intake,52 a strong

26  Herman et al

Table 2  Children With Elevated Adiposity, According to Combined Physical Activity/Sedentary Group

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Active/SED definition

Active/Non-SED

MVPA mean ≥ 60 min/d, Screen time > 2 h/d   BMI OW/OB   WC ≥ 90th %ile   BF% ≥ 85th %ile   WHR ≥ 0.5 MVPA mean ≥ 60 min/d, SED tertiles (3 = SED)   BMI OW/OB   WC ≥ 90th %ile   BF% ≥ 85th %ile   WHR ≥ 0.5 MVPA tertiles (3 = active), SED tertiles (3 = SED)   BMI OW/OB   WC ≥ 90th %ile   BF% ≥ 85th %ile   WHR ≥ 0.5 MVPA tertiles (3 = active), SED tertiles (2 + 3 = SED)   BMI OW/OB   WC ≥ 90th %ile   BF% ≥ 85th %ile   WHR ≥ 0.5

Active/SED

Inactive/Non-SED

Inactive/SED

14.1 a 0a 0a 5.1 a

43.7 b 14.9 b 14.9 b 31.0 b

40.0 b 20.0 b 16.4 b 32.1 b

51.1b 40.4 c 32.4 c 47.3 c

28.7 a 6.3 a 6.3 a 18.2 a

39.1 a,b 17.4 a,b 17.4 a,b 21.7 a,b

41.9 a,b 26.6 b 20.2 b 37.4 b

51.7 b 36.4 b 31.1 b 44.4 b

28.9 a 12.8 a 8.7 a 21.5 a

36.4 a,b 22.7 a,b 18.2 a,b 27.3 a,b

42.1 a.b 22.3 a 18.8 a,b 35.5 b

52.0 b 35.5 b 30.9 b 43.4 b

26.7 a 10.0 a 6.7 a 17.8 a

33.3 a,b 18.5 a,b 13.6 a,b 27.2 a,b

41.2 a,b 17.5 a,b 18.8 a,b 35.0 a,b

48.0 b 31.2 b 25.7 b 40.1 b

Note. c2: P < .001 for all. Similar superscript letters denote proportions for each adiposity outcome that do not differ from each other at P < .05; adjusted for multiple comparisons. Abbreviations: SED, sedentary; MVPA moderate-to-vigorous physical activity; BMI, body mass index; OW, overweight; OB, obese; WC, waist circumference; %ile, percentile; BF%, percent body fat; WHR, waist-to-height ratio.

definitive link between energy intake and obesity in children has generally not been found.53 It is possible that currently available tools to measure food intake are not accurate enough, especially in children, to discern associations with obesity.53 Because screen time is associated with adiposity in this and other studies,50 and excess energy intake is known to be associated with screen time,6,24 the influence of energy intake on the PA/SED-adiposity relationship is a question that deserves further study.

Strengths and Limitations The strengths of this study include the use of objective PA and SED data as well as subjective screen-time reports; the direct measurement of height, weight, waist circumference, and body fat; and the ability to assess multiple methods of categorization of combined PA/SED behaviors and their associations with multiple indices of adiposity. The QUALITY cohort in general benefits from the breadth and multiplicity of data available in a relatively homogeneous community-based sample, minimizing confounding. Several limitations exist. Because of the cross-sectional nature of the data, causal associations between PA/SED category and adiposity cannot be confirmed: the association between PA, SED, and adiposity may be bidirectional, such that a child’s weight status may also predispose them to various combinations of PA and SED behaviors.54,55 The use of 1-minute epochs (vs. shorter ones) for accelerometry, necessary because of device memory limits for 7-day monitoring, may underestimate MVPA in children, who often exhibit shorter bursts of activity compared with adults, leading to underestimates

in the magnitude of associations reported. Further, it is possible that shorter periods of nonwear might have been mistaken for SED time; however, a shorter time definition for nonwear might equally have led to SED time mistaken for nonwear time. Self-reported measures (eg, screen time) are inherently problematic, especially in young children; however, information detailing the specific nature of SED and PA can help describe behavior when collected in combination with objectively quantified total time spent, facilitating the development of meaningful public health messages. Finally, the QUALITY children are a largely francophone-Canadian sample of Western European descent with a parental history of obesity and are of higher socioeconomic status, are more likely to live with both parents, are more likely to reside in urban regions, are more likely to be overweight or obese, are more likely to have a worse lipid profile, and are more likely to report less TV time compared with a representative sample of the Quebec population.25 Patterns of combined PA/SED described in this study should therefore be further assessed across various ethnic/cultural and socioeconomic groups.

Conclusions High levels of PA and high levels of SED are not mutually exclusive, and being sedentary does not simply equate to a lack of PA. Therefore, it should not be assumed that children who spend a lot of time sedentary are not already spending an adequate amount of time in MVPA, or that a child who meets PA recommendations is therefore not still spending too much time sedentary or in front of a

27

1.11 (0.32–3.87) 0.95 (0.49–1.85) 1.00 (0.49–2.04)

0.96 (0.27–3.41) 1.14 (0.61–2.11) 1.16 (0.59–2.31)

1.48 (0.61–3.61) 1.62 (0.69–3.80) 1.31 (0.64–2.68)

1.29 (0.47–3.56) 1.85 (1.19–3.14) 2.38 (1.37–4.12)

1.21 (0.43–3.44) 1.75 (1.08–2.84) 2.27 (1.34–3.85)

1.37 (0.67–2.78) 2.07 (1.03–4.15) 2.27 (1.28–4.02)

2.12 (0.82–5.47) 1.97 (0.76–5.12) 3.73 (1.70–8.20)

1.66 (0.48–5.73) 1.84 (0.99–3.41) 3.17 (1.68–5.99)

2.24 (0.53–9.48) 5.36 (2.43–11.83) 7.57 (3.38–16.95)

[n = 0 with WC ≥ 90th %ile in referent active/ non-SED group]

(a) OR (95% CI)

2.17 (0.68–6.95) 0.93 (0.28–3.08) 2.17 (0.68–6.95)

1.58 (0.32–7.66) 1.04 (0.48–2.28) 1.65 (0.73–3.70)

2.39 (0.39–14.49) 3.17 (1.20–8.39) 3.92 (1.47–10.48)

(b) OR (95% CI)

WC ≥ 90th %ile

2.04 (0.66–6.30) 3.46 (1.19–10.11) 4.09 (1.61–10.37)

1.78 (0.43–7.35) 2.27 (1.11–4.66) 3.85 (1.87–7.93)

2.13 (0.50–9.21) 3.66 (1.62–8.31) 5.75 (2.53–13.09)

[n = 0 with BF% ≥ 85th %ile in referent active/ non-SED group]

(a) OR (95% CI)

1.63 (0.38–6.98) 2.16 (0.55–8.47) 1.91 (0.63–5.84)

2.12 (0.31–14.25) 1.41 (0.55–3.62) 2.16 (0.83–5.64)

2.95 (0.44–19.86) 1.60 (0.54–4.75) 2.39 (0.81–7.05)

(b) OR (95% CI)

BF% ≥ 85th %ile

5.66 (1.53–20.87)

(b) OR (95% CI)

1.82 (0.82–4.03) 2.85 (1.32–6.16) 2.86 (1.49–5.50)

1.08 (0.34–3.41) 1.98 (1.17–3.36) 2.32 (1.32–4.09)

0.86 (0.25–2.96) 2.56 (1.44–4.54) 2.94 (1.61–5.37)

2.24 (0.83–6.05) 2.60 (1.00–6.79) 1.73 (0.78–3.84)

0.89 (0.22–3.53) 1.36 (0.70–2.67) 1.14 (0.55–2.36)

0.65 (0.15–2.90) 1.37 (0.66–2.86) 1.21 (0.56–2.60)

7.57 (2.51–22.80) 3.93 (1.14–13.54) 12.62 (4.25–37.48) 4.56 (1.33–15.65)

7.14 (2.26–22.56)

(a) OR (95% CI)

WHR ≥ 0.5

Note. Referent is active/non-SED. Models: (a) Adjusted for age, sex, Tanner stage, parent education, mother’s BMI, father’s BMI. (b) In addition, adjusted for VO2peak (mL·min–1·kg–1). Abbreviations: BMI, body mass index; OW/OB, overweight/obese; WC, waist circumference; %ile, percentile; BF%, percent body fat; WHR, waist-to-height ratio; OR, odds ratio; CI, confidence interval; MVPA, moderateto-vigorous physical activity; SED, sedentary.

2.17 (0.87–5.39) 1.73 (0.70–4.31)

3.24 (1.23–8.52)

(b) OR (95% CI)

3.81 (1.78–8.14) 4.90 (2.33–10.30)

3.76 (1.68–8.43)

MVPA mean ≥ 60 min/d, screen time > 2 h/d Active/SED

Inactive/non-SED Inactive/SED MVPA mean ≥ 60 min/d, SED tertiles (3 = SED) Active/SED Inactive/non-SED Inactive/SED MVPA tertiles (3 = active), SED tertiles (3 = SED) Active/SED Inactive/non-SED Inactive/SED MVPA tertiles (3 = active), SED tertiles (2 + 3 = SED) Active/SED Inactive/non-SED Inactive/SED

(a) OR (95% CI)

Model

BMI OW/OB

Table 3  Adjusted Odds of Elevated Adiposity by Combined Physical Activity/Sedentary Group for 8- to 10-Year-Old Children at Elevated Risk of Obesity

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28  Herman et al

screen. Children who are both active and non-SED have significantly lower adiposity than their counterparts who are both inactive and SED; however, the adiposity indices of those who are active but SED do not much differ from those who are inactive but non-SED, especially when defined by screen-time guidelines recommending ≤ 2 h/d. Although inactivity appears more important to adiposity than overall total SED time, these associations are mediated by cardiovascular fitness relative to body weight. Public health efforts aimed at fighting the obesity epidemic should continue to focus on increasing youth PA levels, as well as promoting reductions in sedentary time and screen time targeting all children.

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Acknowledgments The research team is grateful to all the children and their families who took part in this study as well as the technicians, research assistants, and coordinators involved in the QUALITY cohort project. Dr. Marie Lambert (July 1952—February 2012), pediatric geneticist and researcher, initiated the QUALITY cohort. Her leadership and devotion to QUALITY will always be remembered and appreciated. We thank Dr. Katherine Gray-Donald for providing the dietary data. The QUALITY study is funded by the Canadian Institutes of Health Research, the Heart and Stroke Foundation of Canada, and the Fonds de la recherche en santé du Québec. Herman was funded by FRSQ and HSFC postdoctoral fellowships. Chaput holds a Junior Research Chair in Healthy Active Living and Obesity Research. Tremblay holds a Canada Research Chair in Environment and Energy Balance. Paradis holds a CIHR Chair in Applied Public Health Research.

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