Original Article

International Journal of Obesity (2008) 32, 1685–1693; doi:10.1038/ijo.2008.171; published online 14 October 2008

A prospective examination of children's time spent outdoors, objectively measured physical activity and overweight

V Cleland1, D Crawford1, L A Baur2, C Hume1, A Timperio1 and J Salmon1

  1. 1Centre for Physical Activity and Nutrition Research, Deakin University, 221 Burwood Hwy, Burwood, Victoria, Australia
  2. 2Discipline of Paediatrics and Child Health, University of Sydney, c/- Clinical School, The Children's Hospital at Westmead, Westmead, New South Wales, Australia

Correspondence: Dr V Cleland, Centre for Physical Activity and Nutrition Research, Deakin University, 221 Burwood Hwy, Burwood, Victoria 3125, Australia. E-mail: verity.cleland@deakin.edu.au

Received 5 May 2008; Revised 5 August 2008; Accepted 1 September 2008; Published online 14 October 2008.

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Abstract

Objective: This study aimed to determine whether time spent outdoors was associated with objectively measured physical activity, body mass index (BMI) z-score and overweight in elementary-school aged children, cross-sectionally and prospectively over 3 years.

Methods: Three-year cohort study with data collected during 2001 and 2004. Nineteen randomly selected state elementary schools across Melbourne, Australia. One hundred and eighty eight 5–6-year-old and 360 10–12-year-old children. Baseline parent reports of children's time spent outdoors during warmer and cooler months, on weekdays and weekends. At baseline and follow-up, children's moderate and vigorous physical activity (MVPA) was objectively assessed by accelerometry, and BMI z-score and overweight was calculated from measured height and weight.

Results: Cross-sectionally, each additional hour outdoors on weekdays and weekend days during the cooler months was associated with an extra 27minweek−1 MVPA among older girls, and with an extra 20minweek−1 MVPA among older boys. Longitudinally, more time outdoors on weekends predicted higher MVPA on weekends among older girls and boys (5minweek−1). The prevalence of overweight among older children at follow-up was 27–41% lower among those spending more time outdoors at baseline.

Conclusion: Encouraging 10–12-year-old children to spend more time outdoors may be an effective strategy for increasing physical activity and preventing increases in overweight and obesity. Intervention research investigating the effect of increasing time outdoors on children's physical activity and overweight is warranted.

Keywords:

youth, physical activity, overweight, longitudinal, accelerometer

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Introduction

A range of effective prevention strategies is required to tackle the significant public health burden of childhood overweight and obesity.1 A focus on increasing physical activity is likely to be important given that this is a potentially modifiable behavior. Ecological models have been proposed to understand the influences on behaviors such as physical activity. These models suggest that there are multiple levels of influence on behaviors, including individual, social and environmental level factors.2 Although individual and social factors have been investigated earlier,3 relatively little is known about environmental influences on children's physical activity, and even less is known about what influences change in physical activity over time. One potential environmental influence on children's physical activity is the amount of time spent outdoors;4 however, this relationship is not well understood. There is evidence among preschoolers that observed physical activity is, on average, higher during outdoor play than during indoor play among both boys and girls.5, 6, 7, 8 There is no evidence of a relationship between time spent outdoors and weight status, but this has only been examined in one study, again among preschool-aged children.9

In addition to the lack of research undertaken in elementary school-aged children, a number of important limitations evident in previous studies highlight the need for further investigation of the relationship between children's time spent outdoors, physical activity and overweight. First, studies that use observational techniques to measure physical activity may overestimate associations between time spent outdoors and physical activity, as measures of both behaviors are taken concurrently, and the same behavior is, therefore, being ‘counted twice’.7 The use of a combination of a reported measure of time spent outdoors and an objective measure of physical activity such as accelerometry has the potential to overcome this issue. Second, to date no studies have investigated the prospective relationship between time spent outdoors, physical activity and weight status among elementary school-aged children, despite this age group being a prime target for obesity prevention.

The aim of this study was to examine the cross-sectional and prospective associations between time spent outdoors, objectively measured physical activity and overweight among elementary school-aged children.

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Methods

Participants were 548 children involved in the Children's Living in Active Neighbourhoods (CLAN) study. The authors certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during this research. This study was approved by the Deakin University Human Research Ethics Committee, Department of Education and Training (Victoria) and the Victorian Catholic Education Office. Informed written consent was obtained from all parents and children participating in the study. The CLAN study design and methods are described in detail elsewhere.10, 11, 12

Sample

Data were drawn from the baseline and first follow-up phases of the CLAN study, conducted in 2001 (T1) and 2004 (T2), respectively. At T1, 19 state elementary schools across high and low socioeconomic areas of metropolitan Melbourne, Australia, were selected using stratified random sampling; five schools that refused to participate were replaced with randomly selected schools. Although all students within the elementary school entry year (5–6 years of age) and final years (grades five and six; 10–12 years of age) were invited to participate, only those that provided active consent by returning a signed consent form were eligible (n=1220; 44% response rate). Those families who agreed to be recontacted for future research (n=698) were subsequently invited to participate in a 3-year follow-up study in 2004 (57% of the baseline sample). The present analyses are based on the 548 children (188 5–6 years old, 360 10–12 years old; 47% boys) for whom baseline reports of time spent outdoors were available, and measures of physical activity and/or body mass index (BMI) were available at both baseline and follow-up. There were no significant differences between the baseline values of key dependent (MVPA, BMI, overweight/obesity) and independent (time spent outdoors) variables of those participants who did and did not partake in follow-up (data not shown). As ethically required in Australia, only those children whose parents provided active consent at both time points were included in the study; existing privacy legislation does not permit access to any further data on non-respondents.

Procedure

During school hours between July and December, children were given a package to take home to their parents inviting their family's participation in the study. During these months (spring–summer in the southern hemisphere), Melbourne average maximum temperatures range from 13 to 24°C. Average maximum temperatures in the cooler months (that is, school terms 2 and 3) range from 13.4 to 20.2°C (average 16.1°C), whereas average maximum temperature in warmer months (that is, school terms 1 and 4) range from 19.6 to 25.8°C (average of 23.5°C). Parents were invited to complete the survey and consent form and return them to the school in a provided envelope. A data collection team then visited the school where participating children were fitted with and instructed on the wear of an accelerometer, and had height and weight measures taken. Accelerometers were collected from children approximately 8 days after they were issued.

Measures

One-week test-retest reliability was examined in a separate sample of parents (n=119) by administering the same survey to parents 1 week apart. The intraclass correlations (ICC) between continuous responses to the first and second administrations were calculated.

Demographic characteristics

Parents reported highest level of maternal education, which was categorized as low (some high school or less), medium (high school or technical college) or high (university or higher), as a proxy for socioeconomic position, consistent with previous studies.13, 14, 15 Parental marital status was reported by parents and categorized as married/living as married and not married (separated/divorced/widowed/never married), and self-reported country of birth was categorized as ‘Australia’ or ‘outside of Australia’.

Time spent outdoors

At T1 and T2, parents were asked ‘In total, how many hours/minutes does your child usually spend outside during a typical week after school’, separately for warmer (‘During the warmer months’) and cooler (‘During the cooler months’) months. The same question was asked about time spent outdoors on a typical weekend. Responses were summed to create six ‘time spent outdoors’ variables for each time point: total hours per week spent outdoors in the warmer months; total hours per week spent outdoors in the cooler months; hours per week spent outdoors on weekdays in the warmer months; hours per week spent outdoors on weekdays in the cooler months; hours per week spent outdoors on weekends in the warmer months; and hours per week spent outdoors on weekends in the cooler months.

One-week test-retest reliability of these items was as follows: time spent outdoors in the warmer months (ICC=0.21), time spent outdoors in the cooler months (ICC=0.46) and overall time spent outdoors (ICC=0.41).

Physical activity

Children's moderate and vigorous physical activity (MVPA) was objectively assessed at baseline and follow-up using uniaxial accelerometers (Manufacturing Technology Inc. (MTI, Fort Walton Beach, FL, USA), Actigraph Model, AM7164-2.2C USA). The MTI accelerometer measures movement in the vertical plane and has been validated as an objective measure for assessing children's physical activity.16, 17 Children were instructed in the use of the accelerometer at school by trained data collectors. They were asked to wear the accelerometer for an 8-day period during waking hours, except during bathing and aquatic activities. Data recorded on the first and last days were discarded for each child due to incompleteness on these days and possible reactivity effects on day 1 (that is, children behaving in a more active way than usual because of the novelty of wearing the measurement device). Only children with at least four complete days of accelerometer data (including one weekend day) were included in the analyses, consistent with recommendations that 4 days is the minimum acceptable amount to typify children's usual activity.18 Days in which total accelerometer counts were less than 10000 or exceeded 20000000 were also excluded from the analyses, as this indicated a possible malfunction of the accelerometer.

Movement count thresholds were applied to the data using a specially designed QBASIC data reduction program to calculate minutes spent in moderate- and vigorous-intensity physical activity. The thresholds were based on an age-specific energy expenditure prediction equation: METs=2.757+(0.0015·counts·min−1)−(0.08957·age (year))−(0.000038·counts·min−1·age(year))19 and were defined in METs (metabolic equivalents of rest) as moderate 3.0–5.9 METs; and vigorous-intensity 6.0+ METs. Average weekly minutes of MVPA on weekdays, MVPA during the ‘critical window’ (after school until 1800 hours) and on weekends was calculated by summing the total minutes on each day, dividing the total by the number of days the accelerometer met the criteria for inclusions, then multiplying by five (for weekday variables) or two (for weekend variables).

BMI and weight status

Height in meters (m) and weight in kilograms (kg) were measured without shoes in private by trained data collectors using a portable stadiometer and digital scales. BMI was calculated using the formula kg/m2, and BMI was converted to BMI z-scores based on the US Centers for Disease Control 2000 reference population (www.cdc.gov/growthcharts). Overweight and obesity were classified as a BMI greater than the internationally accepted age- and sex-specific cutpoints for overweight and obesity in children.20 Because of the small number of participants classified as obese, these were combined with the overweight category, hereafter referred to as ‘overweight’.

Data analysis

Linear regression was used to examine cross-sectional (at T1 and at T2) associations between time spent outdoors (h/week) in warmer and cooler months and total MVPA (min/week) on weekdays, MVPA during the critical window (after school until 1800 hours) on weekdays, MVPA on weekends and BMI z-score. Beta (β) coefficients and 95% confidence intervals are presented. Linear regression was used to predict T2 MVPA and T2 BMI z-score from baseline time spent outdoors, adjusting for baseline MVPA and z-BMI, respectively. Significant results are described in parentheses in the text in relative terms (that is, β-coefficient divided by corresponding mean MVPA value multiplied by 100).

The association between time spent outdoors and overweight was examined cross-sectionally using log binomial regression to calculate prevalence ratios and 95% confidence intervals. For these analyses, the time spent outdoors variables were classified into 3hday−1 categories: <1hday−1 (referent group), 1–1.9hday−1 and greater than or equal to2hday−1 to approximate tertiles of the distribution. Because children spent more time outdoors in the warmer months, the three categories created for time spent outdoors in warmer months were <2hday−1 (referent group), 2–2.9hday−1 and greater than or equal to3hday−1. Because children spent less time outdoors in the cooler months, the three categories created for time spent outdoors in cooler months were <0.5hday−1 (referent group), 0.5–0.9hday−1 and greater than or equal to1hday−1. Log binomial regression was used to predict weight status at T2 from time spent outdoors reported at T1, adjusted for baseline weight status. Relative risks and 95% confidence intervals are presented.

All analyses were conducted in Stata 10.0 (StataCorp, TX, USA), were stratified by sex and age group, adjusted for highest level of maternal education and standard errors were adjusted for clustering within schools to account for within-cluster correlations using the Taylor-series approximation.

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Results

T1 demographic characteristics

At T1, 41% of children were of high socioeconomic position, 35% were of medium socioeconomic position and 24% were of low socioeconomic position. Eighty-six percent of children came from families where parents were married or living as married, 70% of children had both parents born in Australia and 17% had at least one parent born in Australia (similar to national figures of 22% of Australians who were born overseas21).

Time spent outdoors

Children spent significantly (P<0.01) more time outdoors during warmer months compared to cooler months at T1 and T2 (Table 1). Although there was little difference in the amount of time younger boys and girls spent outdoors during the warmer months, older boys generally spent significantly more time outdoors than did older girls in both the warmer and cooler months at both T1 and T2. There was little difference in time spent outdoors between T1 and T2 among younger children. However, time outdoors during the warmer months significantly declined between T1 and T2 among older boys, time outdoors on weekends in the warmer months significantly declined among older girls and time outdoors on weekdays in the colder months significantly increased among older girls.


Physical activity

Compared to girls, boys accumulated significantly higher levels of MVPA on weekdays at T1 and T2 in both age groups (Table 1). There was no difference in MVPA during the critical window (after school until 1800 hours) on weekdays between boys and girls in both cohorts at T1 and in younger boys and girls at T2. However, at T2 older boys accumulated more MVPA than older girls during the critical window. MVPA on weekends was not significantly different between younger boys and girls at T1 or T2, but older boys accumulated significantly more MVPA on weekends than older girls at T1 and T2. In general, average MVPA was significantly lower at T2 than T1.

BMI and overweight

There was little difference in BMI z-score between T1 and T2 among boys (Table 1). However, younger girls had significantly lower BMI z-score values at T2, whereas older girls had significantly higher BMI z-score values at T2. The prevalence of overweight increased significantly between T1 and T2 in all age and sex groups.

Time spent outdoors and physical activity—weekdays

Time spent outdoors on weekdays was cross-sectionally associated with total MVPA on weekdays at T1 among older boys and girls during the cooler months (Table 2). For each additional hour of time spent outdoors, an extra 11.6minweek−1 (1.6%) of MVPA was recorded for boys and an extra 17.0minweek−1 (2.8%) recorded for girls. Cross-sectionally at T2, time spent outdoors in both the warmer and cooler months was significantly associated with an extra 12.7 (2.5%) and 13.6minweek−1 (2.6%) of MVPA on weekdays among older boys. No prospective associations were noted.


Time spent outdoors and physical activity—critical window

At T1, more time spent outdoors on weekdays was cross-sectionally associated with higher levels of MVPA during the critical window among older boys (6.3minweek−1; 4.1%) and older girls (6.1minweek−1; 4.2%) during the cooler months (Table 3). At T2, similar cross-sectional associations were noted at follow-up amongst older children, with more time spent outdoors associated with 4.5minweek−1 (3.2%) and 3.0minweek−1 (2.9%) additional MVPA during the critical window among boys and girls respectively. A prospective association was evident among older boys, with 3.3minweek−1 (2.4%) more MVPA during the critical window at T2 for each extra hour of time spent outdoors at T1.


Time spent outdoors and physical activity—weekends

Cross-sectionally at T1, time spent outdoors on weekends was associated with higher levels of MVPA on weekends among older boys and girls during the warmer (6.1 and 4.7minweek−1, respectively; 2.4 and 2.2%) and cooler (9.4–9.5minweek−1; 3.7–4.4%) months (Table 4). Cross-sectionally at T2, associations were evident only amongst older boys, with 4.5–4.8minweek−1 (3.4–3.6%) more MVPA on weekends among those reporting higher time outdoors. Prospective associations were evident among older boys during the warmer and cooler months, with an extra 4.5–4.8minweek−1 (1.7–1.9%) of MVPA on weekends at T2, respectively, for every additional hour of time spent outdoors on weekends at T1.


Time spent outdoors, BMI and overweight

There was very little evidence of an association between overall time spent outdoors at baseline (weekly, weekdays or weekends) during cooler or warmer months and BMI z-score in younger or older children, cross-sectionally at T1, cross-sectionally at T2, or longitudinally (data not shown). Given the lack of associations observed between time spent outdoors during different times of the week and BMI z-score, only total weekly time spent outdoors (not time spent outside on weekdays and weekends) was examined for associations with the prevalence of overweight.

Cross-sectionally, there were few associations between time spent outdoors at T1 or at T2 and overweight (Table 5). However, a lower prevalence of overweight was noted among older girls who spent more time outdoors in the cooler months at T1, and among younger girls who spent more time outdoors in the cooler months at T2. Older girls who spent a moderate but not the most amount of time outdoors in the warmer months at T2 had an increased prevalence of overweight. There was a prospective inverse association between time spent outdoors overall and during the warmer months at T1 and the prevalence of overweight in older boys and girls at T2. The prevalence of overweight in older boys and girls at T2 who spent more time outdoors at T1 overall was 26–41% and 31–35% lower, respectively, than among those who spent less than 1hday−1 outdoors at T1. The prevalence of overweight among older boys and girls at T2 was 30% lower in those spending 3h or more per day outdoors at T1, compared to those spending less than 2hday−1 outdoors. A significant cross-sectional linear trend for decreasing prevalence of overweight at T1 with increasing time spent outdoors at T1 in the cooler months was noted for older girls.


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Discussion

This study aimed to determine whether time spent outdoors was associated with objectively measured physical activity and overweight in elementary school-aged children, both cross-sectionally and over 3 years of follow-up. Cross-sectional findings suggested that older elementary school-aged children who spent more time outdoors, tended to be more active and had a lower prevalence of overweight than children spending less time outdoors. Associations with physical activity were stronger in the cooler months, where the effect of weather conditions and fewer hours of daylight may have a greater impact on participation in physical activity than during the more temperate months. Adding further strength to the results, time spent outdoors at baseline positively predicted older boys’ physical activity and inversely predicted the prevalence of overweight among older children 3 years later.

While the associations between time spent outdoors and physical activity was small in magnitude, they may have important public health implications. Each extra hour of time outdoors on weekdays and on weekend days during the cooler months was associated with an extra 26.5minweek−1 of MVPA in older girls, and with an extra 21.0minweek−1 of MVPA in older boys. For some children, increasing the time they spend outdoors by this amount of time each week may contribute significantly to their overall physical activity. This may help in the attainment of recommended levels of weekly physical activity. These kinds of increases may well be important at a population level, given that recent national data among adolescents show a low proportion of the population meet current physical activity recommendations.22, 23

The finding that more time spent outdoors was predictive of higher levels of MVPA 3 years later among older boys has important public health implications, particularly because of the well-documented decreases in adolescents’ physical activity levels.24 Although the magnitude of the association observed in this study was marginal, it may be underestimated because of the potential error in the measurement of time spent outdoors, which would result in a shift in estimates towards the null hypothesis. Similarly, the finding that older children who spent more time outdoors had a lower prevalence of overweight 3 years later is important. This suggests that encouraging older children to spend more time outdoors may be one useful strategy to prevent obesity. However, this strategy appears not to have been tested in intervention research and therefore warrants further investigation.

Interestingly, when compared to older children, few associations were noted between time spent outdoors and physical activity, BMI z-score or overweight among younger children. It is possible that this group is relatively homogeneous in terms of their time spent outdoors, physical activity and weight status, which would make differences difficult to detect. This may be because there was little difference between younger boys and girls for most variables, while differences were evident for most variables between older boys and girls. This lack of variability in younger children's physical activity and BMI has been observed previously. Trost et al.19 found little difference in accelerometer-measured MVPA between younger (grades 1–3) boys and girls, but did find differences between older (grades 4 and above) boys’ and girls’ MVPA. A study of Swedish, North American and Australian children observed lower levels of variability in BMI in younger children (6 years old) than in older children (10–12 years).25

Although younger boys were reported to spend approximately 1h less time outdoors than older boys, MVPA among older boys was only about half that of younger boys. It is possible that this difference is due to reporting discrepancies, but considering parents proxy-reported time outdoors for both age groups, this seems unlikely. More plausibly, differences in how older boys accumulate their physical activity compared with younger boys may explain this finding. For instance, older boys are likely to have higher levels of autonomy compared with younger boys. Playing outdoors may therefore be more common among older boys, while younger boys may be accumulating more of their physical activity at school in indoor play or in organized activities. This hypothesis is partially supported by Australian Bureau of Statistics data which suggest that frequency of participation in organized sport cross-sectionally declines with age during childhood.26 Another plausible explanation is the possibility that accelerometers better capture intermittent and incidental physical activity typically observed in younger children.

This study had some limitations. The measure of time spent outdoors was not validated, although 1-week test-retest reliability was fair and similar questions have shown significant correlations with physical activity measured by triaxial accelerometers in preschoolers.27 There is the possibility of a selection bias, as approximately half of the baseline sample did not participate in follow-up. However, as described in the Methods section, baseline values for time spent outdoors, all MVPA variables, BMI z-score and percent overweight did not differ significantly between those who did and those who did not participate at follow-up. Given the relatively temperate weather recorded in Melbourne, the findings may not be generalizable to other countries with much cooler or warmer climates, where the nature of children's outdoor activities may differ markedly.

This study had a number of important strengths. A prospective study design provides insights into the temporal nature of the association between time spent outdoors and physical activity and weight status, adding strength to the findings. While for the cross-sectional analyses it is plausible that being a more active child results in spending more time outdoors, the prospective analyses help to establish the causal direction of this relationship. In addition, accelerometers were used to objectively measure children's physical activity, which, when measured by other instruments—such as self- or parent-report—is prone to measurement error. Furthermore, we examined this relationship in two cohorts of elementary school-aged children, which has provided important information about age groups that may be more amenable to obesity prevention and physical activity promotion interventions.

In conclusion, it appears that encouraging 10–12-year-old children to spend more time outdoors may be a useful strategy for increasing physical activity levels and preventing further increases in the prevalence of overweight. On the basis of our findings, further research investigating the effectiveness of interventions promoting time spent outdoors as a means to increasing older children's physical activity and preventing childhood overweight is warranted.

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Acknowledgements

This research was supported by grants from the Financial Markets Foundation for Children and the National Health and Medical Research Council. David Crawford and Anna Timperio are supported by Public Health Research Fellowships from the Victorian Health Promotion Foundation. Jo Salmon was supported by a National Heart Foundation of Australia Career Development Award. Verity Cleland had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. We acknowledge Michelle Jackson and Sophie Thal-Janzen for coordinating follow-up data collection.

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