Excessive ST during preschool age is a risk factor for increased zBMI at 6 years, regardless of time spent PO. Reducing high levels of ST during preschool age, for e.g. at least 1h per week, could help preventing childhood obesity.

Longer daily ST was associated with a higher zBMI (P = 0.002) and WTH (P = 0.001) at 6 years of age. No significant associations were found for time spent PO. Each additional hour of average ST during the 4 year period resulted in a 66% higher risk of having a zBMI score over 1 (P < 0.001) and almost twice the risk (94% higher risk) of having an zBMI score over 2 (P < 0.001) at 6 years.

PO and ST of 526 children of the European Childhood Obesity Project (CHOP) were annually assessed by questionnaire from 3 until 6 years of age. Body weight, waist circumference and height were measured at 3 and 6 years of age to calculate Body-Mass-Index z-Scores (zBMI) and waist-to-height ratio (WTH). Linear, logistic and quantile regressions were used to test whether average time spent PO and ST in the 4 year period had an effect on anthropometric measures at age 6 years.

In view of the current obesity epidemic, studies focusing on the interplay of playing outside (PO), screen time (ST) and anthropometric measures in preschool age are necessary to guide evidence-based public health planning. We therefore investigated the relationship between average time spent PO and ST from the ages 3 to 6 years and anthropometric measures at 6 years of age.

Funding: The studies reported herein have been carried out with partial support from the Commission of the European Community. BK received grants from within the European Union's Seventh Framework Programme (FP7/2007-2013), project EarlyNutrition under grant agreement no. 289346, the EU H2020 project PHC-2014-DynaHealth under grant no. 633595 and the European Research Council Advanced Grant META-GROWTH (ERC-2012-AdG – no.322605) EC URLS: https://ec.europa.eu/programmes/horizon2020/en/area/funding-researchers . DG received partial funding support from Polish Ministry of Science and Higher Education (2571/7.PR/2012/2) URL: https://www.gov.pl/web/science . The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Data Availability: The authors support sharing data with other researchers for legitimate research purposes. However, the study is still ongoing and data cannot yet be anonymized as we currently plan a further follow-up. Therefore, according to the General Data Protection Regulation and the institution's data protection rules individual study participant data cannot be put in the public domain but can only be shared after establishing a written data sharing agreement ensuring that collaborating researchers do not violate privacy regulations and are in keeping with informed consent that is provided by study participants. Written requests to access the data may be submitted to: office.koletzko@med.lmu.de .

Copyright: © 2020 Schwarzfischer et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Although pathways might be clear, there is a distinctive lack of prospective studies in preschool children examining the relationship between PO, ST and anthropometric measures at later ages. In order to make this gap smaller we aim to test whether time spent PO or ST from 3 to 6 years of age is associated with anthropometric measures at 6 years of age.

While the amount of time spent PO decreases, media use in children and adolescence becomes more and more common [ 8 ], and starts ever earlier in childhood [ 9 , 10 ]. Various guidelines are in place recommending appropriate duration of daily ST for children in different ages [ 11 , 12 ]. In a recently published policy statement the focus of the American Academy of Pediatrics lies on media use in preschool children, recommending a limit of 1 hour or less per day of high-quality media (i.e. content, which is pedagogically valuable and developed for children) use for children older than two years of age [ 13 ]. These recommendations are based on several studies showing that higher ST in younger ages is a risk factor for obesity and delayed development of the child [ 14 – 16 ]. However, evidence from these studies is mostly based on cross-sectional data.

Children’s lifestyle and how they spent their free time have changed in the last decades, with a steady decline of physical activities like outdoor play in 3- to 12-year-olds [ 4 ]. However, an active lifestyle can increase daily energy expenditure and thus might be a viable tool to fight the overweight epidemic in addition to a healthy diet [ 5 ]. Studies on physical activity in the form of PO have shown promising results. For example, in a one year follow-up study, time spent PO was associated with lower body mass index (BMI) in 3- and 4-year-old children [ 6 ]. Another study in 3 year olds reporting decreased overweight risk in children with higher active outdoor play in a cross-sectional analysis [ 7 ]. The evidence from these studies is a good indication for beneficial effects of PO, but is restricted to short-term follow-up or cross-sectional samples. Studies looking at the whole preschool period could give further insight to what extend PO can contribute to the prevention of obesity.

Over the last decades childhood obesity turned out to be one of the major public health concerns, as long-term consequences on health become more obvious [ 1 ]. Changes in body size from 2 to 6 years correlate strongly with adult obesity, which makes this period a prime target for prevention strategies focusing on lifestyle of children [ 2 ]. Data suggest that the decline in preschool children’s time spent playing outside (PO) combined with increased screen time (ST) during preschool age could be a modifiable risk factor for childhood obesity, but more evidence is needed [ 3 ].

Methods

Study subjects and design The study population used is a subset of the “European Childhood Obesity Project” trial, which was designed as a double-blind randomized control trial registered at clinicaltrials.gov as NCT00338689; URL: clinicaltrials.gov/ct2/show/NCT00338689. Details and the primary outcome are published elsewhere [17, 18]. In short, the intervention consisted of two types of infant and follow-on formulae, one with higher and one with lower protein content. Besides those two intervention groups the study also included an observational group of children, who were exclusively breastfed for at least the first three months of life. Healthy full term infants who were born between 1st October 2002 and 31st July 2004 were recruited in five countries (Germany, Spain, Italy, Poland, Belgium; a total of 11 study centers) during their first 8 weeks of life. Part of this secondary analysis were all children with objective height, weight and waist circumference measurements at 3 years as baseline and at 6 years as follow-up with additional annual questionnaire data on PO and ST assessed at 3, 4, 5 and 6 years of age.

Ethics statement The study was conducted according to the principles expressed in the Declaration of Helsinki. The local ethics committees of each study center approved all study procedures: Belgium (Comitè d’Ethique de L’Hopital Universitaire des Enfants Reine Fabiola; no. CEH 14/02), Germany (Bayerische Landesärztekammer Ethik-Kommission; no. 02070), Italy (Azienda Ospedaliera San Paolo Comitato Etico; no. 14/2002), Poland (Instytut Pomnik–Centrum Zdrowia Dziecka Komitet Etyczny; no 243/KE/2001), and Spain (Comité ético de investigación clinica del Hospital Universitario de Tarragona Joan XXIII). Written informed parental consent was obtained for each infant.

Anthropometric measurement Children’s weight, height and waist circumference were measured at study centers at the 3 and 6 years follow-up visit. Standard operating procedures based on the World Health Organization’s Multicenter Growth Reference Study were applied [19]. The same equipment was used in all study centers and study personnel were trained repeatedly during the study to ensure reliable results. All measurements were taken twice, and their means were taken for analysis. BMI was calculated in kg/m2, which has been proven to be a good measure for child overweight and obesity [20]. Age- and sex-specific BMI z-scores (zBMI) were computed based on the World Health Organization’s reference population [21]. Waist to height ratio (WTH) was calculated as ratio between waist circumference [cm] and height [cm]. WTH has been shown to be a reliable measure of abdominal adiposity in children [22].

Activity assessment At the 3 year, 4 year, 5 year and 6 year follow-up questionnaires were handed out, including four questions asking for a typical weekday or weekend day in the last month. A four items questionnaire was used to assess time PO and ST, which was filled by parents before or during study visits. To asses children’s time PO their parents were asked to recall: “How much time would you say your child spends playing outdoors on a typical weekday?” and “How much time would you say your child spends playing outdoors on a typical weekend day?”. For ST similar questions were used: “ How much time would you say your child spends watching TV, playing videogames or using the computer on a typical weekday?” “How much time would you say your child spends watching TV, playing video games or using the computer on a typical weekend day?“. Answers should be entered in free text field in hours and minutes. Identical questionnaires are used in other studies and validated in similar populations [23]. The mean time spent PO and ST per day was calculated as (hours/weekday × 5 + hours/weekend day × 2)/7.

Covariates Additional to gender and study country seven covariates were considered in the analysis of which following covariates were collected at study recruitment: highest education level of mother and father according to International Standard Classification of Education 1997 levels, defined as low (level 0–2), middle (level 3–4) and high (level 5–6) [24], pre-pregnancy BMI, calculated from self-reported height and weight of mothers and dichotomized as BMI above and below 25, smoking status during pregnancy, and the child’s birthweight. At 3 years of age children’s caloric intake was assessed with 3-days weighted food protocols filled in by parents [25]. We defined a “season of measurement” variable (spring [Mar-May], summer [Jun-Aug], autumn [Sept-Nov] and winter [Dec-Feb]) based on the visit date.

Data management Data are reported as mean (μ) ± standard deviation (SD) for continuous variables and as number (n) and percentage (%) for factors. Main predictor variables were time spent PO and ST. Implausible values of more than 10 hours of ST per day were excluded (4 data points). Both PO and ST showed to be relatively stable behaviors over the preschool period: Spearman correlation coefficients of successive PO measurements ranged from 0.51 to 0.60; correlation coefficients of successive ST measurements coefficients ranged from 0.60 to 0.64. We combined consecutive measurements of PO and ST into one average PO and one average ST variable, to estimate the average time spent PO and ST over the whole preschool period. Variables PO and ST used in the data analysis were calculated as individual means of children’s PO and ST measurements over the 4 years period. As follow-up of all participants at all time points was not achieved, we defined that only children with at least two measurement points with both PO and ST data at the respective time point were included in the analysis. We defined overweight as a zBMI at 6 years >1, obesity as a zBMI at 6 years >2 and WTH at 6 years > 0.5 as a measure of increased central adipose tissue associated with worsened cardiometabolic risk [20, 26].