Design This study was embedded in the Generation R Study, a population-based prospective cohort from fetal life onward in Rotterdam, The Netherlands (9). The study has been approved by the Medical Ethical Committee of the Erasmus MC, University Medical Centre in Rotterdam (MEC-2012-165-NL40020.078.12). Women who lived in Rotterdam, The Netherlands, with a delivery date between April 2002 and January 2006, were eligible for participation. Written informed consent was obtained from all participants and their parents or legal representatives. Adolescents visited our research center at the median age of 13 years. From 6,842 participants, twins (n = 172) and adolescents with missing information on physical activity and screen time (n = 3,100) were excluded. Also, children with missing information on BMI, DXA and MRI (n = 312) were excluded. This resulted in 3,258 children for the current analyses (Fig. 1). It should be pointed out that only 2,121 children had MRI-based fat segmentation data available, as in 1,137 children there was no MRI scan acquired (n = 1,092), MRI sequences acquired were incomplete or of insufficient quality (n = 2), or unsatisfactory fat segmentation was achieved judged by visual evaluation (n = 43).
Physical activity and screen time Information on physical activity and screen time was obtained by self-reported questionnaires at age 13 years. This questionnaire was designed based on the Activity Questionnaire for Adults and Adolescents (AQuAA) (10) and the Health Behaviours in School-Aged Children (HBSC) questionnaire (11). The AQuAA questionnaire has been shown to be reproducible although there was low validity measured with the use of accelerometers (10). The widely used HBSC questionnaire, using the Multistage Fitness Test for physical activity and a television watching diary for screen time, has demonstrated adequate reliability and validity (12, 13). To assess the total amount of physical activity, children reported their strenuous physical activity and walking or cycling to and from school respectively. Children were asked to response strenuous physical activity (i.e., ‘How many hours a week do you usually exercise in your spare time in a way that makes you breathless?’). The response categories for strenuous physical activity included: ‘never’, ‘less than 1 hour’, ‘1–2 hour’, ‘2–3 hour’, ‘3–4 hour’, ‘4–6 hour’, ‘more than 7 hours’. The average time per day spent during leisure time to activities that make the respondent running out of breath was calculated using the following formula: hours per day spent on strenuous physical activity / 7. Children also reported the way of traveling to and from school (i.e., ‘How do you usually get from home to school?’, ‘How do you usually go home from school?’) and the travelling time (‘How long does it take you to travel home from school?’). The response categories for the way traveling to and from school included: ‘walk’, ‘bicycle’, ‘bus, train, tram, metro or boat’, ‘car, motorcycle or scooter’, ‘others’. The response categories for the traveling time included: ‘less than 5 minutes’, ‘5 to 15 minutes’, ’15 to 30 minutes’, ’30 minutes to 1 hour’, ‘more than 1 hour’. The average time per day for walking or cycling to and from school was calculated using the following formula: [(hours per time walking or cycling to school + hours per time walking or cycling from school) * 5] / 7. A total daily physical activity time was calculated by adding up children’s strenuous physical activity and walking or cycling to and from school.
Regarding screen time, participants were asked, for weekdays and weekend days separately, for the number of hours during leisure time they played computer games, used a computer for chatting, web-browsing, e-mail or homework, and watched television (including videos/DVDs). A total daily screen time was calculated by using the following formula: [(hours per weekday) * 5 + ((hours per weekend day) *2)] / 7. Information of questionnaires on physical activity and screen time at ages 6 and 10 years was used for additional analysis (Supplemental Appendix 1 and Supplemental Appendix 2).
Body composition measures Children visited our research center at age 13 years, their anthropometrics were measured, and DXA and MRI examinations were performed. DXA-based measures included fat mass index (FMI), lean body mass index (LBMI), android/gynoid fat mass index (A/G ratio), and MRI-based measures included abdominal subcutaneous fat index and abdominal visceral fat index. Particularly, a deep learning-based image segmentation method, using a 2D-Competitive Dense Fully Convolutional Network (CDFNet) as proposed in the FatSegNet method (14), was applied to quantify abdominal subcutaneous fat and visceral fat on MRI scans automatically (Fig. 2). DXA and MRI measurements at age 10 years were used for additional adjustment. Detailed information is described in the Supplemental methods.
Covariates Information on maternal age, pre-pregnancy BMI, educational level (low: no education or primary education; middle: secondary Phase 1 or 2 finished; high: vocational or university degree), parity (nulliparous or multiparous), psychiatric symptoms (yes; no), smoking (yes; no) were obtained from questionnaires during pregnancy.
Child’s age, sex, gestational age at birth, and birth weight were obtained from midwife and hospital registries at birth. Child ethnic background was based on parental countries of birth obtained from questionnaire at enrollment. Ever breastfeeding (yes; no) was obtained from Questionnaires after birth. Puberty stage at the age of 13 years were obtained based on self-reported pubic hair growth according to the Tanner scale (15, 16). According to the distribution within our cohort, puberty status was classified as early puberty (Tanner stages 1–3), or advanced puberty (Tanner stages 4–5). Children’s breakfast skipping behavior was assessed by single-item question in a parent-reported questionnaire. Parents reported the times that their children have breakfast (more than a glass of milk or fruit juice) for weekdays and weekend days separately. We defined breakfast skipping behavior as present if breakfast skipping occurred at least once in a week versus never.
Statistical analysis Characteristics of children included and not included in the study were assessed using Student’s t-tests, Mann-Whitney U tests, and Chi-square tests. We used linear and logistic regression models to examine the associations of physical activity and screen time with body composition at age 13 years. We constructed three sets of models. Model 1 (basic model) was adjusted for child’s sex and age. In model 2 (main model), we additionally adjusted for maternal age, prepregnancy BMI, educational level, parity, psychiatric symptoms (GSI) and smoking during pregnancy, and child’s gestational age at birth, birth weight, ethnic background, breastfeeding, puberty status and breakfast skipping. Potential confounders were selected according to literature, if they were related to both physical activity, screen time and respiratory outcomes, or if the effect estimate of the unadjusted analyses changed ≥ 10% when we additionally adjusted for a confounder. For better visualization of causal assumptions and potential confounders, we created a Directed Acyclic Graph using DAGitty version 2.3 (Supplemental Fig. 1). Consequently, all the potential confounders were included in our models. In model 3 (reverse causation model), we additionally adjusted for body composition outcomes at age 10 years to partly reduce in part the chance of reverse causation based on the main model. In addition, physical activity and screen time were categorized into tertile groups (low, middle and high). Interaction terms were tested in the main model. We tested whether the associations of physical activity and screen time with body composition were modified by child’s sex and puberty stage. If the interaction term was significant (P value < 0.10), we performed further stratified analyses.
Some additional analyses were performed. We excluded adolescents of non-European ethnic background because of their potential different body composition based on our main model in our sensitivity analyses. Longitudinal associations of physical activity and screen time at ages 6 and 10 years with body composition measure at age 13 years were examined separately. Not normally distributed outcomes measures (FMI, A/G ratio, abdominal subcutaneous fat index, abdominal visceral fat index and V/S ratio) were log-natural transformed. To enable comparison of effect estimates, we constructed Z score of outcomes. We tested for non-linearity of the associations via applying natural cubic splines. Missing data in covariates (ranging from 0 to 27.7%) were imputed using the Fully Condition Specification method with ten imputations (17). All measures of associations were presented as pooled estimates from the imputed datasets. Statistical analyses were performed using SPSS, version 28.0 for Windows (IBM Crop., Armonk, NY, USA) and R statistical software, version 4.1.2 (R Foundation for Statistical Computing).