Gender, race, and maternal education (marker for socioeconomic status) were included as covariates in the present study. The questions used to capture screen time have previously been validated ( ( 10)). Time spent watching television/videos and playing video games, on a week night during the school-term, was self-reported by the participants (5 h day −1). The height and weight data were compared to CDC growth charts, to help identify any excessive high and low self-reported values ( ( 9)). It has been shown that self-reported and measured BMI's are highly correlated ( ( 8)). The participants self-reported their height and weight, from which BMI (kg m −2) was calculated. The University of Pennsylvania Institutional Review Board granted ethical approval for the study. The participants who completed the baseline survey were followed-up every 6 months over a 4-year period. A total of 30 adolescents were absent on the assent/survey day, and a further 19 were not interested in the study. Parental consent was obtained on 99% of the eligible adolescents ( n = 1,478), and 97% of the eligible participants completed the baseline survey ( n = 1,429). Based on this selection criteria 98% of the adolescents were eligible ( n = 1,487). Those with a special classroom placement, and who spoke English as a second language were ineligible to participate. A total of 1,517 children were identified through class rosters at the beginning of 9th grade.
The participants were sampled from four high schools in suburban Philadelphia. Therefore, the purpose of the present longitudinal study was to determine if screen time was associated with changes in the BMI distribution in a sample of adolescents from ages 14 to 18. Previous longitudinal studies involving adolescents have modeled average changes in BMI ( ( 4- 6)), but there is evidence from cross-sectional studies that screen time is not uniformly associated with BMI across the BMI distribution in young children ( ( 7)). Further, in one longitudinal study the association between screen time and changes in mean BMI was positive when the participants were children, but null when the participants were adolescents ( ( 4)).ĭisparate findings across studies and developmental periods may be explained, in part, by how changes in BMI were statistically modeled. While these ecological data suggest that greater time spent in screen-based sedentary behavior may have contributed to the high prevalence of adolescent obesity, a recent review of longitudinal association studies concluded that there was insufficient evidence ( ( 3)). In recent decades, increases in the use of electronic media have paralleled the increase in adolescent obesity ( ( 1), ( 2)). Therefore, lowering screen time, especially among overweight and obese adolescents, could contribute to reducing the prevalence of adolescent obesity. Positive associations between screen time and changes in the BMI at the upper tail of the BMI distribution were observed. No associations were observed between screen time and changes at the 10th and 25th BMI percentiles. Increases at all the BMI percentiles over time were observed, with the greatest increase observed at the 90th BMI percentile.
Study wave and screen time were the main predictors, and adjustment was made for gender, race, maternal education, hours of sleep, and physical activity. Longitudinal quantile regression was used to model the 10th, 25th, 50th, 75th, and 90th BMI percentiles as dependent variables. BMI (kg m −2) was calculated from self-reported height and weight. Time spent watching television/videos and playing video games was self-reported (<1 h day −1, 1 h day −1, 2 h day −1, 3 h day −1, 4 h day −1, or 5+ h day −1). Participants ( n = 1,336) were adolescents who were followed from age 14 to age 18 and surveyed every 6 months. Until now, no study has evaluated the longitudinal relationship between screen time and changes in the BMI distribution across mid to late adolescence. Previous research has examined the association between screen time and average changes in adolescent body mass index (BMI).