Presentation Information
[07発-口-01]Seasonal and sex differences in children’s body composition under in-term PA versus vacation non-PA periodsA 40-month natural-experiment observation
*Xiang Pan1,2, Yibo Gao1,2, Mingzhe Li1,2, Koya Suzuki1 (1. Graduate School of Health and Sports Science, Juntendo University, 2. China Institute of Sport Science)
Objectives. To characterise body composition changes in Chinese schoolchildren across the school year, contrasting in-term periods (when a school-based PA programme was active) with vacations, and to test whether BMI, body fat percentage (BFP), and body weight responded uniformly across sex and season.
Methods. Natural-experiment observational analysis of a school-monitored cohort enriched for elevated baseline adiposity (91.5% overweight or obese by Chinese WS/T 586 criteria). The school-based PA programme was delivered by the participating schools to children identified as overweight or obese on their annual screening, who enrolled with parental consent; it ran throughout each in-term period and was fully suspended during vacations. The research team was not involved in its design or delivery. N = 388 (M = 241, F = 147; mean age 10.2 y, range 7.8–12.5) from two Beijing primary schools weremeasured at up to 10 timepoints across a 40-month observation window (September 2021–December 2024) by multi-frequency BIA (1,178 observations across three protocol-batch phases).Three 12-week windows were defined: in-term PA period (W1+W3), winter vacation (W2), summer vacation (W4). Paired changes were tested by Wilcoxon tests;era-stratified longitudinal trajectories were estimated by linear mixed-effects models (Python statsmodels, REML); smooth GAMM curves (R mgcv) were used only for trajectory visualisation.
Results. Across 242 in-term paired observations, BMI declined 0.44 ± 1.08 kg/m² (Cohen d = −0.41), an active reversal during periods when the school-based PA programme was running. Both vacations produced gains (winter ΔBMI +0.84, summer +1.03 kg/m²). Effect sizes diverged systematically across indicators: summer d(weight) = +1.59 > d(BMI) = +0.78 > d(BFP) = +0.54. Summer BFP rose 3.4-fold more in girls than boys (+2.43 vs +0.71 pp, p = 0.013).
Conclusions. Under the alternation between in-term PA periods and vacation non-PA periods, children’s body composition showed marked seasonal directionality, with active reversal during in-term periods and clear gains during vacations. The indicator-specific effect-size divergence and the 3.4-fold female-skewed summer BFP response are consistent with sex-stratified, tissue-specific dynamics that align with peri-pubertal sexual dimorphism.
Methods. Natural-experiment observational analysis of a school-monitored cohort enriched for elevated baseline adiposity (91.5% overweight or obese by Chinese WS/T 586 criteria). The school-based PA programme was delivered by the participating schools to children identified as overweight or obese on their annual screening, who enrolled with parental consent; it ran throughout each in-term period and was fully suspended during vacations. The research team was not involved in its design or delivery. N = 388 (M = 241, F = 147; mean age 10.2 y, range 7.8–12.5) from two Beijing primary schools weremeasured at up to 10 timepoints across a 40-month observation window (September 2021–December 2024) by multi-frequency BIA (1,178 observations across three protocol-batch phases).Three 12-week windows were defined: in-term PA period (W1+W3), winter vacation (W2), summer vacation (W4). Paired changes were tested by Wilcoxon tests;era-stratified longitudinal trajectories were estimated by linear mixed-effects models (Python statsmodels, REML); smooth GAMM curves (R mgcv) were used only for trajectory visualisation.
Results. Across 242 in-term paired observations, BMI declined 0.44 ± 1.08 kg/m² (Cohen d = −0.41), an active reversal during periods when the school-based PA programme was running. Both vacations produced gains (winter ΔBMI +0.84, summer +1.03 kg/m²). Effect sizes diverged systematically across indicators: summer d(weight) = +1.59 > d(BMI) = +0.78 > d(BFP) = +0.54. Summer BFP rose 3.4-fold more in girls than boys (+2.43 vs +0.71 pp, p = 0.013).
Conclusions. Under the alternation between in-term PA periods and vacation non-PA periods, children’s body composition showed marked seasonal directionality, with active reversal during in-term periods and clear gains during vacations. The indicator-specific effect-size divergence and the 3.4-fold female-skewed summer BFP response are consistent with sex-stratified, tissue-specific dynamics that align with peri-pubertal sexual dimorphism.
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