Physical activity, social support and BMI among middle school students in China

Introduction: Adolescent obesity is a growing public health concern in China, driven largely by insufficient physical activity and increasing sedentary behaviors. Evidence on how multiple sources of social support relate to adolescents' physical activity and weight status remains relatively limited within the Chinese educational and cultural context. This study aims to examine the associations between multi-source social support, physical activity, and BMI Z-scores, and to explore gender-specific patterns among Chinese junior high school students. Methods: A cross-sectional survey was conducted among 518 students aged 12-15 years from three junior high schools in Kunming, Yunnan Province. Participants completed the Physical Activity Rating Scale (PARS-3) and the Child and Adolescent Social Support Scale (CASSS), which assesses perceived support from five sources (parents, teachers, classmates, close friends, and school). BMI data were obtained from recent school health assessments. Hierarchical regression and restricted cubic spline analyses were employed to explore the associations and non-linear dose-response patterns between physical activity, multi-source social support, and BMI Z-scores. Results: < 0.05). Parental, classmate, and close-friend support were not significant. Non-linear analyses revealed gender-specific trends: boys displayed a linear dose-response pattern, whereas girls showed a U-shaped curve with optimal BMI at moderate activity levels. The association between teacher support and BMI Z-scores was more evident among girls. The final model explained 14.1% of BMI Z-score variance. Discussion: Physical activity showed the strongest association with BMI Z-scores among adolescents, and teacher support was uniquely associated with weight status in the Chinese school context. Gender-specific non-linear patterns highlight the need for culturally grounded, school-centered, and gender-sensitive strategies to promote healthy weight in early adolescence.