Social anxiety, BMI and cardiorespiratory fitness among schoolchildren: a cross-sectional serial mediation analysis of cognitive reappraisal and sport engagement

Background Social anxiety is a common mental health concern in children and adolescents, but its links with physical health indicators are not well understood. This study examined whether social anxiety was associated with BMI and cardiorespiratory fitness, and whether these associations were explained by emotion regulation strategies and sport engagement. Methods This cross-sectional study included 1108 children in Grades 3 to 5 in Beijing. Social anxiety, cognitive reappraisal, expressive suppression, and sport engagement were assessed by questionnaires. BMI was obtained from school fitness records. Cardiorespiratory fitness was assessed using the standardized 50 m × 8 shuttle-run score in Grade 5 students. Structural equation models were used to examine direct and indirect associations, adjusting for sex, age, school-based sport participation, and out-of-school sport participation. Grade was additionally adjusted for in the BMI model. Results There was no consistent evidence for a direct association of social anxiety with BMI or cardiorespiratory fitness. Higher social anxiety was indirectly associated with higher BMI and lower cardiorespiratory fitness through lower cognitive reappraisal and lower sport engagement. The serial indirect effect was significant for BMI (standardized indirect effect = 0.012, 95% CI [0.006, 0.021]) and cardiorespiratory fitness (standardized indirect effect = − 0.024, 95% CI [− 0.049, − 0.007]). The pathway through expressive suppression and sport engagement was not significant. Conclusions Social anxiety was linked to BMI and cardiorespiratory fitness mainly through cognitive reappraisal and sport engagement, rather than through stable direct associations. These findings highlight cognitive reappraisal and the quality of sport engagement as important targets for supporting children with social anxiety.