Objective: To investigate the association between 24 hour activity behavior and body composition among primary and secondary school students using compositional data analysis method, so as to provide scientific guidance for optimizing daily activity allocation and improving physical health. Methods: In June 2021, following the principle of random cluster sampling, 349 primary and middle school students were selected by lottery method from Taiyuan City, Shanxi Province. Compositional multiple regression analysis was conducted to examine the relationship between different 24 hour activity behaviors and body composition indicators. The isotemporal substitution model was adopted to examine the quantitative effects of substituting 10 or 20 minutes among activity behaviors on body composition. Results: In girls, sedentary behavior (SB) time was negatively correlated with fat free mass (FFM) ( β =-4.81); in boys, moderate to vigorous physical activity (MVPA) time was positively correlated with FFM ( β =7.01), while light physical activity (LPA) time was negatively correlated with FFM ( β =-9.26) and positively correlated with body fat percentage (FAT%) ( β =0.09) (all P <0.01). Isocaloric substitution analysis revealed that replacing the same duration of MVPA with SB, sleep (SL), or LPA for 10 or 20 minutes produced significantly greater adverse effects compared to the beneficial effects of MVPA substitution. Replacing MVPA with LPA for 10 minutes resulted in 1.01% increase in FAT% and 0.91% decrease in reverse substitution for boys; replacing MVPA with SB or SL for 10 minutes resulted in a decrease of 1.10 kg and 1.16 kg in FFM for boys, 0.81 kg and 0.74 kg for girls, and reverse substitution increased by 0.92 and 0.97 kg, 0.65 and 0.58 kg, respectively (all P < 0.05 ). When LPA, SB or SL were replaced with MVPA for 20 minutes, the trend of change were basically consistent (all P <0.05). Conclusions Activity behaviors of 24 hour are associated with body composition among primary and secondary school students. MVPA is the primary influential component.

Objective: The method of compositional data analysis was used to explore the relationship between 24 hour movement behavior and obesity indicators, and to examine the difference of quantitative effect on obesity indicators when one behavior replaced another behavior, so as to provide specific movement behavior advice for weight control in children and adolescents. Methods: In June 2021, 231 students from eight classes in a primary school and a middle school in Taiyuan City, Shanxi Province were voluntarily recruited by using random cluster sampling. ActiGraph GT3X+ accelerometer was used to measure 24 hour movement behavior and Inbody J20 body composition analyzer was used to measure body composition. The relationship between each component and obesity indicators was analyzed by compositional multivariate linear regression model. In addition, 30 minutes of one behavior was used to replace another behavior to predict the effect difference of the outcomes. Results: After adjusting for covariates such as height, weight, age, and sex, compared with time spent in sedentary behavior(SB), sleep (SLP) and light physical activity (LPA), time spent on moderate to vigorous physical activity (MVPA) was positively correlated with fat free mass index (FFMI) ( β= 0.40, P ＜0.05), negatively correlated with waist circumference (WC) ( β=-2.50, P ＜0.05) and waist hip ratio (WHR) ( β= -0.04 , P ＜0.05). Compared with SB, SLP and MVPA, time spent on LPA was positively correlated with WHR ( β=0.06, P < 0.05 ). If MVPA of 30 min/d replaces SLP, SB, and LPA respectively, WC and WHR decrease 1.10,1.10,1.34 cm and 0.02, 0.02 ,0.02 respectively, and FFMI increases 0.19,0.19,0.15 kg/m 2 respectively. Conclusion In 24 h movement behavior, with consistent level of sedentary behavior, sleep or low intensity movement behavior, maintaining a high level of MVPA and replacing sedentary with active activities are crucial for optimal abdominal fat and fat free mass in children and adolescents.