The Association of the Body Mass Index of Children with 24-Hour Activity Composition and Isotemporal Substitution:A Compositional Data Analysis

Objective: To examine the relationships between 24-hour activity behaviors and body mass index(BMI) in Shanghai children by using compositional analyses. Methods: A cross sectional study was conducted on 321 primary school students(158 boys and 163 girls) aged 6-13 years in Shanghai. Time spent in 24-hour activity behaviors was determined by accelerometry,children's height and weight were measured by standard instruments. Compositional linear regression models were used to synthetically analyze the associations between the distribution of time spent 24-hour activity behaviors and zBMI, and the effect of compositional isotemporal substitution of 24-hour activity behaviors with zBMI was investigated as well. Results: 1) By controlling for gender and age, time spent in moderate-to-vigorous physical activity(MVPA)(β =-0.75, P<0.001) and light physical activity(LPA)(β=-0.13, P<0.05) was negatively associated with zBMI in children, while the time spent in sedentary behavior(β=0.61, P<0.001) and sleep(β =0.53, P<0.01) was positively correlated with zBMI; 2) The compositional isotemporal substitution models revealed reductions in zBMI when a 15-min reallocation from LPA, sedentary behavior and sleep to MVPA, and from sedentary behavior to LPA; however, a 15-min reallocation from MVPA to LPA, sedentary behavior and sleep, and from LPA to sedentary behavior was associated with higher zBMI; 3) According to the “dose-response” curves, firstly, the mutual substitution of MVPA and other activities has an asymmetric effect on zBMI and the substitution of LPA and sedentary behavior has a symmetrical effect on zBMI; secondly, the best reduction effect on zBMI is to replace sedentary behavior with MVPA; thirdly, with the increase time of MVPA isotemporal substitute other behaviors, the decrease in zBMI was slowed down, otherwise it increased rapidly. Conclusions:Parents and teachers should take 24-hour activities as a whole and attach great importance to the positive effects of MVPA and LPA,try to reallocate time from ST to MVPA and LPA of children so as to obtain better health benefits.