| Background The problem of obesity in children and adolescents has become a global chronic disease,which can also continue into adulthood,bringing many hidden dangers to life and health.It is worth noting that the overweight and obesity rate of children and adolescents in my country has remained high for several years.With the transformation of modern lifestyles,more and more children and adolescents have insufficient physical activity(PA)or(and)excessive sedentary behavior(SB).A number of studies have pointed out that adequate physical activity time and less sedentary behavior play an important role in preventing the maintenance of a healthy weight.Objective Based on the isotemporal substitution analyze paradigm,the purposes are to comprehensively explore the isotemporal substitution relationship between physical activity,sedentary behavior and body mass index(BMI)of primary and middle school students in Shanghai and to more accurately predict the impact of changes in the time of different activities and behaviors in a day on health outcomes.More targeted suggestions can be made for children and adolescents’one day behavior time.Methods A cross-sectional survey was conducted on 486 primary and middle school students aged 6-16 years old.Their physical activity and sedentary behavior were objectively measured by accelerometers;the standard height and weight measuring instruments were used to measure height and weight data;questionnaires were used to collect other information;The single models,partition models,and isotemporal substitution models were used for mathematical statistics.Results Among the participants,the average age was 11.38±2.16 years,BMI was 18.62±3.51 kg/m~2,96were overweight,accounting for 19.8%,and 25 were obese,accounting for 5.1%.The overweight and obesity rate of primary school students was significantly lower than that of junior high school students(P<0.01).Primary school students participated in LPA significantly longer than junior high school students(t=24.37,P<0.01),and the time spent in SB was significantly shorter than junior high school students(t=-19.69,P<0.01).There was no significant difference in the time of participating in MVPA(t=0.76,P>0.05).The single models(models 1 to 3)show that after adjusting for gender and age,there is a significant negative correlation between LPA and BMI of elementary school students(Β=-0.27,P<0.01,95%CI:-0.43～-0.10),while SB and MVPA were not significantly related to BMI(P>0.05).There is a significant negative correlation between MVPA and BMI of junior high school students(β=-0.23,P<0.01,95%CI:-1.90～-0.56),but there is no significant correlation between SB,LPA and BMI(P>0.05).After adjusting for the effects of gender,age,sedentary behavior,and MVPA in the partition models,the results show that increasing 30 minutes of LPA can reduce BMI by 0.25 kg/m~2(β=-0.25,P<0.05,95%CI:-1.00～0.04)in primary school students,which reduces the overall average BMI by 1.34%.It can find that the correlation between LPA and BMI is independent of SB and MVPA;increasing 30minutes SB or MVPA has no significant effect on BMI.In junior high school students,every 30 minutes of MVPA can reduce BMI by 1.22 kg/m~2(β=-1.22,P<0.01,95%CI:-1.91～-0.52),which reduces the overall average BMI level by 5.90%.It also shows that the correlation between MVPA and BMI is independent of SB and LPA,while the increase of 30 minutes of SB or LPA did not cause significant changes in BMI(P>0.05).The results of isotemporal substitution models show that after controlling for confounding factors,replace 30min of SB or LPA with MVPA for the same time can reduce BMI by 1.25kg/m~2(β=-1.25,95%CI:-1.94～-0.57),1.29 kg/m~2(β=-1.29,95%CI:-2.00～-0.58)separately,in junior high school students.Elementary school students who replaces 30 min of SB with LPA could reduce BMI of 0.26kg/m~2(β=-0.26,95%CI:0.44～-0.09).Although SB does not independently affect the BMI of elementary school and junior high school students,SB replacing LPA in elementary school students and MVPA in junior high school students significantly increase their BMI levels.There are no significant substitution associations between other behavior substitution methods and BMI.Conclusion The level of physical activity of Shanghai children and adolescents measured in this study is low,and the obesity rate is higher than the national average.Primary school students’LPA and BMI have an independent correlation,and junior high school students’MVPA has an independent correlation with BMI.The isochronous substitution relationship between different activity behaviors and BMI is different in school period.It is recommended that Shanghai elementary school students increase LPA time and reduce sedentary behavior time based on existing activities,while junior high school students are advised to try to convert part of LPA or sedentary behavior into MVPA,rather than simply increase or decrease the absolute time of activity,to obtain the highest health benefits. |
