The study on body mass index centile curves and the reference norm for screening overweight and obesity in Kazak primary and middle school students aged 7-18 years in Xinjiang,China

Objective To construct centile curves and cutoff points of body mass index(BMI)for overweight and obesity screening among Kazak children aged 7 to 18 years in Xinjiang,China.Methods The cross-sectional study was conducted in urban and rural areas of Altai city,using random cluster sampling to select 4 schools(a total of 49 classes).Enrolled students who agreed to take part in the study were investigated for demographic and anthropometric data.The BMI centile curves for Kazak children aged 7-18 years were constructed by λ-median-coefficient of variation method.Based on BMI cutoff points for adult overweight and obesity recommended by IOTF and WGOC,the centile curves passing through 25 and 30 kg·m-2,24 and 28 kg·m-2 at age 18 were identified as overweight and obesity cutoff curves for Kazak children,by which the cutoffs of BMI for each age were computed.Results Overall 2 487 students,including 1 158 boys and 1 329 girls were investigated.The cutoff curves for identifying overweight and obesity were P88.63 and P98.28 for boys,and P84.41 and P98.12 for girls based on WGOC cutoffs of adults' BMI,and were P92.96 and P99.28 for boys,P90.53 and P99.38 for girls based on IOTF cutoffs.The cutoff curves for Kazak children differed from that recommended by IOTF and WGOC,as well as differed from cutoff curves for Han and Ugyur children recommended by our earlier studies.The cutoff curves for overweight and obesity for Kazak girls were lower than that of Ugyurs' and Hans' but increased quickly and exceeded Ugyur curves and reached Han curves around age of 14.Conclusions Norms of BMI for screening overweight and obesity in Kazak school-aged children differ from that recommended by IOTF and WGOC,and appear ethnic difference.For Kazak students,the cutoffs of BMI proposed by this study for screening overweight and obesity are recommended.