| Objectives:In recent years, more and more children and adolescents are suffering from the obesity and overweight which not only affect their life, study and health but also are the potential risk factor of suffering many related diseases in the future.Body Mass Index (BMI) is widely used to assess obesity and overweight of children and adolescents because of its convenience and practice."The standard of BMI classification for screening overweight and obesity in Chinese school-aged children and adolescents"was carried out in 2003 in China.However, teenagers'special maturity characteristics make it difficult to get accurate measurement in anthropometric variables, as their growth and development are very different. Some researchers use body height and secondary sexual characters to assess growth and development and think there is relationship between them. .As it is very rough to assess maturity of children and adolescents by body height and secondary sexual characters, the present study are aimed to assess them by height and skeletal age. The paper is to study the relationship between BMI and body height and skeletal age as well as their effects on screening obesity and overweight. Our results can provide reference for better assessment of obesity and overweight of children and adolescents used by"The standard of BMI classification for screening overweight and obesity in Chinese school-aged children and adolescents".Methods: We chose 9841 samples (normal children and adolescents aged from 7 to 18, male 5088, female 4753) from the standard samples according to"The Skeletal Development Standards of Hand and Wrist for Chinese Children-Chinese 05". These subjects lived in Shanghai, Wenzhou, Guangzhou, Dalian and Shijiazhuang respectively. They were chose according to the principle of cluster sampling and divided into every group every year. All children's left wrists were inspected with X ray during 15 days after their birthday, and their body height and weight were measured. All samples were divided into different groups based on their gender, chronological age and growth level. Pearson correlation coefficients and multiple linear regression analysis were used to evaluate the relationships between BMI, height and skeletal age;chi-square criterion was used to make variability comparison of children overweight and obesity among different groups.Then, 370 children in Shijiazhuang city were recruited to measure their body height and weight, skeletal age as well as skinfold thickness. Multiple linear regression analysis were used to evaluate the relationships between BMI, height, skeletal age and skinfold thickness, to make variability comparison of children overweight and obesity among different groups.Results: During the whole process of the growth and development as boys (7-18 years ) the correlation value between height, skeletal age and BMI is 0.267 and 0.343, as girls the correlation value between height, skeletal age and BMI is 0.182 and 0.319, both of the correlation relationships are significant and positive, but along with age increasing, the correlation will decrease after a maximum at a certain age point. As two different indices representing growth level, body height and skeletal age's correlation coefficient with BMI are different in all age-groups. The ages which the correlation coefficient reach the peak are also different.As boys, the highest correlation between height and BMI is the 10-year-old group(r=0.434, P<0.001), and the maximum coefficient correlation between skeletal age and BMI is at 11-year-old(r=0.453, P<0.001). The girls'coefficient correlations between height and BMI reach the maximum at 9-year-old (r=0.345, P<0.001), and their maximum coefficient correlations between skeletal age and BMI are at 12-year-old(r=0.446, P<0.001).It is shown with multiple regression analysis that the multiple correlation coefficient between height, skeletal age and BMI increase along with age increasing in all groups, reaching the peak at adolescence stage(male: R MAX=0.492,11-year old; female:RMAX =0.460,12-year old), then decrease slowly. There is no significant collinearity between height and skeletal age, and the regression coefficient of skeletal age and BMI is higher than that of height and BMI.After grouping according to bone development, coefficient correlation of height and skeletal age with BMI are lower than that before grouping. The coefficient correlation of normal growth adolescence group are significant different in most groups, but in premature and retardation group there are significant differences only in part age stage and age group. The coefficient correlation of height and skeletal age with BMI decrease after the degree of mature is controlled by grouping. It is shown by the results of validation that there are significant correlation between height, skeletal age with skinfold thickness (The male's coefficient correlation of height and skinfold thickness are 0.418,0.282 and 0.400 at 10-,12- and 14- year old respectively, P<0.05; and the female's are 0.576,0.330 and 0.207 at 8-, 10- and 12-year old respectively, P<0.05. The male's coefficient correlation of skeletal age and skinfold thickness are 0.269,0.300 and 0.486 at 10-,12- and 14- year old respectively, P<0.05, and the female's are 0.520,0.326 and 0.249 at 8-,10- and 12- year old respectively, P<0.05). The correlation change trend of height and skeletal age with skinfold thickness are similarly with height and skeletal age with BMI.Whether in whole samples or different age group, the rate of overweight and obesity screened by chronological age are not different from that screened by skeletal age. But the sample of groups have changed in different groups classified by two methods. Among the whole samples, the analysis result screened by chronological age is different from that screened by skeletal age for the 6.0 percent of the boys and the 5.1 percent of the girl, the difference rate is higher for the adolescents group. In chronological age group, after grouping according to development degree, the overweight and obesity rate screened by chronological age are not different from that screened by skeletal age in normal adolescence, but there are difference in several groups of premature and retardation adolescence, especially obesity rate.Conclusions:①During the whole process of the growth and development as boys (7-18 years ) the correlation value between height, skeletal age and BMI is 0.267 and 0.343, as girls the correlation value between height, skeletal age and BMI is 0.182 and 0.319, both of the correlation relationships are significant and positive。The result of multiple regression analysis shows that as an index representing growth and development degree, the effect of skeletal age on BMI is higher than that of height, so it can better reflect the effect the growth and development on BMI.②Among the whole samples, the analysis result screened by chronological age is different from that screened by skeletal age for the 6.0 percent of the boys and the 5.1 percent of the girls. The difference rate is higher for the adolescents group。For prematurity and retardation children and adolescents, the rates of overweight and obesity screened by chronological age are different from that screened by skeletal age.
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