Comparison with the WHO study
The age phases of the pooled analysis were broad; 0 to 60 months range was compared in the WHO study . In our study, the weighted age boundary segmentation was the following: cutoff 1 month before a half year of age, 2 months after a half year old, 3 months at 2 years old, 6 months within 2–6 years of age, and 1 year during the range 7 to 18 years old. The comparison of results showed that there was some crossover with the curve of the WHO study but with the first segment showing values that a few above and then below the WHO study curve with a node located at 3.5 years. The reason may be that the curve of WHO study was obtained with exponential smoothing (http://www.who.int/nutrition/media_page). Many studies in the developing countries compared children’s UAC to WHO references as well. Reports have shown that a ROC curve can be used for evaluation of malnutrition in children under the age of five and UAC by age has better sensitivity than height in Kenya, Africa . There were 26.8 % of children below 2 standard deviations from the mean in a study on children in Malaysia, Asia . Nevertheless, some results implied that the use of the international reference may lead to lower specificity and predictive value in the screening of malnourished children in developing countries such as Uganda . The average of children under 5 years old was at −2 standard deviations from the mean in a study carried out in Kenya . Therefore, that seems to need a local UAC curve for references.
Chinese children UAC curves integration
There were significant differences in adult UAC in China, because China is a large region and includes many nationalities. The Male Xinjiang Khalkhas, Tatar, and Xibo populations had a UAC of about 30 cm, which would be considered as brawnier. Ten southern ethnic groups, i.e., Hainan Han, Hui, Miao, DuLongZu, MuLaoZu, Buyi, Uighur, Tu, Mongolian, and Li had a UAC of about 24~25 cm, which would be considered as smaller. Women were divided into four types, and the strongest groups, in addition to the three of the male groups and ethnic Russians, had a UAC of about 28 cm. The Han, Uzbek, and Kazak urban and rural area populations in Jiangsu had a UAC of about 26 cm. Uely had a below average UAC of about 23~24 cm. The Hui, Soil, and Mongolian populations had a UAC of about 22~23 cm, which would be considered as thin [2–5, 20]. Asian children had less UAC than Caucasians . In the present study, Children UAC development showed some differences in different regions, but the mean gaps between the values of highest and lowest curve of each age groups were all less than the standard deviation of their own study, although those were narrow before puberty and wider in adolescence (Figs. 4 and 5). The charts of the curves showed the merged curve was feasible made by combined studies, and the ligature of weighted mean UAC of age phases is also thought as the mean value of the reference curve. The Z score values could be added as well.
Growth curve for each Z score
Many studies have supported the superiority of Z scores for UAC by age. Use of UAC with age- and sex- adjusted Z scores gave a better sensitivity than absolute cut of values in the identification of children wasting (acute undernutrition) [22–24]. Furthermore, regarding preschool obesity screening, the Z score of UAC by age seemed to be an appropriate alternative, which was better than UAC by height . In addition, Z scores for other anthropometric indicators such as weight have showed symmetry if cut small in the right tail with Z score analysis . UAC Z scores in this pooled analysis showed a normal distribution, similar to the curve for weight, which was a −2~+ 3 right normal curve. The index-smoothing trend showed some regularities with directions going up and down through various points in each Z trend. The development chart can be a representative reference for Chinese children and adolescents.