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Body mass index of children with attention-deficit/hyperactivity disorder.

We read with great interest the article by Dubnov-Raz et al published in this journal. The authors use a cross-sectional method to demonstrate the prevalence of overweight and obesity in children with attention deficit hyperactivity disorder (ADHD) and to examine changes in height and body mass index z scores of children in the ADHD group compared with the controls after a relatively long-term follow-up (17 months). The authors conclude that children with ADHD have lower rates of overweight and obesity compared with controls. In addition, methylphenidate treatment does not significantly affect overweight status. These results are inconsistent with most of previous studies regarding the same issue. This conflicting result is partly explained in the article. However, there are 3 issues not specifically addressed by the authors that should be taken into consideration and clarified when the conclusions are interpreted. The first issue is that the clinical characteristics between the control and ADHD groups are different in some items, which may foster many chances for the introduction of bias. The two groups may not be well-matched on some baseline characteristics, such as age (10.4 vs 8.8, P < .01) and sample size (275 vs 51). Therefore, we are concerned how well matched the ADHD and control groups are on other key characteristics that may confound the results. The children in different age groups have different preferred foods and physical activity levels that would influence the balance of calorie input and consumption. Furthermore, absence of demographic data and markers of socioeconomic status of the participants prevents an analysis of possible mediating factors. Children of different socioeconomic status have different lifestyles and diets, which will greatly affect the prevalence of obesity in these populations. Therefore, lack of demographic information and unmatched clinical characteristics can mislead the conclusion. The second issue, as stated by the authors, is that the present study is not conducted in children who were diagnosed with ADHD for the first time and were medication-naive, which does not minimize the potential influence of drugs on the growth status. The body of literature shows that medications for ADHD would lead to weight reduction. So, if the recruited children with ADHD have been prescribed psychostimulants for a long time, the indexes of weight and body mass index will be changed and not reflect the real growth status of the ADHD group. This selection bias would confound the results of prevalence of obesity in ADHD group. The third issue is the relatively small sample size of children (n 1⁄4 28) with a follow-up of 1 year. Although there is no statistically significant change in mean body mass index z scores between baseline and after methylphenidate treatment (P 1⁄4 .53), the mean body mass index z scores are greatly lower post treatment (1.73 vs 1.47). According to the Centers for Disease Control and Prevention growth charts, a subject with a mean body mass index z score of 1.73 at baseline is considered ‘‘obese,’’ whereas a subject with a score of 1.47 post methylphenidate treatment is considered ‘‘overweight.’’ From this point, the mean body mass index z scores are changed greatly after medication for 1 year. However, the differences have no statistical significance, probably due to the small sample size, which lessens the power of the test. These 3 issues should be taken into consideration when the results are interpreted. Rongwang Yang, MD The Children’s Hospital Zhejiang University School of Medicine, Hangzhou, Zhejiang Province, China