Identifying the best body mass index metric to assess adiposity change in children

Objective Although dual-energy X-ray absorptiometry (DEXA) is the preferred method to estimate adiposity, body mass index (BMI) is often used as a proxy. However, the ability of BMI to measure adiposity change among youth is poorly evidenced. This study explored which metrics of BMI change have the highest correlations with different metrics of DEXA change. Methods Data were from the Quebec Adipose and Lifestyle Investigation in Youth cohort, a prospective cohort of children (8-10 years at recruitment) from Quebec, Canada (n=557). Height and weight were measured by trained nurses at baseline (2008) and follow-up (2010). Metrics of BMI change were raw (Delta BMIkg/m2), adjusted for median BMI (Delta BMIpercentage) and age-sex-adjusted with the Centers for Disease Control and Prevention growth curves expressed as centiles (Delta BMIcentile) or z-scores (Delta BMIz-score). Metrics of DEXA change were raw (total fat mass; Delta FMkg), per cent (Delta FMpercentage), height-adjusted (fat mass index; Delta FMI) and age-sex-adjusted z-scores ( Delta FMz-score). Spearman's rank correlations were derived. Results Correlations ranged from modest (0.60) to strong (0.86). Delta FMkg correlated most highly with Delta BMIkg/m2 (r =0.86), Delta FMI with Delta BMIkg/m2 and Delta BMIpercentage (r =0.83-0.84), Delta FMz-score with Delta BMIz-score (r =0.78), and Delta FMpercentage with Delta BMIpercentage (r =0.68). Correlations with Delta BMIcentile were consistently among the lowest. Conclusions In 8-10-year-old children, absolute or per cent change in BMI is a good proxy for change in fat mass or FMI, and BMI z-score change is a good proxy for FM z-score change. However change in BMI centile and change in per cent fat mass perform less well and are not recommended.