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 Québec, Canada (n=557). Height and weight were measured by trained nurses at baseline (2008) and follow-up (2010). Metrics of BMI change were raw (ΔBMI(kg/m(2))), adjusted for median BMI (ΔBMI(percentage)) and age-sex-adjusted with the Centers for Disease Control and Prevention growth curves expressed as centiles (ΔBMI(centile)) or z-scores (ΔBMI(z-score)). Metrics of DEXA change were raw (total fat mass; ΔFM(kg)), per cent (ΔFM(percentage)), height-adjusted (fat mass index; ΔFMI) and age-sex-adjusted z-scores (ΔFM(z-score)). Spearman's rank correlations were derived. RESULTS: Correlations ranged from modest (0.60) to strong (0.86). ΔFM(kg) correlated most highly with ΔBMI(kg/m(2)) (r = 0.86), ΔFMI with ΔBMI(kg/m(2)) and ΔBMI(percentage) (r = 0.83–0.84), ΔFM(z-score) with ΔBMI(z-score) (r = 0.78), and ΔFM(percentage) with ΔBMI(percentage) (r = 0.68). Correlations with ΔBMI(centile) 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.