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/m²), 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/m² (r = 0.86), ΔFMI with ΔBMI_kg/m² 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.