Is the adiposity rebound a rebound in adiposity?

OBJECTIVE: Early adiposity rebound ([AR], when body mass index [BMI] rises after reaching a nadir) strongly predicts later obesity. We investigated whether the upswing in BMI at AR is accompanied by an increase in body fat. DESIGN: Community-based cohort study. SUBJECTS: A total of 299 first-born children (49% male). Measurements. Six-monthly anthropometry and bioelectrical impedance, 4-6.5 years; lean and fat mass index (kg/m(2)) for direct comparison with BMI. Supplementary (0-2 years) weight and length measures (needed for growth curve modelling) were drawn from subjects' child health records. METHODS: AR was estimated from individually modelled BMI curves from birth to 6.5 years. Two main analyses were performed: 1) cross-sectional comparisons of BMI, fat mass index (FMI), lean mass index (LMI) and percent body fat in children with early (<5 years) and later (>5 years) rebound; and 2) investigation of linear trends in BMI, FMI, LMI and percent body fat before and after AR. Results. The 81 children (27%) experiencing early AR had higher BMI, FMI, LMI and percent fat at 6.5 years. Overall, FMI decreased steeply pre-AR, at -0.56 (0.02) kg/m(2) per year (mean [Standard Error]), then flattened post-AR to 0.07 (0.05) kg/m(2) per year. In contrast, LMI increased pre-AR (0.34 [0.01]) and steepened post-AR (0.47 [0.03] kg/m(2) per year). CONCLUSION: The 'adiposity rebound' is characterised by increasing lean mass index, coupled with cessation of the decline in fat mass index. Understanding what controls the dynamics of childhood body composition and mechanisms that delay AR could help prevent obesity.