Body composition data show that high body mass index centiles over-diagnose obesity in children under 6 years

Background: Most authorities define childhood overweight / obesity as body mass index (BMI) exceeding the same high centile cut-off throughout childhood, but it seems unlikely that true obesity prevalence (excess body fat) is constant though childhood. Objective: We investigated how fat mass and lean mass, adjusted for height, relate to BMI and each other across childhood, using a uniquely large database of body composition measures, estimated using gold standard methods. Design: Cross-sectional and cohort data were collated from representative samples of healthy children aged 6 weeks to 20 years and children attending obesity clinics aged 7-16 years. Body composition was measured by deuterium dilution up to age 4 years, and by either deuterium or the criterion 4-component model from 4-20 years. Fat and lean mass were expressed as fat mass index (FMI; fat mass/height2) and lean mass index (LMI; lean mass/height2). Results: There were 2367 measurements of weight, height, and body composition from 1953 individuals. Before age 6 years, the variability in FMI, LMI and BMI was much less than after, FMI was low (mainly <8 kg/m2) and FMI and LMI were weakly negatively correlated. From mid-childhood, upper limits for both BMI and FMI rose, but FMI in children with BMI < 91st centile still rarely exceeded 8 kg/m2. With increasing age, the correlation of FMI with LMI rose to 0.5-0.7, driven mainly by children with a high FMI also having a high LMI. Conclusions: Raised fat levels are much less common at young compared to older ages, and young children with a high BMI centile have lower FMI than older children with the same BMI centile. Current BMI centile cut-offs thus over-diagnose obesity in younger groups. More stringent cut-offs are required for children under 6 years, matching the World Health Organization recommendation for 0-5 years.