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

Abstract 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.