The contribution of fat and fat-free tissue to body mass index in contemporary children and the reference child.

BACKGROUND: Body mass index (BMI) is widely used to assess the prevalence of childhood obesity in populations, and to infer risk of subsequent obesity-related disease. However, BMI does not measure fat directly, and its relationship with body fatness is not necessarily stable over time.

OBJECTIVE: To test the hypothesis that contemporary children have different fatness for a given BMI value compared to the reference child of two decades ago.

DESIGN: Comparison of children from Cambridge, UK with the reference child of Fomon and colleagues (Am J Clin Nutr 1982; 35: 1169-1175).

SUBJECTS: A total of 212 children aged 1-10.99 y.

MEASUREMENTS: Body composition was assessed by deuterium dilution. Fat-free mass and fat mass were both adjusted for height to give fat-free mass index and fat mass index.

RESULTS: Contemporary Cambridge children have similar mean BMI values to the reference child. However, both boys and girls have significantly greater mean fatness and significantly lower mean fat-free mass than the reference child after taking height into account. Contemporary Cambridge children have greater fatness for a given BMI value than the reference child.

CONCLUSION: BMI-based assessments of nutritional status may be under-estimating the increase in children's fatness. Any change over time in the relationship between BMI and body fatness will create a mismatch between (1) current estimates of childhood obesity and (2) predicted risk of future adult illness, calculated on the basis of longitudinal cohorts recruited in childhood several decades ago. However, an alternative interpretation is that the reference data are inappropriate. Caution should therefore be used in generalizing from this study, and further investigations of the issue are required.