Relation of BMI to fat and fat-free mass among children and adolescents

Objective: Although the body mass index (BMI, kg/m(2)) is widely used as a surrogate measure of adiposity, it is a measure of excess weight, rather than excess body fat, relative to height. We examined the relation of BMI to levels of fat mass and fat-free mass among healthy 5- to 18-y-olds. Methods and Procedures: Dual-energy X-ray absorptiometry was used to measure fat and fat-free mass among 1196 subjects. These measures were standardized for height by calculating the fat mass index (FMI, fat mass/ht(2)) and the fat-free mass index (FFMI, fat-free mass/ht(2)). Results: The variability in FFMI was about 50% of that in FMI, and the accuracy of BMI as a measure of adiposity varied greatly according to the degree of fatness. Among children with a BMI-for-age greater than or equal to85th P, BMI levels were strongly associated with FMI (r=0.85-0.96 across sex-age categories). In contrast, among children with a BMI-for-age <50th P, levels of BMI were more strongly associated with FFMI (r=0.56-0.83) than with FMI (r=0.22-0.65). The relation of BMI to fat mass was markedly nonlinear, and substantial differences in fat mass were seen only at BMI levels ≥85th P. Discussion: BMI levels among children should be interpreted with caution. Although a high BMI-for-age is a good indicator of excess fat mass, BMI differences among thinner children can be largely due to fat-free mass.