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Int J Obes (Lond). 2005 Jan;29(1):1-8.

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

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Division of Nutrition and Physical Activity, Centers for Disease Control and Prevention K-26, Atlanta, GA 30341-3717, USA.



Although the body mass index (BMI, kg/m2) 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.


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/ht2) and the fat-free mass index (FFMI, fat-free mass/ht2).


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 > or =85th 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 > or =85th P.


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.

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