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

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Abstract

OBJECTIVE:

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.

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

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 ≥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 ≥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.

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Author information

Authors and Affiliations

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

    D S Freedman, L M Maynard, Z Mei & W H Dietz

  2. Body Composition Unit, Department of Medicine, Obesity Research Center, St Luke's-Roosevelt Hospital, NY, USA

    J Wang, J C Thornton, R N Pierson Jr & M Horlick

  3. Children's Hospital of New York, Columbia University, NY, USA

    M Horlick

Authors
  1. D S Freedman
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  2. J Wang
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  3. L M Maynard
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  4. J C Thornton
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  5. Z Mei
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  6. R N Pierson Jr
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  7. W H Dietz
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  8. M Horlick
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Corresponding author

Correspondence to D S Freedman.

Additional information

Supported by NIH Grant DK37352.

For overnight deliveries: Rhodes Bldg, Rm 5161, 3005 Chamblee-Tucker Rd, Atlanta, GA 30341-4133, USA.

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Freedman, D., Wang, J., Maynard, L. et al. Relation of BMI to fat and fat-free mass among children and adolescents. Int J Obes 29, 1–8 (2005). https://doi.org/10.1038/sj.ijo.0802735

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  • DOI: https://doi.org/10.1038/sj.ijo.0802735

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