Journal
INTERNATIONAL JOURNAL OF OBESITY
Volume 29, Issue 1, Pages 1-8Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/sj.ijo.0802735
Keywords
body mass index; X-ray densitometry
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Funding
- NIDDK NIH HHS [DK37352] Funding Source: Medline
- NATIONAL INSTITUTE OF DIABETES AND DIGESTIVE AND KIDNEY DISEASES [R01DK037352] Funding Source: NIH RePORTER
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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 &GE;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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