How Accurate Are Systems that Evaluate Fatness in Teens?

Reprinted from Eating Disorders Review
January/February 2005 Volume 16, Number 1
©2005 Gürze Books

Despite widespread publicity and concern about the marked increase in obesity-related problems among the young, no universally accepted classification system currently exists for measuring adolescent obesity. Body mass index, or BMI (kg/m2), is widely used for classifying adult overweight and obesity, but its use with younger populations is still controversial. Among children, BMI is more limited than in adults because it varies with age, sex, and maturation. In addition, for all age groups, relative risks associated with certain BMI values seem to depend upon specific characteristics of individual populations.

International groups urge separate criteria for children, teens

Currently, several classifications systems have been proposed, such as that set forth by the International Obesity Task Force (IOTF) and the World Health Organization (WHO). Both organizations have recommended separate international classification systems to measure childhood and adolescent obesity.

With all this in mind, Dr. Martin G. Neovius and a team of researchers at the Karolinska Institute, Stockholm, designed a cross-sectional analysis of 474 17-year-olds (Am J Clin Nutr 2004;80:597). Body density was measured with densitometry. The international BMI classifications used by the IOTF and WHO were evaluated on the basis of their sensitivity and specificity for detecting excess body fat. True positives were determined by using the percentage body fat cutoffs proposed by Williams et al. (Am J Public Health 1992;82:358).

Lack of accuracy for female adolescents

The authors reported that recommended international classification systems have a very high specificity for both sexes, which results in few cases of non-overweight adolescents being mislabeled as overweight. However, the sensitivity is very low in female adolescents. Thus, many overweight female adolescents could be missed in intervention programs that use the international BMI cutoffs as selection criteria.

The authors point out that the diagnostic demands on a classification system intended for use in clinical practice are different from those on systems intended for public health use or for monitoring. In addition, an international system of reference is a compromise to obtain acceptable, comparable prevalence estimates at a global level. At a national level, given probable population differences in relative risks at certain BMI values and the seriousness of the increasing problem of adolescent obesity, customized systems derived from national data are likely to be more efficient. Such systems should be developed, according to Dr. Neovius and colleagues.

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