Body fat percentages measured by dual-energy X-ray absorptiometry - Body Composition - diagnosing childhood obesity

Nutrition Research Newsletter,  Feb, 2003  

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Although considerable support exists for the use of body mass index as an index of overweight in childhood, the most appropriate cutoffs with which to define overweight at this time of life have not been agreed upon internationally. An expert committee recommended percentile cutoffs based on data from the first National Health and Nutrition Examination Survey, but the use of age-related percentiles has been criticized because the prevalence of overweight is set at a fixed proportion of the population at one point in time. An alternative approach would be to determine the percentiles in late adolescence that correspond to the well-accepted BMI values that denote overweight and obesity in adults. Because the BMI percentile allocation tends to track in childhood, these percentiles could be used to define overweight and obesity in subjects of all ages, thus ensuring continuity in recommended cutoffs from youth to adulthood. Although BMI is strong correlated with adiposity in children, it may not be suitable for use in some subgroups of the pediatric population, because of its inherent status as a weight-based index. The objective of the current study was to estimate the percentage of body fat (%BF) values typically associated with these BMI cutoffs in children and adolescents.

The %BF was measured by dual-energy X-ray absorptiometry in 661 subjects (49% male) aged 3 to 18 y. Regression equations using BMI, age, and sex were developed to predict the %BF associated with BMI cutoffs for overweight (age-specific BMI equivalent to a BMI of 25 in an 18-y-old) and obesity (age-specific BMI equivalent to a BMI of 30 in an 18-y-old) over this age range.

Measurements classified 17.1% of males and 19.8% of females as overweight and 5.5% of males and 7.5% of females as obese. The %BF associated with an obese BMI tended to be higher in peripubertal males than in younger or older males. Although the predicted %BF of young females was similar to that of young males, values rose steadily with age, such that an 18-y-old female with a BMI of 30 had an estimated %BF of 42%, whereas that in males of similar age was 27%.

In conclusion, the %BF values associated with BMI classifications of overweight and obesity vary considerably with age in growing children, particularly in girls.

R Taylor, I Jones, S Williams, A Goulding. Body fat percentages measured by dual-energy X-ray absorptiometry, corresponding to recently recommended body mass index cutoffs for overweight and obesity in children and adolescents aged 3-18 y. Am J Clin Nutr 76:1416-1421 (December 2002) [Correspondence: RW Taylor, Department of Human Nutrition, University of Otago, PO Box 56, Dunedin, New Zealand. E-mail: reachel.taylor@stonebow.otago.ac.nz]

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