Which metric of relative weight best captures body fatness in children?
OBJECTIVE
To evaluate the relative merits of BMI (kilograms per meter squared) and age- and gender-adjusted BMI, age- and gender-specific z score of BMI, and age- and gender-specific percentiles of BMI as surrogate measures of body fatness among a sample of youth.


METHODS
The sample comprised 596 children and adolescents 5 to 18.7 years old and was 40% male and 55% white. Height and weight were measured by trained research staff. DXA was used to determine body fat mass. BMI, age- and gender-specific percentile of BMI, and age- and gender-specific z scores of BMI were computed, and these metrics were compared with measured body fatness.


RESULTS
The BMI values in the sample ranged from 12.9 to 55.0 kg/m(2), with a mean of 24.9 kg/m(2). The Spearman correlations with percentage body fat were similar for all of the BMI metrics (r = 0.82 to 0.88). Linear regression models with age- and gender-specific percentiles of BMI explained significantly less of the variance (65%) than models with log-transformed BMI (81%) or age- and gender-specific z scores of BMI (75% to 79%). z scores were the most accurate at classifying children who were overfat (sensitivity = 0.84, specificity = 0.96 for z score > or = 1). However, using a BMI > or = 85th percentile or a BMI > or = 20 kg/m(2) was also accurate at classifying youth.


CONCLUSIONS
The BMI metrics had similar correlations with body fatness, but age- and gender-specific percentiles of BMI were the least accurate proxy measure of body fatness. However, a BMI z score > or = 1, BMI percentile > or = 85, and BMI> or = 20 kg/m(2) are all useful for identifying children who may be overfat.
Field
Alison E
AE
Division of Adolescent Medicine, Department of Medicine, Children's Hospital Boston and Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA. Alison.Field@TCH.harvard.edu
Laird
Nan
N
Steinberg
Emily
E
Fallon
Erica
E
Semega-Janneh
Mariama
M
Yanovski
Jack A
JA
D000273:Adipose Tissue; D000293:Adolescent; D000367:Age Factors; D001823:Body Composition; D015992:Body Mass Index; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D009765:Obesity / Q000175:diagnosis; D011627:Puberty; D012016:Reference Values; D012737:Sex Factors