title: US pediatric population-level associations of DXA-measured percentage of body fat with four BMI metrics with cutoffs.
abstract: OBJECTIVE
  Four body mass index (BMI) metrics--BMI, BMI z-score, BMI percentile and BMI%--are commonly used as proxy measures for children's adiposity. We sought to determine a BMI metric that is most strongly associated with measured percentage of body fat (%BF) in the US pediatric population stratified by sex, age and race/ethnicity, and to determine cutoffs that maximize the association for each BMI metric. SUBJECTS, DESIGN AND METHODS: %BF was measured by dual-energy X-ray absorptiometry among N=6120 US boys and girls aged 8.0-17.9 years old from the National Health and Nutrition Examination Survey 1999-2004. We fit piecewise linear regression models with cutoffs to %BF data using each BMI metric as the predictor stratified by sex, race/ethnicity and age. The slopes were modeled differently before and after the cutoffs which were determined on the basis of grid searches.
  
  
  RESULTS
  BMI z-score was in general most strongly associated with %BF for both boys and girls. The associations of the four BMI metrics were lowest for boys aged 12-13.9 years and girls aged 16-17.9 years, and strongest for Mexican-American boys and for non-Hispanic Black girls. Overall, the associations were stronger for girls than for boys. In boys, BMI had the lowest association with %BF (R(2)=0.39) for all ages combined. The fold changes in slopes before and after cutoffs were greatest in general for BMI percentiles regardless of age, sex and race/ethnicity. BMI z-score cutoffs were 0.4 for both boys and girls for all ages combined. Except for BMI, the slopes after the cutoffs were in general greater than those before.
  
  
  CONCLUSIONS
  All BMI metrics were strongly associated with %BF when stratified by age and race/ethnicity except that BMI was the least associated with %BF in boys for all ages combined. Overall, BMI z-score was superior for evaluation of %BF, and its cutoff of 0.4 can also serve as a threshold for careful monitoring of weight status.
authors.affiliation: Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY, USA.
authors.forename: M
authors.initials: M
authors.lastname: Heo
authors.affiliation: Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY, USA.
authors.forename: J
authors.initials: J
authors.lastname: Wylie-Rosett
authors.affiliation: 1] University of Verona, Verona, Italy [2] Pennington Biomedical Research Center, Baton Rouge, LA, USA.
authors.forename: A
authors.initials: A
authors.lastname: Pietrobelli
authors.affiliation: Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY, USA.
authors.forename: G C
authors.initials: GC
authors.lastname: Kabat
authors.affiliation: Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, NY, USA.
authors.forename: T E
authors.initials: TE
authors.lastname: Rohan
authors.affiliation: University of North Carolina-Chapel Hill, Chapel Hill, NC, USA.
authors.forename: M S
authors.initials: MS
authors.lastname: Faith
mesh_terms: D015502:Absorptiometry, Photon; D000273:Adipose Tissue / Q000473:pathology*; D050154:Adiposity / Q000208:ethnology; D000293:Adolescent; D001741:Black or African American / Q000706:statistics & numerical data*; D017677:Age Distribution; D000367:Age Factors; D001823:Body Composition; D015992:Body Mass Index; D001835:Body Weight / Q000208:ethnology; D002648:Child; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D017058:Mexican Americans / Q000706:statistics & numerical data*; D009749:Nutrition Surveys; D012016:Reference Values; D015203:Reproducibility of Results; D017678:Sex Distribution; D012737:Sex Factors; D014481:United States / Q000453:epidemiology; D044465:White People / Q000706:statistics & numerical data*