Fat and lean BMI reference curves in children and adolescents and their utility in identifying excess adiposity compared with BMI and percentage body fat

Abstract

Background: Body mass index (BMI) and percentage body fat (%BF) are widely used to assess adiposity. These indexes fail to account for independent contributions of fat mass (FM) and lean body mass (LBM) to body weight, which vary according to age, sex, pubertal status, and population ancestry in the pediatric population.

Objective: The objective was to develop pediatric reference curves for fat mass index (FMI) and lean body mass index (LBMI) and evaluate the effects of population ancestry and LBM on measures of excess adiposity (BMI, %BF, and FMI).

Design: Sex-specific FMI and LBMI reference curves relative to age for children and adolescents aged 8-20 y were generated from cross-sectional body-composition data measured by dual-energy X-ray absorptiometry from NHANES.

Results: The mean LBMI z score was higher in blacks (males: 0.26; females: 0.45) than in whites (males: -0.07; females: -0.09) and Mexican Americans (males: 0.05; females: -0.09). The positive predictive value of overweight by BMI to identify excess adiposity defined by FMI was lower in blacks (males: 35.9%; females: 30.3%) than in whites (males: 65.4%; females: 52.2%) and Mexican Americans (males: 73.3%; females: 68.3%). Participants classified as having excess adiposity by FMI but normal adiposity by %BF had significantly higher BMI, LBMI, and height z scores than did those classified as having excess adiposity by %BF but normal adiposity by FMI.

Conclusions: Relative to FMI, the prevalence of excess adiposity is overestimated by BMI in blacks and underestimated by %BF in individuals with high LBM. The use of FMI and LBMI improves on the use of %BF and BMI by allowing for the independent assessment of FM and LBM.

FIGURE 1.

Reference curves for FMI in males and females; 5th, 10th, 25th, 50th, 75th, 90th, and 95th centiles are shown. FMI, fat mass index.

FIGURE 2.

Reference curves for LBMI in males and females; 5th, 10th, 25th, 50th, 75th, 90th, and 95th centiles are shown. LBMI, lean BMI.

FIGURE 3.

Mean (±SE) BMI-Z, LBMI-Z, and height-Z in NHANES participants concordantly and discordantly classified as having excess adiposity by FMI and %BF. BMI-Z (*), LBMI-Z (**), and height-Z (***) were all significantly greater in male and female participants classified as having excess adiposity by FMI but normal adiposity by %BF than in those classified as having excess adiposity by %BF but normal adiposity by FMI (P < 0.001). Two-sample t tests were used to determine significance. n = 7095. BMI-Z, BMI z score; height-Z, height z score; FMI, fat mass index; LBMI-Z, lean BMI z score; %BF, percentage body fat.