Screening Accuracy of BMI for Adiposity Among 8- to 19-Year-Olds

Abstract

Background and objectives: Although the limitations of BMI have long been recognized, there are recent concerns that it is not a good screening tool for adiposity. We therefore examined the cross-sectional relation of BMI to adiposity among 6923 8- to 19-year-olds in the National Health and Nutrition Survey from 2011 through 2018.

Methods: Participants were scanned with dual-energy x-ray absorptiometry. Adiposity was expressed as fat mass index (FMI, fat mass kg ÷ m2) and percentage of body fat (%fat). Lean mass was expressed as lean mass index (LMI, lean mass ÷ m2). Regression models and 2 × 2 tables were used to assess the relation of BMI to FMI, %fat, and LMI.

Results: Age and BMI accounted (R2) for 90% to 94% of the variability of FMI and LMI in each sex. Associations with %fat were weaker (R2s ∼0.70). We also examined the screening abilities of a BMI ≥ Centers for Disease Control and Prevention 95th percentile for high levels of adiposity and LMI. Cut points were chosen so that prevalences of high values of these variables would be similar to that for high BMI. Of participants with a high BMI, 88% had a high FMI, and 76% had a high %fat. Participants with a high BMI were 29 times more likely to have a high FMI than those with lower BMIs; comparable relative risks were 12 for high %fat and 14 for high LMI.

Conclusions: Despite its limitations, a high BMI is a very good screening tool for identifying children and adolescents with elevated adiposity.

FIGURE 1

Relation of BMI, FMI, %fat, and LMI to age. Note that the y axes differ across panels. Lines were smoothed with locally weighted scatterplot smoother, using a span of 0.3, and accounted for the sample weights (see Methods).

FIGURE 2

Prediction of FMI by BMI among 300 16-year-old girls. The left panel shows differences in FMI from the mean FMI for this sex/age. The right panel is a Bland-Altman plot showing the differences between DXA-calculated FMI and BMI-predicted FMI. The orange circle represents a girl with an FMI of 28.3 kg/m² and a BMI of 55.3.

FIGURE 3

Probability of a high value of FMI, LMI, and %fat by extended BMI-for-age z-score among participants with a high BMI. Logistic regression was used to model the relation of the 3 metrics to BMIz within each sex and to obtain the predicted probabilities. BMI percentiles corresponding to the extended z-scores are shown at the bottom of the figure.