The Cross-Sectional Relation of Body Mass Index to Adiposity among 8- to 19-Year-Olds within Several Races and Ethnicities

Objective: To assess the screening ability of a high body mass index (BMI) for high adiposity among 8- to 19-year-olds.

Study design: This cross-sectional study included 6454 National Health and Nutrition Survey participants from 2011 through 2018. Fat and lean mass were measured with dual-energy X-ray absorptiometry. We expressed adiposity as fat mass index (FMI, kg ÷ m²) and %fat.

Results: Based on the Centers for Disease Control and Prevention 95th percentile, a high BMI correctly classified a high FMI for about 95% of participants in each racial and ethnic group. About 81% (Blacks) to 90% (Hispanics) of participants with a high BMI also had a high FMI. Further, children with a high BMI were 17 (Hispanics) to 46 (Blacks) times more likely to have a high FMI than those with a "normal" BMI. The screening ability of high BMI for high %fat was weaker because levels of %fat are influenced by both fat mass (numerator) and lean mass (denominator).

Conclusions: Despite differences in body composition, a high BMI is a very good screening tool for identifying high FMI not only among White 8- to 19-year-olds but also among Asians, Blacks, and Hispanics. Compared with %fat, FMI is likely a better adiposity metric among children and adolescents.