Estimating Fitness Bias in Body Mass Index of Middle School Students

Background. Since muscle is more dense than fat, athletes tend to have greater mass and BMI than similarly sized non-athletes. Comparing direct adiposity measures and BMI confirms that BMI is a biased proxy for adiposity for elite athletes. A similar, but less pronounced bias should exist for athletes of non-elite status. More generally, BMI may overstate the adiposity status of physically fit individuals.

Methods. This paper provides a methodology for estimating the size of the fitness bias in BMI using physical activity performances. Students were placed in an eight cell partition based on their performances on three physical activities. Various definitions of fit and not-fit based on this partition are examined. Middle school students are categorized as fit if their performances exceed median mile run and push-up levels by sex × grade. Approximately 30% of females and 33% of males are fit using this definition.

Results. Using data from 9062 students, regressions suggest 3.1, 95% CI [0.9, 5.3], of a female's BMI percentile of 85, and 3.6% of her weight, CI [1.6%, 5.6%], is due to being fit, but 5.6, CI [3.3, 7.9], of a male's BMI percentile of 85, and 5.9% of his weight, CI [3.9%, 7.9%], is due to being fit. These indirectly measured increases in weight are smaller than, but consistent with, the bias of more than 20% obtained via direct comparisons of adiposity with BMI for elite athletes.

Conclusions. BMI report cards should include the caveat that BMI may overstate the adiposity status of fit children.