Estimating Fitness Bias in Body Mass Index of Middle School Students

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 bias should exist for non-elite athletes as well as fit individuals. This paper provides a methodology for indirectly estimating the size of the fitness bias in BMI using median physical activity performances. Approximately 30% of females and 33% of males are fit using this definition. Using data from 9 062 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 increases in weight are smaller than, but consistent with, the bias of more than 20% for elite athletes. Strong performance on individual physical activities decreases BMI percentile and BMI, but doing well on multiple physical activities has the reverse effect. This provides evidence of a fitness bias. BMI report cards should include the caveat that BMI may overstate the adiposity status of fit children.