Classifying preadolescent boys based on their weight status and percent body fat produces different groups.

This cross-sectional pilot study investigated the relationship between weight status using body mass index (BMI) and percent body fat in preadolescent boys and examined grouping of boys based on BMI and percent body fat. Height and weight were measured to calculate BMI in 50 white 8- to 10-year-old preadolescent boys. Percent body fat was measured using air-displacement plethysmography. Boys were grouped based on their BMI status into an average BMI group and a high BMI group and based on their percent body fat into a lower percent body fat group and an upper percent body fat group. Boys' participation on sports teams and in recreational classes or lessons and boys' sedentary activity was reported by their mothers. Boys in the upper percent body fat group participated in more hours of sedentary activity than boys in the lower percent body fat group (P<0.05). Although percent body fat was significantly correlated with BMI (r=0.74, P<0.0001), a significant difference was found in the classification of boys into groups (chi(2)=13.52, P<0.0001). Six of the 25 boys in the high BMI group were also classified in the lower percent body fat group, and six of the 25 boys in the average BMI group were also classified in the upper percent body fat group, indicating that six boys from each group were mislabeled. Practitioners should use BMI as a screening tool and recognize its limitations for assessing adiposity.