BMI does not capture the high fat mass index and low fat-free mass index in children with cerebral palsy and proposed statistical models that improve this accuracy

Background/objectives: Children with cerebral palsy (CP) are at risk for having a misclassified overweight/obesity status based on BMI thresholds due to their lower fat-free mass and similar fat mass compared with typically developing children. The primary objective was to determine if BMI could predict fat mass index (FMI) and fat-free mass index (FFMI) in children with CP. Subjects/methods: Forty-two children with CP and 42 typically developing children matched to children with CP for age and sex participated in the study. Dual-energy X-ray absorptiometry was used to assess body composition. Children with CP who could ambulate without assistance were considered ambulatory (ACP) and the rest were considered nonambulatory (NACP). Results: Children with CP had higher percent body fat (%Fat) and FMI and lower fat-free mass and FFMI than controls (p < 0.05) but no difference in fat mass (p = 0.10). When BMI was statistically controlled, NACP had higher %Fat, fat mass and FMI and lower FFMI than ACP and controls (p < 0.05). NACP also had lower fat-free mass than controls (p < 0.05). ACP had higher %Fat and FMI and lower fat-free mass and FFMI than controls (p < 0.05). BMI was a strong predictor of FMI (r ² = 0.83) and a moderately strong predictor of FFMI (r ² = 0.49) in children with CP (both p < 0.01). Prediction of FMI (R ² = 0.86) and FFMI (R ² = 0.66) from BMI increased (p < 0.05) when age, sex and ambulatory status were included. Conclusion: Compared with typically developing children, children with CP have a higher FMI and lower FFMI for a given BMI, which is more pronounced in NACP than ACP. The finding suggests that the prevalence of overweight/obesity status may be underestimated in children with CP.