Body Composition Assessment in Pediatric Cystic Fibrosis in the Highly Effective Modulator Therapy Era—A Pilot Study

Background/Objectives: The prevalence of obesity in cystic fibrosis (CF) has increased following the introduction of highly effective modulator therapy (HEMT). However, the impact of increased adiposity on the lung function of pediatric patients with CF (ppwCF) in the era of HEMT remains incompletely understood. Furthermore, while the body mass index (BMI) is the most commonly used nutritional marker in CF care, it has important limitations and may not accurately reflect body composition. Methods: In this cross-sectional pilot study, ppwCF aged 5–18 years completed Bioelectrical Impedance Analysis (BIA) while a subset completed a dual-energy X-ray absorptiometry (DXA) scan and spirometry. Normality was assessed using Shapiro–Wilk tests. Correlations, categorical association, agreement tests, Firth penalized logistic regression, and exploratory linear regression of continuous FEV1% predicted were used to examine relationships among anthropometric measurements, body compositions, and pulmonary function. Results: Thirty-four individuals completed study procedures (19 female (56%); mean age 9.85 (SD 3.58)). BIA and DXA-derived body fat percentages were strongly correlated, and categorical agreement was moderate (Cohen’s Kappa 0.574; p-value 0.005). In age and sex-adjusted linear regression, BIA fat percentage (β = −0.23; 95% CI, −1.09 to 0.64; p = 0.595) and DXA fat percentage (β = −0.47; 95% CI, −2.14 to 1.21; p = 0.552) were not significantly associated with FEV1% predicted. Similarly, Firth logistic models showed no statistically significant association between the adiposity category and reduced lung function. Conclusions: BIA-derived body fat measurements correlate with DXA-derived measurements in ppwCF and demonstrate feasibility for use in routine CF clinic visits. This pilot cohort did not show a statistically significant relationship between higher adiposity and poorer pulmonary function. However, the wide confidence intervals indicate substantial uncertainty. Larger prospective studies are needed to evaluate longitudinal relationships between body composition and lung function in the HEMT era.