Densitometry Versus Bioimpedance for Modeling Vitamin D–Endocrine and Metabolic Associations in Pediatric Obesity: A Cross-Sectional Parallel-Modality Analysis

<b>Background/Objectives</b>: It has been previously shown that bioimpedance assessment (BIA) systematically underestimates adiposity compared to densitometry analysis (DXA), though the methods correlate strongly. However, whether DXA outperforms BIA for physiology modeling-using vitamin D as a sentinel signal-remains uncertain. We compared DXA and BIA side-by-side to model (i) adiposity-25(OH)D associations, (ii) mediation-style links with metabolic outcomes, and the vitamin D-PTH-calcium axis. <b>Methods</b>: We performed a cross-sectional analysis of 165 children with simple obesity and no vitamin D prophylaxis collected between July 2022 and July 2025. We measured adiposity through DXA and BIA methods, laboratory 25(OH)D, and associated biochemical and clinical parameters: PTH, calcium, phosphate, glucose/insulin/HOMA-IR, lipids. Information on age, sex, and season was recorded and used to adjust for potential covariates. Parallel analyses included partial correlations, linear regression, mediation models, and Bland-Altman analysis for DXA-BIA agreement. <b>Results</b>: The cohort median age was 13 years; median 25(OH)D level was 21.9 ng/mL. DXA fat % exceeded BIA (46.6% vs. 36.7%). Univariately, 25(OH)D correlated inversely with adiposity (DXA rho = -0.16, BIA rho = -0.19), but adiposity was not a significant determinant of 25(OH)D after season/age adjustment with either modality. No mediation of vitamin D to metabolic associations via adiposity were detected. The vitamin D-PTH-calcium axis was robust across modalities. <b>Conclusions</b>: In children with established obesity, seasonal and age factors dominate 25(OH)D variability, while the adiposity contributes little within-group. Vitamin D shows endocrine but not metabolic associations, and within this homogenous pediatric obesity cohort, DXA does not outperform BIA for physiologic modeling.