DXA-Derived Total Body Fat Percentage and Serum 25(OH)D in Overweight and Obese Children: A Cross-Sectional Study

Background/Objectives: Lower serum 25-hydroxyvitamin D [25(OH)D] is common in pediatric overweight and obesity, but it remains unclear whether direct adiposity assessment adds information beyond an age- and sex-adjusted model containing BMI z-score and season. We tested whether DXA-derived total body fat percentage is associated with 25(OH)D and whether it adds explanatory power to that model. Methods: We analyzed cross-sectional data from 768 children/adolescents (10–18 years) with overweight/obesity. Anthropometry was available for all; DXA data for 489. The primary analysis used complete-case DXA data (n = 485). Vitamin D supplementation status was not available. Results: Participants with and without DXA were similar in age, sex, BMI z-score, and serum 25(OH)D, but blood sampling season differed (p < 0.001). Adding DXA-derived body fat percentage to the age- and sex-adjusted base model increased R2 from 0.0498 to 0.0657 (ΔR2 = 0.0159). Within the complete-case DXA subgroup (n = 485), DXA-derived total body fat percentage was inversely associated with 25(OH)D after adjustment for BMI z-score, season, age, and sex (B = −0.265; 95% CI, −0.448 to −0.082; p = 0.005). BMI z-score was significant in the base model but not after adjustment for DXA fat percentage. Conclusions: DXA-derived total body fat percentage added explanatory value for serum 25(OH)D beyond BMI z-score and season, but the incremental gain was small (ΔR2 = 0.0159). The findings are consistent with BMI z-score acting as a less direct proxy for the adiposity compartment that DXA quantifies more specifically. Causal interpretation is precluded by the cross-sectional design, unknown supplementation status, and non-random DXA availability.