BMI, Vitamin D Status, and Biochemical Markers of Mineralization in Children with Low-Energy Fractures: A Prospective Case–Control Study

Background: Pediatric fractures are common, but the relative contributions of BMI-related body size and vitamin D–calcium metabolism remain uncertain. This study examined anthropometric and biochemical factors associated with fracture status in otherwise healthy children after low-energy trauma. Methods: This prospective case–control study included 100 children aged 3–15 years: 50 with radiographically confirmed fractures and 50 trauma controls without fracture. BMI and age- and sex-standardized BMI measures were recorded. Serum 25OHD, PTH, ALP, calcium, and phosphate were measured within seven days of injury, and dietary calcium intake was estimated. Associations were evaluated using correlation analyses, logistic regression, and BMI-adjusted analysis of covariance. Results: Fracture cases had higher BMI z-scores than controls (median 0.53 [IQR −0.53 to 1.44] versus −0.16 [IQR −0.89 to 0.38]; p = 0.006) and lower 25OHD concentrations. After adjustment for BMI z-score, the estimated geometric mean 25OHD concentration was 58.0 nmol/L (95% CI 53.0–64.1) in cases and 66.7 nmol/L (95% CI 60.3–73.7) in controls (p = 0.046). The Group × BMI z-score interaction was not significant (p = 0.327). Vitamin D deficiency (<30 nmol/L) affected only 3% of participants, and consistent biochemical abnormalities of mineralization were uncommon. Conclusions: Fracture status was associated with higher standardized BMI measures and lower 25OHD concentrations, but there was no evidence that the BMI–25OHD relationship differed by fracture status. These observational findings do not establish a predominant causal mechanism and support cautious, clinically targeted metabolic evaluation.