Vitamin D Status Among Infants Aged 1–5 Months in Northwestern Romania: A Single-Center Study and Implications for Infant Care and Management

Background/Objectives: Vitamin D is essential for calcium and phosphorus homeostasis and bone mineralization in infancy; serum 25-hydroxyvitamin D [25(OH)D] is its main biomarker. In Romania, data on vitamin D status among infants younger than six months remain limited, despite evidence of suboptimal status in other Romanian pediatric and perinatal populations. This study aimed to assess vitamin D status in infants aged 1–5 months in northwestern Romania and its associations with supplementation dose, feeding type, and season of blood sampling. Methods: This single-center observational study included infants aged 1–5 months. Serum 25(OH)D was interpreted as deficient (<12 ng/mL), insufficient (12–<20 ng/mL), sufficient (20–100 ng/mL), or potentially excessive (>100 ng/mL). Statistical analyses used parametric or non-parametric methods as appropriate to data distribution. Results: The cohort included 124 girls (56.6%) and 95 boys (43.4%; median age 4 [3–5] months). Median serum 25(OH)D was 61.24 [49.53–75.32] ng/mL; 94.5% of infants had sufficient concentrations, deficiency and insufficiency each occurred in 0.9%, and 3.7% had potentially excessive concentrations. Serum 25(OH)D increased progressively with reported supplementation dose (p < 0.0001) and differed by feeding type (p = 0.0279), with breastfed infants having higher concentrations than formula-fed infants. No significant seasonal difference was observed (p = 0.4218). Conclusions: In this hospital-based cohort, most infants had sufficient 25(OH)D concentrations, in a context of near-universal supplementation. Higher reported doses were associated with higher 25(OH)D concentrations, and concentrations >100 ng/mL occurred almost exclusively in infants receiving 2000 IU/day. Without data on calcium metabolism, these concentrations do not indicate vitamin D toxicity, but they support adherence to recommended prophylactic doses.