Impact of different vitamin D supplementation doses on vitamin D status and bone metabolism in preterm infants.

BACKGROUND: Preterm infants are at high risk for vitamin D deficiency due to limited transplacental transfer and increased postnatal requirements. Optimal vitamin D supplementation dosing in this population remains controversial. OBJECTIVE: To compare the effects of 400 IU/day versus 800 IU/day vitamin D supplementation on serum 25-hydroxyvitamin D [25(OH)D] levels, mineral metabolism, renal outcomes, and neonatal morbidities in preterm infants. METHODS: In this prospective cohort study, 66 preterm infants born at ≤ 34 weeks’ gestation and admitted to the neonatal intensive care unit (NICU) were followed. Infants were categorized into two groups according to the documented daily vitamin D supplementation regimen used in routine clinical practice (400 IU/day or 800 IU/day). Serum 25(OH)D, calcium, phosphorus, and alkaline phosphatase (ALP) levels were measured at admission and at 1 and 2 months postnatally. Renal ultrasonography and urinary calcium-to-creatinine (Ca/Cr) ratios were evaluated at 2 months. Neonatal morbidities, including bronchopulmonary dysplasia (BPD), were recorded. RESULTS: At admission, vitamin D deficiency or insufficiency (< 20 ng/mL) was highly prevalent in both groups. By postnatal 1 month, more than 90% of infants achieved vitamin D sufficiency, and all infants were sufficient by 2 months. The increase in serum 25(OH)D from admission to 2 months was significantly greater in the 800 IU/day group (p = 0.040). Calcium, phosphorus, and ALP levels remained within physiological ranges in both groups. No cases of vitamin D toxicity were observed, and renal ultrasonography findings were comparable between groups. In the unadjusted analysis, the incidence of BPD was lower in infants receiving 800 IU/day compared with those receiving 400 IU/day. CONCLUSION: Vitamin D supplementation with 800 IU/day appeared metabolically safe in preterm infants and resulted in greater increases in serum 25(OH)D levels compared with 400 IU/day. A lower incidence of BPD was observed in the higher-dose group, although randomized controlled trials are needed to confirm these findings. CLINICAL TRIAL REGISTRATION: Not applicable.