A Prospective Cross-Sectional Study on the Vitamin D Status of Neonates and the Impact of Neonates' Standard Vitamin D Supplementation on Neonatal Morbidities

(1) Background: This study aimed to determine whether standard-dose vitamin D supplementation could provide adequate levels in exclusively breastfed infants born with different cord 25(OH)D levels and to evaluate related neonatal morbidities. (2) Methods: A prospective cross-sectional study was designed in term infants. Three groups were formed based on cord 25(OH)D levels; Group1 Deficiency:25(OH)D < 12 ng/mL, Group2 Insufficiency:25(OH)D = 12-19 ng/mL, and Group3 Optimum:25(OH)D = 20-100 ng/mL. Cord and 1st month 25(OH)D levels, after receiving standard-dose vitamin D prophylaxis, neonatal outcomes and anthropometric measurements were compared. (3) Results: The study involved 86 infants. Group1 Deficiency had also significantly lower 25(OH)D levels at 1st month compared to the others (p < 0.001). There was a significant positive correlation between cord and 1st month 25(OH)D levels (r = 0.78, p < 0.001). Despite the fact that the mother's age and clothing style were similar, Group1 Deficiency mothers had higher parity numbers and used less vitamin D during pregnancy (p = 0.03, p = 0.04). Neonatal outcomes analysis revealed that newborns in Group1 Deficiency experienced more respiratory distress, transient tachypnea, and early-onset sepsis, as well as more and longer hospital stays in the first-month of life (p < 0.05). (4) Conclusions: Infants with low cord 25(OH)D levels had also lower 25(OH)D values in the 1st month of life and experienced higher rates of neonatal morbidities. Given its numerous biological activities and impact on neonatal morbidities, determining an individualized dose of vitamin D supplementation may be more accurate than using the standard approach.