Bidirectional changes in serum 25-hydroxyvitamin D in newborns during the first four days of life: A longitudinal cohort study

BACKGROUND & AIMS: Little is known about short-term vitamin D dynamics during the immediate neonatal period. Most neonatal studies have relied on single-point measurements of 25-hydroxyvitamin D (25(OH)D), limiting understanding of early postnatal changes. In Japan, healthy full-term infants typically remain hospitalized for 4-5 days, receiving minimal vitamin D intake from small amounts of formula and experiencing virtually no sunlight exposure. This clinical setting provides a unique opportunity to investigate early postnatal vitamin D dynamics under conditions of minimal exogenous input. This study aimed to examine short-term changes in serum 25(OH)D from birth to day 4 of life (Δ25(OH)D) and determine how baseline vitamin D status influences these changes. METHODS: We analyzed 40 infants with paired cord blood and day 4 serum 25(OH)D measurements obtained by chemiluminescence immunoassay. Linear regression evaluated the association between cord blood 25(OH)D concentrations and Δ25(OH)D. RESULTS: Estimated vitamin D intake during hospitalization was 64 ± 18 IU/day, substantially below the 400 IU/day recommended by the American Academy of Pediatrics. Baseline-dependent bidirectional changes in serum 25(OH)D were observed: infants with lower cord blood concentrations increased, whereas those with higher baseline concentrations declined. Cord blood 25(OH)D concentrations were strongly inversely correlated with Δ25(OH)D (r = -0.745, p < 0.001). Regression analysis indicated an equilibrium point of approximately 21.4 ng/mL, at which no net change in serum 25(OH)D was expected. CONCLUSION: This study provides the first quantitative description of short-term changes in neonatal 25(OH)D after birth and suggest dynamic physiological regulation of vitamin D during early life.