Dynamics of Vitamin D metabolism in maternal-fetal dyads

Objectives: Metabolites of vitamin D in maternal-neonatal dyads remain relatively unexplored. The goal of this study was to evaluate concentrations of 25(OH)D₃, 24,25(OH)₂D₃, and 3-epi-25(OH)D3 in maternal-infant pairs at delivery. Methods: Serum samples of maternal and infant cord blood were collected on 131 mother-infant pairs at delivery. Vitamin D metabolites were analyzed in triplicate using liquid chromatography-tandem mass spectrometry. Statistical analysis was conducted using the Fisher exact test, Wilcoxon rank sum test, and Spearman correlation coefficients. Results: Mean 25(OH)D₃ concentrations in maternal and cord blood were 32.9 and 18.5 ng/mL, respectively; mean maternal and cord 24,25(OH)2D₃ were 2.0 versus 1.1 ng/mL, respectively. Absolute concentrations of 3-epi-25(OH)D3 were similar in maternal and cord samples (2.4 vs 2.2 ng/mL), whereas the proportion of the total 25(OH)D as the 3-epimer was 6.5% in maternal samples and 10.5% in cord samples. This suggests that the fetus contributes significantly to 3-epi-25(OH)D3 production. In contrast, the ratio of 25(OH)D3:24,25(OH)2D3 was identical in maternal and cord samples (18.5) suggesting equivalent CYP24A1 activity in mother and fetus. Maternal and cord metabolite levels were highly correlated (r=0.78, 0.90, 0.89 for 25(OH)D3, 24,25(OH)2D3, and 3-epi-25(OH)D3, respectively, P=0.001 for all). Serum concentrations of all metabolites were lower in nonwhite infants compared with white infants. Maternal and cord concentrations of 25(OH)D3 were positively associated with birth weight (r=0.21, P=0.02; r=0.25, P=0.003, respectively). Conclusions: This data suggests that although maternal and cord concentrations of vitamin D metabolites are highly correlated, regulation of specific vitamin D metabolites in the mother and the neonate may be mediated independently.