Endocrine Abstracts

Low maternal vitamin D (VD) status is associated with adverse fetal development. However gestational changes in VD metabolites and their relationship with cord concentrations are unclear.

The plasma concentration of 25-hydroxyvitamin D₃ (25(OH)D₃) was measured using UPLC-MS/MS, and free 25(OH)D and vitamin D binding protein (DBP) concentrations were measured by ELISA. Plasma samples were collected longitudinally from women resident in rural Gambia with abundant exposure to UVB (n=21), before pregnancy, at gestational week 13, 20, 30 and in cord blood. Data were analysed in Stata/SE 14.0 using mixed effects models and linear regression.

Concentrations (mean±S.D.) of 25(OH)D₃ (64±13, 71±14, 81±22, 100±23 nmol/l) and DBP (356±47, 488±104, 570±100 and 625±105 mg/l) increased across gestation (P<0.0001), before pregnancy, at week 13, 20 and 30, respectively. Free 25(OH)D concentration remained unchanged (13.2±3.0, 12.2±2.0, 12.5±2.0, 12.7±3.0 pmol/l) (P=0.2), measured at the same time-points. Cord plasma concentrations of 25(OH)D₃ (58±15 nmol/l) and DBP (270±48 mg/l) were lower than maternal plasma at week 30 (P<0.0001), and free 25(OH)D (15.5±5.0 pmol/l) was higher (P=0.03).

Maternal (week 30) and cord concentrations were highly correlated for free 25(OH)D (β (95% CI)) (1.38 (0.94, 1.82)) (P<0.0001), but not 25(OH)D₃ (0.2 (−0.09, 0.52)) (P=0.2), suggesting efficient placental transfer of free 25(OH)D. The slope of the regression of free 25(OH)D on 25(OH)D₃ differed between cord, (0.09 (0.06, 0.11)) and maternal plasma (0.04 (0.02, 0.06)) (P<0.05), suggesting that free 25(OH)D contributes a higher proportion of total 25(OH)D in cord than maternal plasma, possibly reflecting relative DBP concentration.

Gestational changes in VD metabolism were apparent; the increasing 25(OH)D₃ and DBP concentration was in keeping with the unchanging free 25(OH)D. These data suggest that mechanisms exist to regulate placental VD transfer and the availability of free 25(OH)D to the fetal circulation and contribute to our understanding of maternal and neonatal VD requirements.