Determinants of the Maternal 25-Hydroxyvitamin D Response to Vitamin D Supplementation During Pregnancy

Context: individual differences in 25-hydroxyvitamin D (25(OH) D) response. Objective: We assessed which maternal and environmental characteristics were associated with 25(OH) D after supplementation with cholecalciferol. Design: Within-randomization-group analysis of participants in the Maternal Vitamin D Osteoporosis Study trial of vitamin D supplementation in pregnancy. Setting: Hospital antenatal clinics. Participants: Atotal of 829 pregnantwomen(422 placebo, 407 cholecalciferol). At 14 and 34 weeks of gestation, maternal anthropometry, health, and lifestyle were assessed and 25(OH) D measured. Compliance was determined using pill counts at 19 and 34 weeks. Interventions: 1000 IU/d of cholecalciferol or matched placebo from 14 weeks of gestation until delivery. Main Outcome Measure: 25(OH) D at 34 weeks, measured in a single batch (Diasorin Liaison). Results: 25(OH) D at 34 weeks of gestation was higher in the women randomized to vitamin D (mean [SD], 67.7 [21.3] nmol/L) compared with placebo (43.1 [22.5] nmol/L; P < .001). In women randomized to cholecalciferol, higher pregnancy weight gain from 14 to 34 weeks of gestation (kg) (beta = -0.81 [ 95% confidence interval -1.39, -0.22]), lower compliance with study medication (%) (beta = -0.28 [-0.072, -0.48]), lower early pregnancy 25(OH) D (nmol/L) (beta = 0.28 [0.16, 0.40]), and delivery in the winter vs the summer (beta = -10.5 [-6.4, -14.6]) were independently associated with lower 25(OH) D at 34 weeks of gestation. Conclusions: Women who gained more weight during pregnancy had lower 25(OH) D in early pregnancy and delivered in winter achieved a lower 25(OH) D in late pregnancy when supplemented with 1000 IU/d cholecalciferol. Future studies should aim to determine appropriate doses to enable consistent repletion of 25(OH) D during pregnancy.