Determinants of the maternal 25-hydroxyvitamin D response to vitamin D supplementation during pregnancy.

Context Current approaches to antenatal vitamin D supplementation do not account for inter-individual differences in 25-hydroxyvitamin D [25(OH)D] response. Objective We assessed which maternal and environmental characteristics were associated with 25-hydroxyvitamin D [25(OH)D] following supplementation with cholecalciferol. Design Within-randomisation-group analysis of participants in the MAVIDOS trial of vitamin D supplementation in pregnancy. Setting Hospital antenatal clinics Participants 829 pregnant women (422 placebo, 407 cholecalciferol). At 14 and 34 weeks’ 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 1000IU/day cholecalciferol or matched placebo from 14 weeks’ gestation until delivery. Main outcome measure 1000IU/day cholecalciferol or matched placebo from 14 weeks’ gestation until delivery. Conclusions 25(OH)D at 34 weeks’ gestation was higher in the women randomised to vitamin D [mean (SD): 67.7 (21.3) nmol/l] compared with placebo [43.1 (22.5) nmol/l, p<0.001]. In women randomised to cholecalciferol, higher pregnancy weight gain from 14 to 34 weeks’ gestation [kg] (β=-0.81 (95%CI -1.39, -0.22)), lower compliance with study medication [%] (β=-0.28 (-0.072, -0.48)), lower early pregnancy 25(OH)D [nmol/l] (β=0.28 (0.16, 0.40)) and delivery in the winter vs the summer (β=-10.5 (-6.4, -14.6)) were independently associated with lower 25(OH)D at 34 weeks’ gestation. 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/day cholecalciferol. Future studies should aim to determine appropriate doses to enable consistent repletion of 25-hydroxyvitamin D during pregnancy.