effect of high-dose vitamin d supplementation on compensation for vitamin d deficiency in pregnancy and incidence of gestational diabetes mellitus: a randomized clinical trial

Background: although studies have shown that vitamin d deficiency in pregnancy is associated with gestational diabetes mellitus (gdm), the clinical evidence of the preventive role of high-dose vitamin d supplementation in pregnancy is in doubt. objectives: the study aimed to evaluate the effect of vitamin d supplementation on the gdm occurrence. methods: in a randomized clinical trial conducted in semnan, iran, in 2014, we recruited 175 eligible pregnant women at 8 - 12 weeks with vitamin d deficiency and normal fasting blood sugar (fbs) and randomly divided them into intervention (n = 87) and control groups (n = 88). from the 14 to 16 weeks, both groups received the 400 iu/daily doses of vitamin d. the intervention group received an additional 50,000 iu every two weeks for six weeks. in the 24 - 26 weeks, we measured fbs, serum vitamin d level, and oral glucose tolerance (75 g). results: the baseline levels of serum vitamin d (mean ± sd) were 15.38 ± 7.69 and 14.00 ± 8.81 (p = 0.728) in the intervention and control groups, respectively. there was no difference between the two groups in severe vitamin d deficiency (27.6% vs. 30.7%, p = 0.652). after the intervention, the serum vitamin d level was significantly different between the two groups (16.63 ± 6.77 vs. 66.96 ± 21.13, p < 0.001). however, it did not reach the toxic serum level in any of the participants. the level of vitamin d in 85 (97.7%) and 0 (0.0%) participants reached the normal range; there were 0 (0.0%) and 16 (18.8%) cases of severe deficiency in the groups. twelve women (13.6%) in control and eight (9.2%) in intervention groups developed gdm (p = 0.356). conclusions: a high dose of vitamin d supplementation at 14th - 16th weeks of pregnancy could improve vitamin d deficiency effectively and safely. despite the decreased incidence of gdm, there was no significant evidence of its preventive role.