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

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.

The study aimed to evaluate the effect of vitamin D supplementation on the GDM occurrence.

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).

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).

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.