Background: Health benefits of vitamin D has been proved by a large number of studies, however, to the best of our knowledge there has been no study investigating the effect of mega dose of vitamin D on gestational diabetes mellitus (GDM). This study was the first to assess the effect of postpartum injection of mega dose of vitamin D on blood pressure (BP) in GDM. Methods: This is a randomized controlled clinical trial conducted on 58 pregnant women suffering from GDM who were randomly assigned into control (CG, n = 24) and intervention group (IG, n = 24). Patients in intervention group (IG) received an intramuscular injection of 300,000 IU of vitamin D. BP, Serum concentration of 25 (OH) D3, parathyroid hormone (PTH), calcium, phosphor, diastolic (DBP) and systolic blood pressure (SBP) were measured at the baseline and after 3 months. Results: Mega dose supplementation resulted in increased serum
25-hydroxy vitamin D concentrations in IG compared with the CG (62.10 nmol/l compared with 24.10 nmol/l, P < 0.001). Additionally, injection of vitamin D significantly reduced SBP (98.1 ± 9.0 mmHg compared with 106.9 ± 15.9 mmHg, P = 0.02) and slightly decreased DBP but it was not statistically significant (63.3 ± 1.5 mmHg compared with 73.6 ± 10.3 mmHg, P = 0.13). Serum PTH significantly decrease after intervention in IG compared with CG (2.88 ± 1.60 pmol/l compared with 4.78 ± 2.4 pmol/l P = 0.003). Conclusions: This study strongly improved vitamin D status in women with GDM and consequently confirmed the efficacy of a mega dose of vitamin D injection on decreasing of BP.