AIM: This study was performed to determine the effect of a single, large, 
intramuscular injection of vitamin D post-partum on glucose tolerance and 
insulin resistance in women with gestational diabetes.
METHODS: Forty-five participants in a randomized controlled trial on gestational 
diabetes mellitus were divided into an intervention group and a control group. 
Only subjects in the intervention group received one intramuscular injection of 
300,000 IU of vitamin D3. HbA(1c), serum 25-hydroxyvitamin D3, fasting insulin 
and blood glucose, C-peptide, homeostasis model assessment insulin resistance 
index (HOMA-IR), β-cell function, insulin sensitivity and the Quantitative 
Insulin Sensitivity Check Index (QUICKI) were measured at baseline and after 3 
months of intervention.
RESULTS: Approximately 80% of the mothers had a degree of vitamin D deficiency. 
Post-intervention, this was found in 4.2 and 71.4% in the intervention and 
control groups, respectively. The medians of HOMA-IR indices before and after 
intervention were 0.6 and 0.5 (P = 0.7), respectively, in subjects in the 
intervention group, and 0.5 and 0.9 (P = 0.01) in subjects in the control group. 
The mean of the QUICKI fell only in the control group (P = 0.008). In the 
control group, β-cell function increased by ~8% (P = 0.01) and insulin 
sensitivity decreased after 3 months (P = 0.002). Post-intervention, the median 
C-peptide decreased in the intervention group and increased in the control 
group, but the change was significant only in the control group (P = 0.03).
CONCLUSIONS: A single injection of 300,000 IU of vitamin D3 achieves a 3-month 
serum 25-hydroxyvitamin D range of 50-80 nmol/l and is an efficient, effective 
and safe procedure for improving the vitamin status and indices of insulin 
resistance in mothers with gestational diabetes after delivery.