A study published in The European Journal of Medical Sciences found that large doses of vitamin D improved insulin sensitivity and led to a positive trend in HbA1c levels.
Type 2 diabetes (T2D) makes up 90% of diabetes cases. In 2010, it was estimated that there were 285 million people with T2D globally.
People with T2D have poor insulin sensitivity, which means that their cells are less responsive to insulin. Insulin transports glucose into cells, and poor insulin sensitivity means insulin isn’t transporting glucose effectively, which leads to abnormally high blood glucose levels. Glucose is dangerous when it is circulating in the blood in high levels for too long. Chronically high blood glucose levels can increase the risk of cardiovascular disease.
Observational studies suggest that low vitamin D levels may increase the risk for T2D and may contribute to impaired tolerance of glucose.
Past research has found that vitamin D is required for and improves the production of insulin. However, randomized controlled trials evaluating the effects of vitamin D on glucose and insulin sensitivity in T2D have not always shown clear benefit. A recent systematic review of 15 newer published studies found discrepancies in outcomes, with some studies showing benefit in vitamin D, others little benefit. The review concluded these discrepancies could be due to diverse study populations (sample size, stage of diabetes, etc.) and mixed interventions (oral, dose, duration, etc.).
In this recent double-blinded, randomized controlled trial, 55 patients who had T2D for longer than 10 years were recruited from ambulatory care facilities in Swiss hospitals. They were randomized into either a group receiving an initial injection of 300,000 IU of vitamin D3 intramuscularly or a group receiving a placebo injection.
After 3 months, patients in the placebo group or in the vitamin D group who had reached sufficient levels (above 32 ng/ml) received the placebo injection and patients in the vitamin D group with levels still below 32 ng/ml received 150,000 IU of D3.
The researchers took baseline measurements of HbA1c and insulin sensitivity, calculated by the homeostasis model assessment insulin resistance (HOMA-IR), in all patients. HbA1c is a lab test that shows the average level of blood sugar (glucose) over the previous 3 months. These measurements were taken again at 3 months into the trial and at the end of the trial 3 months later.
Here were the results:
- In both groups, HbA1c levels after 6 months increased non-significantly compared to baseline.
- HbA1c levels increased significantly less in patients treated with vitamin D3 than in the placebo group.
- HOMA-IR decreased significantly by 12.8% in the D3 group compared to a 10% increase in the placebo group (p=0.032).
“Based on this calculation, D3 administration ameliorated insulin resistance and significantly limited the rise in HbA1c as compared to placebo during this 6-month intervention trial,” the researchers concluded.
The researchers noted that the effects observed could potentially be due to the reported anti-inflammatory effects of vitamin D3.
Although the researchers aimed to enroll all patients between October and November to limit the effects of sunlight and consequent production of vitamin D in the skin, enrollment didn’t finish until December, meaning the study didn’t finish until July. The effect of vitamin D3 administration could have been masked by the “spontaneous” increase in 25(OH)D3 levels in the placebo group, most likely due to increased sun exposure during the spring and summer months.
Larger future trials should give us greater insight into if and how vitamin D can help benefit T2D patients.
Jehle, S. et al. Effect of large doses of parenteral vitamin D on glycemic control and calcium/phosphate metabolism in patients with stable type 2 diabetes mellitus: a randomized, placebo-controlled, prospective pilot study. The European Journal of Medical Sciences, 2014.