Vitamin D supplementation can improve blood sugar control among some people living with type 2 diabetes

Wu C, Qiu S, Zhu X, et al. Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systematic review and meta-analysis Metabolism. 2017;73:67-76.

Review question

What are the effects of vitamin D supplementation on blood sugar control in people with type 2 diabetes? 

Background

Diabetes effects an estimated 400 million people globally, with that number expected to rise to 642 million by 2040. Vitamin D supplementation has emerged as a possible strategy for the treatment of type 2 diabetes. Past observational studies show a relationship between vitamin D levels and diabetes risk. However, randomized controlled trials on the effect of vitamin D supplementation in people with type 2 diabetes have presented mixed findings, while a previous systematic review and meta-analysis provided evidence of a weak association between reductions in fasting glucose and improvements in insulin resistance, and vitamin D supplementation in this population.

How the review was done

This is a systematic review and meta-analysis of 26 randomized controlled trials published between 2007 and 2016, including a total of 1874 participants.

• Participants were adults aged 49 - 67 years old with type 2 diabetes, and most had a vitamin D deficiency.
• Study participants took vitamin D supplements at varying doses (400 IU - 300,000 IU), frequencies (daily, weekly, monthly or one time) and durations (4 - 48 weeks). The supplements came in the form of tablets, drinks, or injections.  
• Researchers measured change in blood sugar levels among participants, i.e. HbA1c and fasting blood glucose.
• Results were compared to people in control groups who did not receive vitamin D supplementation.

What the researchers found

Overall, vitamin D supplementation reduced HbA1c but not fasting blood glucose in people with type 2 diabetes, compared to no vitamin D supplementation. When further categorizing people with type 2 diabetes into groups based on their vitamin D levels and obesity status, different impacts were seen on blood sugar control outcomes. For instance, vitamin D supplementation reduced both HbA1c and fasting blood glucose in people with type 2 diabetes who were diagnosed as vitamin D deficient, but not among those who were not deficient.

More so, there was a relationship between vitamin D and weight, whereby non-obese individuals with type 2 diabetes experienced improved blood sugar control whereas obese individuals did not. For example, non-obese individuals who took vitamin D supplements experienced a reduction in HbA1c, while vitamin D deficient non-obese individuals with type 2 diabetes experienced a reduction in both HbA1c and fasting blood glucose. Positive reductions in blood sugar were seen when taking vitamin D supplementation at a dosage of ≥ 1000 IU/day and for ≥ 12 weeks. More research is needed to further confirm these findings.

Conclusion

Vitamin D supplementation may be an effective strategy to improve blood sugar control in people with type 2 diabetes through reduced HbA1c and/or fasting blood glucose levels. Specifically, vitamin D supplementation has the greatest benefit for those with type 2 diabetes who are vitamin D deficient, as well as those who are non-obese.  

Effect of vitamin D supplementation on blood sugar control in people with type 2 diabetes

*FBG = fasting blood glucose