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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

Population

HbA1c

FBG

People with type 2 diabetes

Reduction

No effect

People with type 2 diabetes and vitamin D deficiency

Reduction

Reduction

People with type 2 diabetes and vitamin D insufficiency

No effect

No effect

People with type 2 diabetes and vitamin D sufficiency

No effect

No effect

Non-obese people with type 2 diabetes

Reduction

No effect

Obese people with type 2 diabetes

No effect

No effect

Non-obese people with type 2 diabetes and vitamin D deficiency

Reduction

Reduction

*FBG = fasting blood glucose




Glossary

Control group
A group that receives either no treatment or a standard treatment.
Meta-analysis
Advanced statistical methods contrasting and combining results from different studies.
Observational studies
Studies where the treatment that each person receives is beyond the control of the researcher.
Randomized controlled trials
Studies where people are assigned to one of the treatments purely by chance.
Systematic review
A comprehensive evaluation of the available research evidence on a particular topic.

Related Web Resources

  • Type 2 diabetes: Screening for adults

    Health Link B.C.
    People at average risk for type 2 diabetes should be tested every 3 years after age 40. You may need to be tested more frequently if you are at higher risk. Find out your risk with the Canadian Diabetes Risk Assessment Questionnaire (link in this resource).
  • High blood sugar can increase cognitive decline

    Berkeley Wellness
    New research shows that if you have high blood sugar, you might be more at risk for cognitive decline as you age. Whether or not you have diabetes, it is important to keep your blood sugar under control.
  • Prediabetes: Which Treatment Should I Use to Prevent Type 2 Diabetes?

    OHRI
    This patient decision aid helps People with prediabetes considering treatment to help prevent type 2 diabetes decide on whether to make a major lifestyle change or take the medicine metformin by comparing the benefits, risks, and side effects of both options.
DISCLAIMER These summaries are provided for informational purposes only. They are not a substitute for advice from your own health care professional. The summaries may be reproduced for not-for-profit educational purposes only. Any other uses must be approved by the McMaster Optimal Aging Portal (info@mcmasteroptimalaging.org).

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