Vitamin D supplementation of 1000 IU or more per day may reduce the risk of type 2 diabetes in patients with prediabetes

Barbarawi M, Zayed Y, Barbarawi O, et al. Effect of vitamin D supplementation on the incidence of diabetes mellitus J Clin Endocrinol Metab. 2020;105:dgaa335.

Review question

Does vitamin D supplementation reduce the risk of type 2 diabetes?

Background

Globally, over 500 million people live with diabetes, and millions more are at an increased risk of developing it. Over the past 15 years, some research has suggested that low levels of vitamin D are associated with the development of type 2 diabetes. In response, vitamin D supplementation was put forward as a potential preventative strategy. However, the evidence is unclear on the effectiveness of vitamin D supplementation in a variety of populations—such as those with prediabetes or those who do not have higher-than-normal blood sugar levels. Additionally, some more recent studies have been small or only focused on low-dose vitamin D supplementation. Therefore, a comprehensive look at the topic is needed.

How the review was done

This is a systematic review and meta-analysis of nine randomized control trials. The studies were published between 2008 and 2019 and included 43, 559 participants.

Key features of the included studies:

• Participants were patients with prediabetes or populations at an average-risk of diabetes. The average age of participants ranged between 46 to 77 years old.
• Participants received vitamin D (i.e., vitamin D3, vitamin D2, or an osteoporosis medication comparable to vitamin D called eldecalcitol) supplementation in various forms, such as tablets, pre-filled syringes, and sachets. Some participants also received calcium supplements. Vitamin D doses were either low (less than 1000 IU/day) or moderate to high (equal to or more than 1000 IU/day). The frequency of supplementation ranged from daily to weekly. 
• Researchers measured the incidence of type 2 diabetes.
• Results were compared to people in control groups receiving a placebo on its own or in combination with calcium supplements.
• Participants were followed for one to seven years.

What the researchers found

While vitamin D supplementation does not appear to reduce the risk of developing type 2 diabetes, compared to placebo, among those at average-risk of diabetes taking a low dose of vitamin D supplements, in those with prediabetes, vitamin D supplementation at a moderate or high dose may lower the risk of developing type 2 diabetes. Furthermore, among those with prediabetes, those with an average body mass index (BMI) of less than 30 kg/m2 may be more likely to benefit from vitamin D supplementation than those whose average BMI is 30 kg/m2 or more. More research is needed to understand if and how age, sex, baseline BMI, dosing schedules, and pre-treatment vitamin D levels influence the impact of vitamin D supplementation on diabetes.

Conclusion

Vitamin D supplementation at a moderate or high daily dose may reduce the risk of type 2 diabetes in people with prediabetes, and particularly in those with a non-obese BMI, compared to placebo. Results for people at average-risk of diabetes using vitamin D supplementation at a low daily dose are not as promising.