Large Trial Suggests Vitamin D Supplements Can’t Prevent Type 2 Diabetes

Everyday supplements of vitamin D offer no protection against the onset of type 2 diabetes in high-risk populations. That’s the conclusion of a new study involving over 1200 Japanese adults. The trial, published in The BMJ, suggests that the supplements may offer a benefit for a small subset of individuals who have insufficient insulin secretion, although the authors state that further research will be required to confirm this.

Vitamin D deficiency and type 2 diabetes

Type 2 diabetes affects nearly 500 million people worldwide. This is predicted to increase to 700 million by 2045. Another half a billion people have impaired glucose tolerance or pre-diabetes. Pre-diabetes is defined as having higher than normal blood sugar levels that, left untreated, can develop into type 2 diabetes.

Weight loss and exercise can lower the risk of progression to type 2 diabetes, but as these are difficult to sustain, researchers are looking for new ways to tackle the problem.

Previous studies had shown that vitamin D deficiency is associated with an increased risk of future diabetes. But trials investigating vitamin D supplements for the prevention of type 2 diabetes have shown inconsistent results.

To address this knowledge gap, researchers set out to assess whether eldecalcitol (an active form of vitamin D used to treat osteoporosis) could reduce the risk of developing type 2 diabetes among people with impaired glucose tolerance.

Vitamin D proves ineffective at preventing diabetes

The study cohort was recruited from three Japanese hospitals over a six-year period. Their average age was 61 (range 30-78) years, 46% were women, and 59% had a family history of type 2 diabetes.

Participants were randomly assigned to receive either a standard daily dose of eldecalcitol (630 participants) or placebo (626 participants) and were assessed for diabetes every three months over a three-year follow-up period.

Unsurprisingly, given its role in treating osteoporosis, the researchers found that those taking eldecalcitol had a significant increase in both lower back and hip bone mineral densities compared with placebo. 

However, the difference in those that developed diabetes between the placebo group (14%) and the eldecalcitol group (12.5%) was not significant.

“Although treatment with eldecalcitol did not significantly reduce the incidence of diabetes among people with pre-diabetes, the results suggested the potential for a beneficial effect of eldecalcitol on people with insufficient insulin secretion,” they write.

The findings, say the authors, are robust, with a large sample size and high adherence to treatment. Nevertheless, it is unclear whether the results would apply to all ethnicities and whether the dosing used was appropriate for the prevention of diabetes.

What is vitamin D good for?

This new trial was well conducted and results are consistent with two other recent trials, says Queen Mary University of London’s Tatiana Christides in a linked editorial. 

However, several questions remain, she writes, including whether vitamin D supplementation may be more effective for particular populations, and whether longer duration of treatment or younger age at initiation might be more beneficial.

Until further data are available from high quality randomized trials, she suggests healthcare professionals “should continue to discuss with patients the musculoskeletal health benefits of vitamin D and support them to achieve and maintain lifestyle changes that, although challenging to sustain, are known to decrease development of type 2 diabetes.”

Reference:  Kawahara T, Suzuki G, Mizuno S, et al. Effect of active vitamin D treatment on development of type 2 diabetes: DPVD randomised controlled trial in Japanese population. BMJ. 2022;377:e066222. doi: 10.1136/bmj-2021-066222.

This article is a rework of a press release issued by the BMJ. Material has been edited for length and content.