People who are already at a high risk of developing type 2 diabetes do not seem to benefit from taking vitamin D supplements every day, according to a trial from Japan that was published today in The BMJ.

The findings reveal that supplements have no clinically meaningful effect in high-risk adults, but they do imply that people with low insulin secretion may benefit from them, though this is yet uncertain.

Around 480 million people worldwide have type 2 diabetes, which is expected to rise to 700 million by 2045. Another half-billion people have glucose intolerance or are diabetic (higher than normal blood sugar levels that, if left untreated, can develop into type 2 diabetes).

Weight loss and exercise can reduce the risk of type 2 diabetes progression, but they are difficult to maintain, thus new approaches are needed to combat the disease.

Although some studies have linked vitamin D deficiency to an increased risk of type 2 diabetes in the future, trials of vitamin D supplementation to prevent type 2 diabetes have yielded mixed results.

To fill this knowledge vacuum, researchers set out to see if eldecalcitol (an active form of vitamin D used to treat osteoporosis in Japan) could help persons with poor glucose tolerance avoid developing type 2 diabetes.

Their results are based on data from 1,256 Japanese adults with poor glucose tolerance who were enrolled from health centers in Japan between June 2013 and August 2019. They were 61 years old on average (range 30-78), 46 percent of whom were female, and 59 percent of whom had a family history of type 2 diabetes.

Over a three-year follow-up period, participants were randomly randomized to receive either a conventional daily dose of eldecalcitol (630 participants) or a placebo (626 participants) and were tested for diabetes every three months.

Those who developed diabetes (12.5 percent in the eldecalcitol group and 14 percent in the placebo group) or whose blood sugar levels returned to normal (23 percent in the eldecalcitol group and 20 percent in the placebo group) showed no significant differences between groups over this time.

However, after controlling for 11 potentially significant characteristics such as age, gender, blood pressure, body mass index, and diabetes family history, the results revealed that eldecalcitol could prevent type 2 diabetes in pre-diabetic patients with inadequate insulin production. However, the experts stress that further research is needed before they can come to any definitive conclusions.

However, when eldecalcitol was compared to placebo, they discovered a significant increase in both lower back and hip bone mineral density.

There was no discernible difference in major adverse events between the two groups.

The researchers admit that there are several unknowns, such as whether the dose of eldecalcitol employed was appropriate for diabetes prevention and whether the results apply to all ethnicities. Nonetheless, because this was a big trial with frequent follow-up and good treatment adherence, the findings appear to be reliable.

“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. They also advocate for more research to see if vitamin D is useful to patients who have pre-diabetes.

According to Tatiana Christides of the Queen Mary University of London, the new trial was well-run, and the results are consistent with two previous recent trials.

However, she argues that other uncertainties remain, including whether vitamin D supplementation is more effective for specific populations and whether longer treatment durations or younger initiation ages are more advantageous.

She recommends that healthcare professionals “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 the development of type 2 diabetes” until more data from high-quality randomized studies becomes available.

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