Yet another study has found that vitamin D supplementation doesn’t reduce the risk of developing type 2 diabetes in the general population with prediabetes, but it does leave the door open for benefit in those with low insulin secretion.
The new findings come from the prospective Diabetes Prevention With Active Vitamin D (DPVD) trial of more than 1,200 Japanese participants with impaired glucose tolerance.
The data were published online in The BMJ by Tetsuya Kawahara, MD, PhD, of Shin Komonji Hospital, Kitakyushu, Japan, and colleagues.
Treatment with 0.75 μg/day of eldecalcitol, an active vitamin D analogue, for 3 years did not prevent progression from prediabetes to type 2 diabetes, nor did it improve the rate of regression to normoglycemia, compared with placebo.
However, “we showed a preventive effect of eldecalcitol after adjusting for covariables ... ,” wrote Dr. Kawahara and colleagues.
‘Remarkably similar’ results in several trials
The new trial is “well conducted, with rigorously defined and tested diagnostic criteria, and of sufficient duration, but it may have been underpowered to detect a small effect,” Tatiana Christides, MD, PhD, of Queen Mary University of London, wrote in an accompanying editorial.
Dr. Christides notes that a recent meta-analysis of intervention trials did find a significant 10% reduction in risk of type 2 diabetes with vitamin D supplementation, “a difference too small to be detected by the new trial ... Although a 10% risk reduction is modest, it may be valuable at the population level and justifies further study.”
The new finding, a nonsignificant 13% relative reduction in risk, is similar to the 13% relative risk reduction found in the Vitamin D and Type 2 Diabetes (D2d) trial reported in 2019.
But in that study as in this one, there was a suggested benefit in a subset of people. In D2d, it was in those who were vitamin D deficient.
Asked to comment, D2d lead investigator Anastassios G. Pittas, MD, chief of the division of diabetes, endocrinology, and metabolism at Tufts University, Boston, pointed out that the results were also “remarkably similar” to those of a third study from Norway published in 2014, which also found a 13% relative risk reduction.
“The nearly identical results from the three trials that were specifically designed and conducted to test whether vitamin D supplementation lowers diabetes clearly points to a beneficial effect of vitamin D for diabetes risk reduction. However, the overall effect in people not selected for vitamin D insufficiency seems to be less than hypothesized in each trial,” Dr. Pittas said in an interview.
He added, “there will be no more specific vitamin D and diabetes prevention trials, so we need to continue gaining insights from these three trials.”