Results of the analysis, led by Anastassios G. Pittas, MD, MS, with the division of endocrinology, diabetes, and metabolism at Tufts Medical Center, in Boston, were published online in Annals of Internal Medicine (2023 Feb 7. doi: 10.7326/M22-3018).
Dr. Anastassios G. Pittas
All three eligible trials included in the analysis were randomized, double blinded, and placebo controlled. The three eligible trials tested three oral formulations of Vitamin D: cholecalciferol, 20,000 IU (500 mcg) weekly; cholecalciferol, 4,000 IU (100 mcg) daily; or eldecalcitol, 0.75 mcg daily, against placebos.
The authors of the new paper found that vitamin D reduced the risk for diabetes in people with prediabetes by a statistically significant 15% in adjusted analyses. The 3-year absolute risk reduction was 3.3%.
They found no difference in the rate ratios for adverse events (kidney stones, 1.17, 95% confidence interval, 0.69-1.99; hypercalcemia, 2.34; 95% CI, 0.83-6.66]; hypercalciuria, 1.65; 95% CI, 0.83-3.28]; death, 0.85; 95% CI, 0.31-2.36]) when study participants got vitamin D instead of placebo.
Differences from previous analyses
The relationship between vitamin D levels and risk for type 2 diabetes has been studied in previous trials and results have been mixed.
The authors note that two previous meta-analyses included trials “that had relatively short durations for assessment of diabetes risk (for example, ≤ 1 year), had high risk of bias (for example, open-label trials), or were not specifically designed and conducted for primary prevention of type 2 diabetes, potentially undermining the validity of the results.”
Each of the trials in this meta-analysis had a low risk of bias as determined by the Cochrane risk-of-bias tool, Dr. Pittas and colleagues said.
“The present study does not reach an opposite conclusion from the D2d study,” said Dr. Pittas, who coauthored that paper as well. “Rather, it confirms the results of the D2d study. In D2d and two other similar vitamin D and diabetes prevention trials (one in Norway and one in Japan), vitamin D reduced the rate of progression to diabetes in adults with prediabetes, but the observed differences were not statistically significant because the reported relative risk reductions (10%-13%) were smaller than each trial was powered to detect (25%-36%).”
“Individual participant data meta-analyses increase the statistical power to detect an effect. After combining data, we found that vitamin D reduced the risk of progression from prediabetes to diabetes by 15% and this result was statistically significant. So, the conclusion of the meta-analysis is essentially the same conclusion as in D2d and the other two trials. The difference is that the result is now statistically significant,” Dr. Pittas added.
Small reduction but large population
The authors acknowledged that the absolute risk reduction number is small, especially when compared with the risk reduction seen with intensive lifestyle changes (58%) and metformin (31%), as reported in an article published in the New England of Journal of Medicine (2002 Feb 7;346:393-403). But “extrapolating to the more than 374 million adults worldwide who have prediabetes suggests that inexpensive vitamin D supplementation could delay the development of diabetes in more than 10 million people,” they said.