In adults with prediabetes, can vitamin D supplementation decrease their likelihood of developing type 2 diabetes over several years?
Vitamin D supplementation, over a median of 3 years, slightly reduces the diagnoses of diabetes in older people at risk of developing type 2 diabetes (NNT = 18 - 115). The benefit is greater in patients who achieve higher serum levels of vitamin D.
Self-funded or unfunded
This meta-analysis combined data from 3 randomized controlled studies that evaluated the role of vitamin D in preventing the development of diabetes. The studies included 4190 at-risk adults. The authors searched 3 databases (but not Cochrane CENTRAL) and were able to combine individual patient data from the trials rather than simply using the summary results from each study. Risk of bias for the studies was low. Each study used a different version and dose of vitamin D. The patients were an average of 60 years of age and approximately 30% of participants had low serum 25-hydroxyvitamin D levels. Over a median follow-up of 3 years, intention-to-treat analysis indicates new-onset diabetes occurred in 25% of participants receiving placebo and 22.7% of participants receiving vitamin D. This difference translates to a number needed to treat (NNT) of 30 (95% CI 18 - 115). In a subgroup analysis, the risk of diabetes development was lowest in patients with the highest serum levels of vitamin D (at least 125 nmol/L), though this level was only achieved in 22.5% of treated patients. Kidney stone development was not higher with supplementation, though the study may not have been powered to find a difference if one exists.
Allen F. Shaughnessy, PharmD, MMedEd
Professor of Family Medicine