Vitamin D supplementation may reduce diabetes risk for the more than 10 million adults with prediabetes

A review of clinical trials found that higher vitamin D intake was associated with a 15% reduction in the likelihood of developing type 2 diabetes in adults with prediabetes. The review is published in Annals of Internal Medicine.

Vitamin D is a fat-soluble vitamin available in or added to some foods, as a supplement, or produced by the body when ultraviolet rays from sunlight hit the skin. Vitamin D has many functions in the body, including a role in insulin secretion and glucose metabolism. Observational studies have found an association between having a low level of vitamin D in the blood and a high risk of developing diabetes.

Tufts Medical Center researchers conducted a systematic review and meta-analysis of three clinical trials comparing the impact of vitamin D supplements on diabetes risk. The authors found that during a three-year follow-up period, new-onset diabetes occurred in 22.7% of adults who received vitamin D and in 25% of those who received placebo, suggesting a relative risk reduction of 15%. According to the authors, extrapolating their findings 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.

In an accompanying editorial, authors from University College Dublin and the Food Safety Authority of Ireland point out that previous data have shown significant adverse effects for high intakes of vitamin D. They argue that professional societies that promote vitamin D therapy have an obligation to warn physicians about both the necessary intake of vitamin D and the safe limits. They advise that this very high-dose vitamin D therapy could prevent type 2 diabetes in some patients, but it can also cause harm.