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D2d Study: Vitamin D supplementation does not significantly reduce risk of type 2 diabetes


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Anastassios G. Pittas MD, MS
Anastassios G. Pittas, MD, MS

Among vitamin D-sufficient adults at high risk for type 2 diabetes, vitamin D3 supplementation at a dose of 4000 IU per day did not significantly lower the risk of diabetes compared to placebo, according to the results of the Vitamin D and Type 2 Diabetes (D2d) Study – A Multicenter Randomized Controlled Trial for Diabetes Prevention. The results of the study were presented Friday at the Scientific Sessions.

In the study, the investigators randomly assigned adults who met at least two of three glycemic criteria for prediabetes (fasting plasma glucose level of 100 to 125 mg per deciliter; plasma glucose level two hours after a 75 g oral glucose load of 140 to 199 mg per deciliter; and glycated hemoglobin level of 5.7 to 6.4 percent) and no diagnostic criteria for diabetes to receive 4000 IU per day of vitamin D3 or placebo, regardless of the baseline serum 25-hydroxyvitamin D level. A total of 2,423 participants at 22 U.S. study sites were randomized to take vitamin D3 or placebo (1,211 to the vitamin D group and 1,212 to the placebo group) and followed for new-onset diabetes for a median of 2.5 years.

“The primary outcome in this time-to-event analysis was new-onset diabetes, and the trial design was event-driven, with a target number of diabetes events of 508,” said Anastassios G. Pittas, MD, MS, Co-Director of the Diabetes and Lipid Center at Tufts Medical Center and principal investigator of the D2d study. “To maximize the study’s ability to observe a treatment effect, participants were asked to refrain from using diabetes-specific and/or weight loss medications during the study and to limit the use of outside-of-study vitamin D to 1000 IU per day from all supplements, including multivitamins.”

By month 24, the mean serum 25-hydroxyvitamin D level in the vitamin D group was 54.3 ng per milliliter (from 27.7 ng per milliliter at baseline), compared to 28.8 ng per milliliter in the placebo group (from 28.2 ng per milliliter at baseline). After the median follow-up of 2.5 years, the primary outcome of diabetes occurred in 293 participants in the vitamin D group and 323 in the placebo group (9.39 and 10.66 events per 100 person-years, respectively). The hazard ratio for vitamin D as compared with placebo was 0.88 (0.95 percent confidence interval, 0.75 to 1.04; P=0.12). The incidence of adverse events did not differ significantly between the two groups.

“Moving forward, our group plans to use study data to answer ancillary questions of importance, such as the effect of vitamin D supplementation on cardiac risk factors, kidney disease, and cancer and bone health among people at risk for diabetes,” Dr. Pittas said.