Rationale and design of the Vitamin D and Type 2 Diabetes (D2d) study: a diabetes prevention trial

Diabetes Care. 2014 Dec;37(12):3227-34. doi: 10.2337/dc14-1005. Epub 2014 Sep 9.


Objective: Observational studies suggest that vitamin D may lower the risk of type 2 diabetes. However, data from long-term trials are lacking. The Vitamin D and Type 2 Diabetes (D2d) study is a randomized clinical trial designed to examine whether a causal relationship exists between vitamin D supplementation and the development of diabetes in people at high risk for type 2 diabetes.

Research design and methods: D2d was designed with support from a U34 planning grant from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The final protocol was approved by the D2d Research Group, the data and safety monitoring board, and NIDDK. Key eligibility criteria are age ≥30 years, BMI of 24 (22.5 for Asian Americans) to 42 kg/m(2), increased risk for diabetes (defined as meeting two of three glycemic criteria for prediabetes established by the American Diabetes Association [fasting glucose 100-125 mg/dL (5.5-6.9 mmol/L), 2-h postload glucose after 75-g glucose load 140-199 mg/dL (7.7-11.0 mmol/L), hemoglobin A₁c 5.7-6.4% (39-46 mmol/mol)]), and no hyperparathyroidism, nephrolithiasis, or hypercalcemia. D2d participants are randomized to once-daily vitamin D₃ (cholecalciferol 4,000 IU) or placebo and followed for an average of 3 years. The primary end point is time to incident diabetes as assessed by laboratory criteria during the study or by adjudication if diagnosed outside of D2d. Recruitment was initiated at the end of 2013.

Conclusions: D2d will test whether vitamin D supplementation is safe and effective at lowering the risk of progression to diabetes in people at high risk for type 2 diabetes.

Trial registration: ClinicalTrials.gov NCT01942694.

Publication types

  • Multicenter Study
  • Randomized Controlled Trial
  • Research Support, N.I.H., Extramural
  • Research Support, N.I.H., Intramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Blood Glucose / drug effects
  • Cholecalciferol / administration & dosage*
  • Diabetes Mellitus, Type 2 / prevention & control*
  • Dietary Supplements*
  • Double-Blind Method
  • Female
  • Glycated Hemoglobin / analysis
  • Humans
  • Male
  • Middle Aged
  • Placebos
  • Prediabetic State / drug therapy*
  • Research Design
  • Treatment Outcome


  • Blood Glucose
  • Glycated Hemoglobin A
  • Placebos
  • Cholecalciferol

Associated data

  • ClinicalTrials.gov/NCT01942694