Vitamin D and Type 2 Diabetes Study (D2d Study)

The purpose of this study was to evaluate the effect of taking vitamin D supplements on the prevention of diabetes in people who are at high risk for diabetes. D2d was a multi-center trial and enrolled over 2,400 participants.

Eligible Participants

Eligible participants for this study met the following criteria:

  • Both men and women
  • 30 years of age or older
  • Has prediabetes (“borderline” diabetes)
  • Were willing to be randomly assigned to take either a vitamin D supplement (4000 international units) or a placebo for roughly 4 years

Study Design

Participation in the study required up to 13 scheduled visits to the Clinical Translational Unit (CTRU) at Stanford Hospital & Clinics, which included blood draws at certain time points.


Among people at high risk for type 2 diabetes, vitamin D3 supplementation at a dose of 4000 IU per day did not result in a significantly lower risk of diabetes than placebo. The results of this study were published in the New England Journal of Medicine in August, 2019.