To the Editor: In the randomized, placebo-controlled Vitamin D and Type 2 Diabetes (D2d) trial involving persons at risk for type 2 diabetes, Pittas et al. (Aug. 8 issue)(1) found that vitamin D-3 supplementation at a dose of 4000 IU per day did not result in a significantly lower risk of diabetes than placebo. Investigators emphasized that the trial was planned not only for participants with vitamin D insufficiency, given ethical considerations. Although the mean baseline vitamin D levels are well balanced between the trial groups and above insufficiency limits, more than 20% of the participants in the placebo group . . .