A Summary of the D2d Study Results and What They Mean


Why and how did we do the D2d Study?

Observational studies have shown that people who have lower blood levels of vitamin D are more likely to develop type 2 diabetes, but it is not known if taking vitamin D supplements lowers the risk of developing diabetes. This is an important question because more than 84 million Americans are at high risk of developing diabetes (called prediabetes), and the number of people with diabetes is expected to increase. We did the D2d study to see if vitamin D supplementation lowers the risk of diabetes in people who have prediabetes.

2,423 participants from 22 locations across the US took part in the study, and they were randomly divided into two groups: one group took a pill with 4,000 units of vitamin D each day (vitamin D group) and the other group took an identical pill that did not have vitamin D (placebo group). We monitored participants for diabetes with blood tests every 6 months and most participants were followed in the study between 2 and 5 years.

The size and length of the study would allow us to detect a drop in the risk of developing diabetes with vitamin D of 25 percent or more. If the difference in the number of diabetes cases between the vitamin D and placebo groups was less than 25 percent, the study would be less likely to detect such a difference.


What did the study results show about diabetes risk?

The study was unable to show that vitamin D decreases the risk of diabetes by 25 percent. At the end of the study, fewer participants in the vitamin D group (293 out of 1,211 participants; 24.2 percent) developed diabetes compared to placebo (323 out of 1,212 participants; 26.7 percent), a small difference which was not statistically significant.

The way the body responds to taking a vitamin (like vitamin D) likely depends on how much of the vitamin D is already in the body. If vitamin D can help prevent diabetes, people with lower blood vitamin D levels at the start of the study might benefit more than those who have higher vitamin D levels. When they started the study, about 80 percent of participants had a vitamin D level that would be considered “sufficient” by current Institute of Medicine recommendations (equal to or greater than 20 ng/mL). This high percentage of participants with sufficient levels may have reduced our ability to detect an overall benefit of vitamin D when we looked at all participants in the study.

When we looked at only participants who had very low blood vitamin D levels – less than 12 ng/mL – at the start of the study, vitamin D supplementation appeared to reduce risk of diabetes in these participants; however, no conclusion can be drawn due to the very few participants with these low levels of vitamin D.


Is vitamin D safe to take?

During the study, more participants in the vitamin D group (5 out of 1,211 participants in the vitamin D group; 0.42 percent) developed a high blood calcium level compared to placebo (3 out of 1,212; 0.24 percent) and more participants in the vitamin D group (25 out of 1211 participants in the vitamin D group; 2.1 percent) developed kidney stones compared to placebo (21 out of 1,212; 1.7 percent). These differences were small and not statistically significant. There were no other safety issues during the study.


What do the results mean for me?

We found that among people with prediabetes and sufficient vitamin D level, vitamin D supplementation at 4,000 units per day did not significantly reduce the risk of diabetes. These findings do not change the need for all individuals to meet the vitamin D requirements set out by the Institute of Medicine. Adults up through age 70 need 600 units of vitamin D intake daily and those older than 70 years need 800 units daily. People get vitamin D from food and sunlight. However, very little vitamin D is found in food, and your body’s ability to make vitamin D from sunlight depends on many factors, including your exposure to the sun, where you live, the time of year, and the time of day. Supplements can be used to help you meet the daily requirement.

The results of the D2d study do not change the recommendation that people at high risk for diabetes should continue to work on lifestyle changes aimed at weight loss and increased physical activity.


Are there plans for future studies to learn more about vitamin D?

We will continue to analyze the information we collected during the study to see if vitamin D supplementation has an effect on how the body makes insulin or responds to insulin. We also plan to combine results from the D2d study and several other large studies that also examined the effect of vitamin D supplementation on diabetes to better understand the effect of vitamin D on diabetes. Additionally, D2d investigators are looking at the impact of vitamin D supplementation on other conditions, such as heart disease, cancer, and kidney health using information that has been collected as part of the D2d study.