Short answer: Not significantly, in the largest trial. D2d gave 2,423 adults with prediabetes 4,000 IU of vitamin D3 a day or placebo for a median of 2.5 years. Diabetes developed in 293 people on vitamin D and 323 on placebo, a hazard ratio of 0.88 that was not statistically significant. NIH says trials give little support for vitamin D to control blood sugar or prevent diabetes, especially in people who are not low.
D2d was funded by the NIH diabetes institute (NIDDK). It enrolled adults who met at least two of three prediabetes measures: fasting glucose 100 to 125 mg/dL, a 2-hour glucose test of 140 to 199 mg/dL, or an HbA1c of 5.7% to 6.4%. They were not chosen for low vitamin D, and ODS says 78% had adequate levels at the start.
| Measure | Vitamin D3, 4,000 IU/day | Placebo |
|---|---|---|
| People | 1,211 | 1,212 |
| Blood vitamin D at 24 months | 54.3 ng/mL (from 27.7) | 28.8 ng/mL (from 28.2) |
| New diabetes | 293 (9.39 per 100 person-years) | 323 (10.66 per 100 person-years) |
| Hazard ratio | 0.88 (range 0.75 to 1.04), not significant | — |
| Side effects | No significant difference | — |
ODS notes that observational studies link low vitamin D with more diabetes, but many people in them were overweight or had obesity, which goes with both lower vitamin D and diabetes. Trials like D2d were run to test cause and effect.
ODS also mentions a later analysis by the D2d authors: among people who started with a very low vitamin D level, below 12 ng/mL, diabetes was 62% less common on vitamin D than on placebo. This was a post hoc look, done after the trial, in a small slice of the people, so it is a lead for future research rather than a trial result. ODS also describes a Norwegian trial in 511 people with prediabetes, given 20,000 IU a week for 5 years, that found no difference in progression to diabetes.
ODS reviews trials in people who already have diabetes too. A pooled analysis of 35 trials in 43,407 adults found no effect of vitamin D on glucose, insulin, insulin resistance or HbA1c. Another, of 20 trials in 2,703 adults with diabetes, found a small drop in insulin resistance but no change in fasting glucose, HbA1c or fasting insulin. ODS sums up that trials provide little evidence vitamin D helps manage diabetes, particularly in people with enough vitamin D.
Tested ways to slow the move from prediabetes to diabetes come from other trials, such as the Diabetes Prevention Program, which tested lifestyle change and metformin (see metformin and weight).
If a test shows low vitamin D, the amount to take is a decision for the prescriber. Dose Track can log a daily vitamin D next to metformin, insulin or a GLP-1, with HbA1c results noted by date.
Read more on Dose Track: Vitamin D3 · Blood sugar · HbA1c · Hazard ratio · Metformin side effects