Vitamin D helps regulate glucose metabolism and insulin secretion. Observational studies point to an association between low levels of the active form of vitamin D, 25-hydroxyvitamin D, in the blood and a high risk of developing type 2 diabetes. That led researchers to wonder, could vitamin D reduce prediabetics' risk of developing type 2 diabetes? The answer to that question, according to a team led by researchers at Tufts University in Boston, is yes — with a few reservations. They found that vitamin D reduced the risk of developing type 2 diabetes in adults with prediabetes by 15 percent.

“Although this relative reduction in risk may seem small, when extrapolated to the more than 374 million adults worldwide with prediabetes, it suggests inexpensive vitamin D supplements may delay the development of type 2 diabetes in more than 10 million people,” the authors write.

Participants in all three studies who took vitamin D were 30 percent more likely to have returned to normal blood glucose control than those who took placebo.

Three randomized trials that enrolled almost 4,200 adults with prediabetes, a condition characterized by blood sugar that is higher than normal, but not high enough yet for a diabetes diagnosis, were used in the study. In a Norwegian study, participants took either 20,000 IU (500 mcg) per week of cholecalciferol, an inactive form of vitamin D, or a placebo. An American study randomized participants to either 4,000 IU (100 mcg) of cholecalciferol daily or a placebo. In a Japanese study, participants took .75 mcg per day of eldecalcitol, a synthetic version of vitamin D approved for use in Japan, or a placebo.

Cholecalciferol is broken down to 25-hydroxyvitamin D in the liver. Participants in the Norwegian and American studies who maintained a mean serum level of 125 nmol/L or higher of 25-hydroxyvitamin D during the study had a 76 percent lower risk of developing type 2 diabetes compared to those whose levels were between 50 to 74 nmol/L. The results suggests that prediabetics need to maintain a 25-hydroxyvitamin D level near or above 125-150 nmol/L.

It is not yet clear if this level could be tolerated by most people. The tolerable upper intake level of vitamin D, or the highest dose with no risk of adverse effects, is currently set by government agencies at 4,000 IU (100 mcg) per day. As an editorial related to the study points out, to reach or maintain this 25-hydroxyvitamin D level might require a dose above the tolerable level.

What is needed to test the finding that high levels of vitamin D reduce type 2 diabetes risk in those with prediabetes is a well-designed trial of treatment to a target level of 25-hydroxyvitamin D, the researchers said. This trial would also provide important evidence about the tolerable upper intake level of vitamin D.

Kidney stones and excess calcium in the blood and urine are associated with high dose vitamin D therapy. Unfortunately, scientists do not know if high doses of vitamin D can have other adverse effects, such as increasing the risk of falls. Observational studies can provide more information about adverse effects associated with high doses of vitamin D, according to the researchers.

The ability to adequately control blood glucose levels is very important to good health, so the risks of high dose vitamin D therapy may be worth it: Prediabetes puts a person at greater risk of cardiovascular disease, kidney disease and retinopathy, compared to those without prediabetes. Participants in all three studies who took vitamin D were 30 percent more likely to have returned to normal blood glucose control than those who took placebo.

The study, patient summary, and editorial are published in Annals of Internal Medicine.