Vitamin D supplements are typically used to guard against bone loss and fractures, but new research offers up another possibility: For folks with pre-diabetes, they may help lower the chances of a full diabetes diagnosis.

Across three clinical trials, investigators found that vitamin D supplements were modestly effective in curbing the risk of pre-diabetes progressing to type 2 diabetes. Over three years, just under 23% of study patients using vitamin D developed diabetes, versus 25% of those given placebo pills.

On average, the study found, supplements lowered the risk of progressing to type 2 diabetes by 15%.

“It’s pretty clear vitamin D has a moderate effect on reducing the risk of type 2 diabetes, if you’re at high risk,” said lead researcher Dr. Anastassios Pittas, of Tufts Medical Center, in Boston.

The findings do not apply to people at average risk of the disease, he stressed, and it’s still unclear what the optimal dose of vitamin D is for people with pre-diabetes.

Plus, Pittas said, no supplement would be a replacement for lifestyle changes, including a healthy diet and regular exercise.

“We don’t want the message to be, take a pill and you won’t need to do the hard work of changing your diet and exercising,” Pittas said.

Type 2 diabetes arises when the body’s cells no longer properly respond to the hormone insulin, which helps shuttle sugars from food into cells to be used as energy. As a result, blood sugar levels remain chronically high, which over time can damage the blood vessels and lead to heart, kidney and eye disease, among other complications.

Pre-diabetes is a state where blood sugar is abnormally high, but not yet high enough to diagnose type 2 diabetes. In the United States alone, about 96 million adults have pre-diabetes, according to the U.S. Centers for Disease Control and Prevention.

The vitamin D study started with the observation that diabetes prevalence is typically greater in places farther from the equator. That, Pittas said, hinted that sunlight exposure — which spurs the body to naturally produce vitamin D — might play a role in diabetes risk.

Subsequent studies found a link between people’s blood levels of vitamin D and their risk of type 2 diabetes. Meanwhile, lab research pointed to some potential reasons: vitamin D can, for instance, restore normal insulin production in animals.

So far, there have been three clinical trials that directly tested whether vitamin D supplements can lower the odds of pre-diabetes progressing to type 2. Each found that participants given vitamin D did have a somewhat lower risk, versus those given a placebo. But the difference was not significant in statistical terms, meaning the supplement could not be declared effective.

So Pittas and his colleagues conducted a “meta-analysis” that pulled together the data from all three trials. The idea is that, with a larger number of patients, it will be easier to detect a moderate effect of vitamin D.

The analysis, published online Feb. 6 in the Annals of Internal Medicine, included just over 4,000 adults with pre-diabetes. Half were randomly assigned to take vitamin D. In two trials, participants took vitamin D3, at a dose of either 4,000 IU a day, or 20,000 IU per week. The third trial used eldecalcitol, a vitamin D “analogue” prescribed for osteoporosis.

Over three years, just under 23% of supplement users developed type 2 diabetes, versus 25% of placebo users.

That’s a modest difference, but the researchers pointed to the bigger picture: There are 374 million people worldwide with pre-diabetes, and these findings suggest that vitamin D can at least delay diabetes in 10 million of them.

“I’d say that’s pretty significant,” said Dr. Isaac Dapkins, chief medical officer of NYU Langone’s Family Health Centers, in New York City.

Dapkins, who was not involved in the study, said it gives him incentive to measure blood vitamin D levels in his patients with pre-diabetes. There were indications that supplements were more effective for participants who started off with vitamin D deficiency (below 12 ng/mL).

Like Pittas, Dapkins stressed the importance of overall lifestyle in halting the progression of pre-diabetes.

“If you were to start an exercise program, it would be more effective [than vitamin D],” he pointed out.

But, Dapkins said, adding a vitamin D supplement could be an easy, low-cost way to get further protection. His advice was for people with pre-diabetes to talk to their doctor, and get a blood vitamin D measurement if they haven’t already.

Pittas said more work is needed to figure out the optimal dose of vitamin D for people with pre-diabetes.

In general, 4,000 IU per day — the dose used in one trial — is considered the upper limit for vitamin D intake. Vitamin D is stored in body fat, Dapkins noted, and there is the potential for very high levels to cause problems, such as kidney stones.

Across the three trials, just over 1% of participants developed kidney stones, and supplement users were not at greater risk.

More information

The U.S. National Institutes of Health Office of Dietary Supplements has more on vitamin D.

SOURCES: Anastassios Pittas, MD, MS, chief, division of endocrinology, diabetes & metabolism, Tufts Medical Center, Boston; Isaac Dapkins, MD, chief medical officer, Family Health Centers, NYU Langone, New York City; Annals of Internal Medicine, Feb. 6, 2023, online