Many studies that track healthy people for years have reached the same conclusion: “People with higher levels of vitamin D have a lower risk of developing type 2 diabetes in the future,” says Tufts Medical Center’s Anastassios Pittas, who is also a professor of medicine at Tufts University’s Sackler School of Graduate Biomedical Sciences.
But those studies can’t tell if something else about people with low vitamin D increases their risk. So researchers have looked further.
“We and others have done short-term intervention studies looking at insulin sensitivity, insulin secretion, or similar outcomes,” says Pittas. “Some have not shown any difference, but others show promise.”
For example, Pittas and his colleagues gave 92 overweight or obese adults with prediabetes vitamin D (2,000 IU a day), calcium (400 mg twice a day), both, or a placebo for four months. Those taking vitamin D (either alone or with calcium) had better pancreatic beta-cell function than those who didn’t take vitamin D.
“Beta-cell function is one of the best measures we have for future risk of getting type 2 diabetes,” notes Pittas.
But in other studies, vitamin D didn’t help. Why?
“I think that it is very difficult to show any results in people with normal glucose tolerance or established type 2 diabetes,” says Pittas. “If you’re normal, vitamin D will not make you any more normal. And if you have established type 2 diabetes and you’re treated with insulin or other medications, vitamin D cannot compete.”
That’s why Pittas has launched D2d, a large trial testing vitamin D (4,000 IU a day) in people with prediabetes.
“We’re targeting people at high risk for diabetes,” he explains. “D2d is a national study taking place in 21 cities. There is a questionnaire online, at d2dstudy.org, that people can complete to find out their risk of diabetes. Even if people are not interested in participating in the study, they can go there to evaluate their risk by answering a few questions.”
Meanwhile, the VITAL trial will test whether a lower dose of vitamin D (2,000 IU a day) can prevent diabetes in people who aren’t at high risk.
“Between the two trials, I think there will be a clear answer as to whether vitamin D supplementation is promising for preventing diabetes,” says Manson.
How might vitamin D work?
“There’s preliminary evidence that it may improve insulin secretion,” says Pittas.
“The enzyme that converts vitamin D to its active form is expressed in beta cells, which means that the beta cell needs an adequate vitamin D supply to function. Only a few organs outside of the kidney have the ability to activate vitamin D so they don’t have to depend on the circulating levels of the active vitamin.”
Sources: Diabetes Care 36: 1422, 2013; Am. J. Clin. Nutr. 94: 486, 2011.