US National Institutes of Health (NIH) said Monday that it's to launch a large-scale clinical trial to investigate if a vitamin D supplement helps prevent or delay type 2 diabetes in adults who have prediabetes, and are at high risk for developing type 2.

The multiyear study will include about 2,500 people and its goal is to learn if vitamin D, specifically D3 (cholecalciferol), will reduce the risk of developing type 2 diabetes in adults aged 30 or older with prediabetes.

People with prediabetes have blood glucose levels that are higher than normal but not high enough to be called diabetes.

"Vitamin D use has risen sharply in the US in the last 15 years, since it has been suggested as a remedy for a variety of conditions, including prevention of type 2 diabetes," said Myrlene Staten, Vitamin D and Type 2 Diabetes (D2d) project officer at the National Institute of Diabetes and Digestive and Kidney Diseases ( NIDDK), part of the NIH.

"But we need rigorous testing to determine if vitamin D will help prevent diabetes. That's what D2d will do," Staten said in a statement.

Past observational studies have suggested that vitamin D could reduce the diabetes risk by 25 percent, but the NIH said it will be not able to make any definitive conclusion until the clinical trial is complete.

The trial will directly examine if a daily dose of 4,000 International Units (IUs, 100 mcg) of vitamin D, greater than a typical adult intake of 600 to 800 IUs (15 to 20 mcg) a day, but within limits deemed appropriate for clinical research, helps keep people with prediabetes from getting type 2 diabetes.

Half of the participants will receive vitamin D. The other half will receive a placebo, a pill that has no drug effect. Participants will have check-ups for the study twice a year, and will receive regular health care through their own health care providers, the NIH said.

The study will be double-blinded, so neither participants nor the study's clinical staff will know who is receiving vitamin D and who is receiving placebo, it said.

The study will continue until enough people have developed type 2 diabetes to be able to make a scientifically valid comparison between diabetes development in the two groups, likely about four years, it added.

According to data from the 2011 National Diabetes Fact Sheet, an estimated 79 million Americans have prediabetes, and nearly 26 million more have diabetes.