One in four Americans is estimated to have borderline diabetes. Researchers at the Maine Medical Center Research Institute want to help them avoid a full-blown diagnosis of the insulin disorder, a leading cause of death in the U.S.

Vitamin D might just hold a clue. The Scarborough institute has been selected to participate in a large-scale clinical trial, funded by the National Institutes of Health, to study the effectiveness of vitamin D in reducing the risk of developing type 2 diabetes. As one of 20 participating sites in the country, MMCRI is helping to recruit a total of 2,400 volunteers for the study.

Early research shows Vitamin D holds promise for staving off diabetes, said Dr. Irwin Brodsky, principal investigator for the study at MMCRI. The vitamin’s gotten plenty of attention for other supposed benefits as well, such as boosting the immune system and the brain, he said. But while some medical professionals sing the vitamin’s praises, those effects remain unproven, he said.

“The interest level around the country is very, very high … I hate to say it but we give advice more on the basis of faith than the basis of medicine,” Brodsky said.

People with diabetes tend to have low Vitamin D levels, and animal studies have linked severely low Vitamin D to insulin problems, Brodsky explained. The randomized trial is designed to pin down whether that circumstantial evidence underlies a real connection between vitamin D and diabetes. Study participants with pre-diabetes will take a Vitamin D supplement, hopefully postponing or preventing diabetes down the road.

If the study shows a benefit, Vitamin D could one day provide a low-risk, inexpensive, and easy way to help prevent the disease. While plenty of research already supports lifestyle changes — eating well, exercising, and losing weight — as ways to decrease the chances of developing diabetes, Vitamin D could complement such efforts, Brodsky said.

The study, known as D2d, is the first definitive, large-scale clinical trial of its kind. MMCRI, which was awarded a $1.6 million NIH grant, hopes to recruit about 150 local participants. The institute’s looking for adults at least 30 years old with risk factors for developing diabetes, among other criteria. Mainers get limited sun exposure in these cold northern latitudes, which may decrease vitamin D levels, making them important participants in the study, according to Brodsky.