Researchers enrolled 126 healthy young adults from the University of California. All the subjects had normal glucose tolerance and normal blood pressure. Thirty seven percent of these young adults tested vitamin D deficient (serum 25-hydroxyvitamin D level less or equal to 20 ng/ml).
Researchers measured serum (blood) vitamin D levels, glucose levels, and insulin levels at fasting and after an infusion of a high amount of glucose into the bloodstream. The people with the highest vitamin D levels also had the best insulin sensitivity; less insulin was required to control blood sugar levels. Also, the results indicated that low vitamin D levels have a negative effect on the function of the cells that release insulin.
Based on their study findings and other previous research studies, researchers concluded that low levels of vitamin D is a risk factor for Type-2 diabetes and the metabolic syndrome. They also conclude that increasing serum vitamin D levels can improve insulin sensitivity by 60%, which is a greater improvement than treatment with the prescription drugs trolitazone or metformin.
Chiu, K et al, "Hypovitaminosis D is associated with insulin resistance and b cell dysfunction1-3". Am. J. Clin. Nutr., 2004;79:820-5. http://www.ajcn.org/