Catherine Peterson of the University of Missouri and colleagues studied 35 pre-diabetic obese children and adolescents who were undergoing treatment in the university's Adolescent Diabetic Obesity Program.
All had insufficient or deficient vitamin D levels and had similar diets and activity levels.
Half of the study participants were randomly assigned either a high-dose vitamin D supplement or a placebo daily for six months, Peterson said.
The study, published in the American Journal of Clinical Nutrition, found those who took the supplement developed sufficient vitamin D levels and lowered the amount of insulin in their blood.
"By increasing vitamin D intake alone, we got a response that was nearly as powerful as what we have seen using a prescription drug," Peterson said in a statement. "We saw a decrease in insulin levels, which means better glucose control, despite no changes in body weight, dietary intake or physical activity."
The vitamin D dosage given to the obese adolescents in the study was not something recommended for everyone, Peterson said.
"For clinicians, the main message from this research is to check the vitamin D status of their obese patients, because they're likely to have insufficient amounts," Peterson said. "Adding vitamin D supplements to their diets may be an effective addition to treating obesity and its associated insulin resistance."