Effect of Vitamin D Supplementation on Oral Glucose Tolerance Among Obese Adolescents

The investigators intend to recruit 20 obese adolescent subjects from the PI's obesity clinic to participate in the study. Investigators expect 5 to drop out, therefore leaving 15 subjects to complete the study. Eligible and assenting subjects (with consent from a parent) will be admitting to the Pediatric Procedure Suite and MU Women's and Children's Hospital. They will have an IV placed in the arm by a nurse experienced with working with children. The subjects will have blood drawn from the IV checking for vitamin D level, insulin, glucose, and c-peptide level (another marker for insulin status). The subject will then be asked to drink a 75 gram glucose solution. Additional blood will be drawn from the IV site to check for glucose, insulin, and c-peptide levels at 30 minutes, 60 minutes, 90 minutes, and 120 minutes. A total of 30 mL (2 tablespoons) of blood will be drawn that day. The subject will have the IV removed and will be discharged to then take two 2,000 IU vitamin D3 pills (total 4,000 IU) daily for six months.

At a routine 3 months clinical visit, the subject will be tested for routine, standard of care, basic metabolic profile (BMP) with 4 ml of blood (less than 1 teaspoon) to assess for high calcium levels, a potential complication of vitamin D therapy. In the investigators' previous study of adolescents taking 4,000 IU vitamin D daily for six months, no subject developed a high calcium level.

At six months, the subject will return to the Pediatric procedure suite to have another OGTT and labs via IV as described above.

Inclusion criteria

• Obese adolescent (BMI >85th percentile for age)
• 9-19 years of age
• attending the ADOBE clinic at the University of Missouri
• 25OH vitamin D level within past 3 months

Exclusion criteria

• use of vit D supplements other than standard multi-vitamin preparation (i.e., should not be receiving vit D > 1000 IU/d)
• use of medications that interfere with vit D metabolism (e.g., anti-convulsive)
• history of hepatic or renal disorders, hypercalciuria, or hypercalcemia
• undergoing UV radiation as medical therapy
• pregnancy; cigarette smoking; current use of a tanning bed
• current type 2 diabetes
• any current antihyperglycemic medication use (e.g. metformin, insulin) less than one month prior to initial OGTT.