The discovery that the vitamin D receptor is expressed in more than 30 tissues indicates that the physiologic functions of vitamin D are much broader than its well-known role in the regulation of calcium absorption and bone metabolism. There is evidence that vitamin D is involved in the pathogenesis of cancer, cardiovascular disease, multiple sclerosis, and type I diabetes. Recent epidemiological evidence points to a strong association between vitamin D insufficiency and insulin resistance, the metabolic syndrome, and type II diabetes. The investigators would like to examine the role of vitamin D in the development of insulin resistance in overweight children and adolescents, which represent a high risk population for cardiovascular and metabolic complications. The investigators propose a prospective randomized clinical trial of vitamin D supplementation in overweight, insulin resistant, vitamin D deficient children. The investigators objective is to assess if changes in insulin resistance, fasting lipid profiles, blood pressure, and inflammatory markers occur in these patients post treatment with vitamin D. Additionally, concomitant changes in calcium and bone metabolism after vitamin D treatment will be evaluated. This is because, contrary to adults, the optimal vitamin D concentrations that regulate calcium and bone metabolism have not been established in pediatrics.

Inclusion Criteria:

1. BMI>85th% for age & sex
2. Vitamin D25 between 10-20ng/ml
3. Normal serum Ca concentrations >8.5mg/dl
4. Evidence of insulin resistance (measured by HOMA-IR, and QUICKI indices)

Exclusion Criteria:

1. Vitamin D25<10ng/ml
2. No parental consent
3. No evidence of insulin resistance
4. BMI < 85th percentile
5. Known diagnosis of type 1 or 2 diabetes
6. Severe underlying disease such as liver disease, end-stage renal disease, or malignancy
7. Present medication that affects insulin sensitivity such as steroids or Metformin
8. Any chronic illness or administration of medications that is associated with fat malabsorption as they may interfere with vitamin D absorption.
9. Known history of hypocalcemia, calcium disorder (such as Di George syndrome)
10. Serum Calcium concentration < 8.5mg/dl
11. Other drugs that might effect vitamin D metabolism due to induction of P450 enzyme activity.