Vitamin D Deficiency, Insulin Resistance and FGF-23

Inclusion Criteria: * Age 18 to 45 yrs * Serum 25-OHD \< or = 20 ng/mL * At least 1 menses in the last 3 months (females) and normal serum testosterone (males) Exclusion Criteria: * Significant cardiac, hepatic, oncologic, or psychiatric disease * History of diabetes mellitus, malabsorption, kidney stones, or recent alcohol excess/abuse (15 drinks per week in the last month) * Fasting glucose \> 126 mg/dl or 2 hour OGTT \> 200 mg/dl * Use of medications known to affect serum phosphate levels including phosphate-binding antacids, sodium etidronate, calcitonin, excessive doses of vitamin D (\> 1000 units per day), excessive doses of vitamin A (\> 20,000 units/day), calcitriol, growth hormone, or anti-convulsants * Use of metformin or insulin sensitizing agents * Serum calcium \< 8 or \> 11 mg/dL, creatinine \> 1.5 mg/dL, or Hgb \< 11 gm/dL * Liver function tests \> 2 times the upper limit of normal * TSH \< 0.1 or \> 7 uU/mL * WBC \< 2,000 or \> 15,000/cmm * Platelet count \< 100,000 or \> 500,000/cum * Hormone replacement therapy or testosterone use * Urine uhCG positive (females), testosterone \< 270 ng/dL (males)