Vitamin D Oral Replacement in Asthma

This study has two parts. In part 1, study participants will be randomized to receive one of four doses of vitamin D supplementation in international units (IU) over at 16-week period: 1) Single 50,000 IU loading dose + 6000 IU daily dose; 2) Single 50,000 IU loading dose + 10,000 IU daily dose: 3) 6000 IU daily dose; or 4) 600 IU daily dose. Based on pharmacokinetic analysis, one of the doses (1-3) will be selected to use in part 2. In part 2, study participants will be randomized to the dose selected in part 1 or the 600 IU daily dose of vitamin D supplementation to be administered over a 16-week dosing period. Across both parts, safety of each dose regimen of vitamin D supplementation will be evaluated, and the effectiveness of each dose to achieve a serum level of 25(OH)D greater than or equal to 40 ng/ml will be assessed.