The purpose of this study is to compare the effects of two different forms of vitamin D supplements (vitamin D3 and 25-hyrdroxyvitamin D3 [25(OH)D3]) on vitamin D levels in the blood, and on markers of skeletal and immune health. This study is designed as a pilot study. A multi-ethnic cohort of 48 patients will be included (12 Caucasian, 12 African American, 12 Hispanic/Latino, 12 Asian/Asian American). Potential study candidates will undergo a screening visit during which a medical history will be taken, a physical exam performed, a dietary questionnaire administered, and blood collected. The purpose of the screening visit is to identify vitamin D-deficient (25-hydroxyvitamin D <20 ng/ml) individuals who are candidates for vitamin D supplementation. Eligible patients will then be randomly assigned to receive either vitamin D3 (2400 IU/day) or 25(OH)D3 (20 mcg/day) for 16 weeks. After initiating supplementation, study patients will return for follow-up visits at weeks 4, 8, and 16. At each visit, blood will be collected to assess vitamin D levels in the blood, as well as markers of skeletal and immune function. Participation in this study will therefore involve a total of 5 visits (1 screening visit, 1 randomization visit during which participants will receive their study supplements, 3 follow-up visits). After all data is collected, changes in vitamin D levels in the blood following supplementation with either vitamin D3 versus 25(OH)D3 will be determined, and correlated to markers of calcium balance and immune function. Knowledge gained from this study may have a significant impact on how vitamin D status is defined, and how vitamin D repletion is administered.

Inclusion Criteria:

• Age > 18
• 25D < 20 mg/ml

Exclusion Criteria:

• Age < 18
• 25D > 20 ng/ml at time of screening
• Clear signs of infection at time of screening (coughing, sneezing, wheezing, frequency, dysuria)
• History of conditions that would influence intestinal absorption of vitamin D3 or 25OHD3 supplements
• History of nephrolithiasis, primary hyperparathyroidism, or other metabolic bone disease
• History of chronic kidney disease (defined as CrCl < 30 ml/min)
• History of rheumatologic or autoimmune conditions
• History of sarcoidosis
• History of active or latent tuberculosis
• History of HIV
• History of hyperthyroidism
• History of chronic glucocorticoid use defined as the equivalent of 5 mg of prednisone per day for more than 2 months within the last 6 months
• History of use of medications known to affect calcium/vitamin D metabolism, bone metabolism, and immune response
• Hypercalcemia
• Hypercalcinuria