This study is to research two questions. First, is vitamin D3 more effective than vitamin D2 in raising 25-hydroxyvitamin D [25(OH)D] levels in chronic kidney disease (CKD) patients? And secondly, what are the differential effects of vitamin D2 and vitamin D3 on other mineral metabolism parameters?

Vitamin D helps form and strengthens bones by allowing the body to absorb calcium. Vitamin D helps the immune system fight infection as well as helps keep muscles strong. Without enough vitamin D, bones can become weak, thin and brittle.

Vitamin D is useful in people with various types of health issues. Patients with CKD exhibit an unusually high rate of vitamin D deficiency, which may contribute to some of the poor clinical outcomes in this group.

This study will randomize patients with CKD and low vitamin D levels to two groups; one group will be treated with vitamin D2 (ergocalciferol) and the other group will be treated with vitamin D3 (cholecalciferol). The purpose of this study is to compare the effects of the two different forms of vitamin D specifically in patients chronic kidney disease.

Inclusion Criteria:

• Age 18 and above
• Chronic kidney disease with an estimated glomerular filtration rate (GFR) between 15-60 ml/min (CKD stage III-IV)
• Vitamin D insufficiency (25-hydroxyvitamin D level < 30 ng/mL) that has not been treated with vitamin D replacement since the acquisition of this level

Exclusion Criteria:

• Current treatment with cholestyramine
• Presence of GI disorders such as short bowel, history of gastrectomy, colectomy, gastric bypass, inflammatory bowel disease, celiac disease, disorders of fat absorption, chronic diarrhea.
• Liver cirrhosis
• Known current substance abuse
• Current treatment with immunosuppressant medications
• Presence of chronic infection
• History of chronic inflammatory disease (i.e. - lupus, active rheumatoid arthritis, Crohns disease)
• Currently receiving high-dose vitamin D replacement (avg dose of ≥ 3,000 U per day) or "active" vitamin D analogue (e.g., calcitriol, which is 1,25-dihydroxyvitamin vitamin D).