This investigation will consist of a prospective study utilizing two separate populations of patients with 25(OH)D deficiency, one population with chronic kidney disease (CKD) and one with normal renal function.

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 all different types of health issues. CKD is the slow loss of kidney function over time. The main function of the kidneys is to remove wastes and excess water from the body. This loss of function usually takes months or years to occur Patients with CKD often have low levels of vitamin D in their blood.

This study will have two groups of patients with CKD and one group of patients that have normal kidney function, but all groups will have low levels of vitamin D. The two groups with CKD (group 1 and group 2) will receive either cholecalciferol or calcitriol. The purpose of having a control group (group 3) without CKD will be to evaluate if any changes that are witnessed in response to vitamin D therapy are specific to patients with kidney disease or apply to all patients with vitamin D deficiency who receive vitamin D supplements.

There are two different drugs in this study. One is called Calcitriol and is approved by the Food and Drug Administration (FDA) as a vitamin D supplement. The other drug is called cholecalciferol and it is approved by the FDA. Unlike calcitriol, cholecalciferol is a nutritional form of vitamin D that can often be found in various types of foods and its chemical structure must be changed by the body to become the active form of vitamin D. It is believed that cholecalciferol may have different effects in the body compared to calcitriol. It is possible that CKD patients would benefit from receiving both of these drugs, but this is currently unclear. While on this study you will receive either one of these study drugs, depending on which group you are assigned to.

Inclusion Criteria:

• Chronic kidney disease w/GFR<35ml/min
• Nutritional vitamin D deficiency, defined as 25(OH)D < 25ng/ml
• Secondary hyperparathyroidism, PTH>75pg/mL

Exclusion Criteria:

• Active infection
• Recent hospitalization for acute illness (within last month)
• Refusal to study participation
• History of chronic inflammatory disease process (i.e. inflammatory bowel, rheumatoid arthritis, SLE, etc.)
• Allergy to cholecalciferol or calcitriol
• History of parathyroidectomy
• Functional renal transplant within 5 years
• Current treatment with immunosuppressant medications
• Noncompliance with prescribed medications