This is a Phase II study involving Stage IV colorectal cancer patients with serum vitamin D deficiency, to determine the ability to correct vitamin D deficiency and to maintain serum vitamin D levels (25-hydroxy vitamin D) once achieved.

Patients will be given vitamin D repletion with Cholecalciferol (vitamin D3 50,000 International Units) three times per week until the target vitamin D level of 40 ng/ml is achieved, and vitamin D3 maintenance initiated at 2,000 International Units daily thereafter.

To determine the ability of achieving the target serum 25-hydroxy vitamin D level of 40 ng/ml within 6 weeks of beginning vitamin D supplements in patients with metastatic colon cancer. A response is defined as achieving serum vitamin D levels ≥40 ng/ml at least once at any point during the first 6 weeks

Stage IV colorectal cancer patients will be given vitamin D repletion with Cholecalciferol (vitamin D3 50,000 International Units) three times per week until the target vitamin D level of 40 ng/ml is achieved, and vitamin D3 maintenance initiated at 2,000 International Units daily thereafter.

To determine the ability of achieving the target serum 25-hydroxy vitamin D level of 40 ng/ml within 6 weeks of beginning vitamin D supplements in patients with metastatic colon cancer. A response is defined as achieving serum vitamin D levels ≥40 ng/ml at least once at any point during the first 6 weeks

Stage IV colorectal cancer patients will be given vitamin D repletion with Cholecalciferol (vitamin D3 50,000 International Units) three times per week until the target vitamin D level of 40 ng/ml is achieved, and vitamin D3 maintenance initiated at 2,000 International Units daily thereafter.