Inclusion Criteria:
- > 18 years of age
- Diagnosed with stage III colon or stage II/III rectal cancer that will receive neoadjuvant or adjuvant chemotherapy but have not yet started
- Baseline serum vitamin D level below 52 ng/ml
- Willing to stop herbal medications as directed by physician
- Willing to stop current supplemental vitamin D (Multivitamin with vitamin D component is acceptable)
Ineligibility Criteria
- Patients who do not undergo chemotherapy
- Patients with prior chemotherapy for this cancer
- No other prior malignancy except for adequately treated basal cell or squamous cell skin cancer, in situ cancer, or other cancer for which the patient has been disease-free for > 3
- years
- Unwilling or unable to stop oral supplemental vitamin D
- Patients taking high-dose vitamin D supplementation (50,000 IU weekly) prior to enrollment
- Patients with vitamin D levels above 52 ng/ml at baseline testing
- Patients with hypercalcemia and/or any condition resulting in malabsorption
Patients will be randomized to one of two supplementation options. One group will take a 2,000 IU capsule daily and the other group will take 50,000 IU weekly.