Effectiveness of High Dose Vitamin D Supplementation in Stage III Colorectal Cancer

Inclusion Criteria: * ≥ 18 years of age * Willing to stop herbal medications as directed by physician * Willing to stop current supplemental Vitamin D (Multivitamin with Vitamin D component is acceptable) * Willing to travel to Legacy Health/OHSU facility if necessary * Agree to attend study visits outside of standard of care visits, if necessary * 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 Exclusion Criteria: * ≤ 18 years of age * Colon cancer stages I-II and IV or Rectal cancer stage I or IV * 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 * Unable to comply with protocol * Unable to provide written informed consent * 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 * Investigator does not believe study participation, for any reason, is in the best interest of the patient