This is a pilot study to test whether there is an association between baseline Vitamin D levels, Vitamin D supplementation and survival in patients with stage III colon and stage II/III rectal cancer receiving chemotherapy. 70 patients with colon stage III or rectal stage II or III cancer that require chemotherapy will be screened and 60 patients will be enrolled. Patients will be randomized to standard dose (2000 IU daily) or high-dose (50,000 IU weekly) Vitamin D supplementation for 1 year after initiation of chemotherapy. Patients' Vitamin D levels will be checked throughout supplementation then followed for 5 years with occasional Vitamin D testing and surveying in order to collect information on recurrence and survival outcomes.

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