Study of Vitamin D in Untreated Metastatic Colorectal Cancer

In this research study, the investigators are comparing standard and higher dose Vitamin D treatment when given in combination with standard treatment for metastatic colorectal cancer. Standard treatment includes the chemotherapy combination of 5-FU, Leucovorin and Oxaliplatin (FOLFOX) with bevacizumab. Participants will be randomized into one of the study groups-Arm A: Vitamin D (standard dose of 400 IU/day), FOLFOX and Bevacizumab or Arm B: Vitamin D (higher dose of 8000 IU/day for 2 weeks followed by 4000 IU/day), FOLFOX and Bevacizumab. Study Treatment (A cycle of treatment is 14 days): Vitamin D Cycle 1: You will take two capsules of Vitamin D orally, once a day (at the same time), every day. Participants randomized to Arm A will be taking one capsule with 400 IU of Vitamin D and one capsule with placebo (pills with no medicine) so that neither you nor your doctor will know what group you have been assigned to. Participants randomized to Arm B will be taking two capsules of 4000 IU each. Subsequent Cycles: You will take one capsule orally, once a day (at the same time), every day. Participants randomized to Arm A will be taking one capsule with 400 IU of Vitamin D. Participants randomized to Arm B will be taking one capsule with 4000 IU of Vitamin D. FOLFOX and bevacizumab FOLFOX and bevacizumab will be given intravenously (IV, through a vein in your arm) on Day 1 of every cycle for all participants in both Arms A and B. The infusions will take several hours, in addition to your doctor's visit, so you should plan on being in clinic most of the day. Note that the 5-FU is given bolus on day 1 (given as one dose), and is then given as a continuous IV infusion over 2 days. You will need to have a port-a-cath placed. A port-a-cath is a medical device that is placed under the skin. The continuous infusion is delivered by a pump that is inserted into the port-a-cath. The pump will be carried in a pouch that you can hook around your waist. Arrangements will be made for you to have the pump disconnected after 2 days. You may need to return to clinic to have it disconnected.