The purpose of this study is to find out what effects, good and/or bad, vitamin D blood levels has on stage IV colorectal cancer. Tbe doctors want to see if it is possible to increase low vitamin D levels into normal range using vitamin D supplements taken by mouth. Low vitamin D levels have been associated with worse outcomes in persons who have cancer. Low vitamin D may also cause people to have symptoms such as pain and fatigue. We want to see if increasing low vitamin D levels will help improve cancer outcomes. Vitamin D is routinely repleted in all subjects known to be vitamin D deficient. Therefore, the treatment given would be considered standard of care.