Prospective studies suggest that vitamin C deficiency may be associated with the development of colorectal cancer, whereas vitamin D supplementation may reduce the risk of colorectal cancer. 3 prospective studies have shown that low levels of vitamin D are associated with high mortality in patients with colorectal cancer, especially in stage III and IV patients. In addition, a study enrolling 515 patients with stage IV colorectal cancer showed that 82% of patients were vitamin D deficient (<30 ng/ml) and 50% were vitamin D deficient (<20 ng/ml). However there has not been a single study confirming that vitamin D supplementation improves patient prognosis. In a recent report, the Institute of Medicine noted that the only role of vitamin D supported by current data is for bone health, which has not been demonstrated in cancer or other diseases. Given this report and the lack of Class 1 evidence, the panel does not currently recommend routine screening for vitamin D deficiency in patients with colorectal cancer, nor does it advocate routine vitamin D replacement therapy.