The results of a phase 2 randomized controlled trial (RCT) of high- vs. low-dose vitamin D3 supplementation for 139 patients with advanced colorectal cancer taking chemotherapy was recently reported in JAMA [Ng et al., 2019]. The high-dose vitamin D treatment was 8000 IU/d vitamin D3 for two weeks, then 4000 IU/d thereafter. The low-dose treatment group received 400 IU/d vitamin D3. The length of time before the disease worsened was 13 months in the high-dose treatment group and 11 months in the low-dose group. Further, high-dose vitamin D3 supplementation significantly reduced the risk of death by 36% (P = 0.02). In secondary analyses, the length of time before the disease worsened was significantly longer for patients in the treatment group who maintained a healthy weight, or had more metastatic sites.