The randomized phase 2 SOLARIS trial failed to demonstrate improved disease control with high-dose (4000 IU daily) versus standard-dose (400 IU daily) vitamin D3 supplementation concurrent to first-line chemotherapy for metastatic colorectal cancer, despite median baseline levels being insufficient, suggesting a low D3 level is more biomarker than culprit: While cancer biology can plausibly diminish circulating D3, correcting the number did not meaningfully diminish the cancer. | Ng, JAMA 2026
