There is a known association between low vitamin D levels and increased risk of colorectal cancer. So along with general prevention, keeping it supplemented post treatment seems a good strategy against metastasis. Vitamin D activates our T cells. There are also some small trials underway to investigate the benefits of high-dose vitamin D3 supplementation during chemo. There seems to be some good evidence it helps long term outcomes, although they're still unsure why. But it seems low risk. [Effect of High-Dose vs Standard-Dose Vitamin D3 Supplementation on Progression-Free Survival Among Patients With Advanced or Metastatic Colorectal Cancer: The SUNSHINE Randomized Clinical Trial | Colorectal Cancer | JAMA | JAMA Network](https://jamanetwork.com/journals/jama/fullarticle/2730112) >Vitamin D is a steroid hormonewith the main function of regulating bone metabolism. However, it has many physiological functions, such as anti-inflammatory, immunomodulatory, proapoptotic, and antiangiogenic effects. Preclinical studies have suggested that it might act as a carcinogenesis inhibitor, slowing tumor progression by promoting cell differentiation and inhibiting cancer cell proliferation. Vitamin D may, therefore, inhibit tumor invasiveness and its propensity to metastasize, potentially leading to reduced cancer mortality. > #Source [Vitamin D and colorectal cancer – A practical review of the literature - ScienceDirect](https://www.sciencedirect.com/science/article/pii/S2468294222001071)