Vitamin D supplements prevent cancer and heart disease.

Low vitamin D is associated with many diseases, so supplements were widely expected to prevent cancer and heart disease. But low vitamin D might be a consequence of poor health (people who are ill go outside less), not a cause — only a randomized trial can tell.

The VITAL trial (Manson et al., 2019) randomized nearly 26,000 US adults to vitamin D or placebo for about five years. It found no significant reduction in the primary outcomes of invasive cancer or major cardiovascular events. Some secondary analyses hinted at a possible reduction in cancer deaths over longer follow-up, keeping the door open, but the headline preventive claims were not confirmed. For people who are not deficient, routine high-dose vitamin D to prevent cancer or heart disease is not supported; correcting genuine deficiency is a separate, valid use.