Vitamin D has been investigated for its potential role in supporting or treating cancer, primarily due to its effects on cell growth, immune modulation, and inflammation. Observational studies have found that low serum levels of vitamin D are associated with increased risks of certain cancers, such as colorectal, breast, and prostate cancer. Laboratory and animal studies suggest that the active form of vitamin D (calcitriol) can inhibit cancer cell proliferation, induce apoptosis (programmed cell death), and reduce tumor angiogenesis.

However, clinical trials in humans have produced inconsistent results. Some randomized controlled trials (RCTs) and meta-analyses suggest a modest reduction in cancer mortality (but not necessarily incidence) with vitamin D supplementation, especially in individuals who were deficient at baseline. The evidence is not strong enough to recommend vitamin D as a stand-alone cancer treatment, but it may be considered as an adjunct to standard therapy, particularly for those with low vitamin D levels.

Major guidelines currently do not endorse vitamin D supplementation for cancer prevention in the general population, but acknowledge its importance for overall health, especially bone health and immune function. In summary, while there is some scientific basis for the use of vitamin D in cancer support, the evidence is limited (rated 2 out of 5), and more robust clinical trials are needed to clarify its role.