Vitamin D is commonly known for its crucial role in bone health, but the authors point out it also regulates the immune system, and its anti-cancer effects are mediated mainly by immune cells, such as monocytes and T cells. Vitamin D exerts its effects via the vitamin D receptor (VDR), which is a transcription factor involved in the expression and epigenetic regulation of numerous genes.

According to the review, studies focusing on the effect of vitamin D on different types of cancers provide the strongest evidence of its benefits in colorectal cancer and in blood cancers, such as leukemias and lymphomas. Vitamin D is important both for the differentiation of blood cells during hematopoiesis as well as adult stem cells in rapidly regenerating tissues, such as colon or skin. A too low vitamin D status leads to a suboptimal function of the VDR and in an increased risk that these cells are not fully differentiating and start to turn into uncontrolled growing cancer cells.

Even in other types of cancer, such as breast and prostate cancer, a low vitamin D status, measured as the level of 25-hydroxyvitamin D in the blood, has been associated with a higher cancer incidence and a poorer prognosis. However, vitamin D supplementation has not been consistently shown to reduce cancer mortality in randomized controlled trials. According to the authors of the review, the impact of vitamin D could be shown more clearly if the participants were stratified according to their individual vitamin D responsiveness and the health outcomes analyzed in relation to changes in individual vitamin D status.

Professor Carlberg’s research group has earlier shown that individuals differ in their molecular response or sensitivity to vitamin D supplementation. For example, 25% of the Finnish population seem to be low responders, needing a higher dose of vitamin D supplementation to reach the full clinical benefit. In terms of cancer risk, being a high responder can be expected to have a protective effect.

According to the review, a good vitamin D status is beneficial in general cancer prevention. There is less evidence of its usefulness in the treatment of cancer.