Fact Check: Does Vitamin D Protect Against Cancer?

Certainly, people have been recommended to take vitamin D, and there is often talk of a worldwide vitamin D deficiency. In a global study from 2014, about 45% of volunteers were found to have inadequate vitamin D levels. Some people believe this vitamin deficiency to be associated with reduced protection against diseases. Therefore, vitamin D is claimed to have many preventive properties against depression, diabetes, and cancer.

But what is true about this myth?
Vitamin D is a hormone produced when the skin is exposed to sunlight, whereas only a fraction be absorbed through food sources. Factors influencing vitamin D supply include geographical location, which determines exposure to sun rays, local air pollution and other factors like age, skin pigmentation, and pregnancy. Life habits such as diet, cultural dress codes, and daily outdoor activities also play an important role in vitamin D supply.
Its main functions are in bone mineralisation and maintaining the calcium balance in our body. It likely plays a role in many other cellular processes, as vitamin D can be recognized/taken up by nearly every cell in the body. In experiments with animal models and cell cultures, anticarcinogenic properties, such as promotion of cell death and inhibition of cell growth, were found. Based on these results, researchers hope to exploit a cancer-preventing effect in humans.
In the report "Vitamin D and cancer prevention - between truth and speculation" (written by the authors of this article), the current state of studies on the issue of cancer prevention through vitamin D was outlined in detail. One of the biggest problems in this area of research is that we can't experiment with humans the way we do with animal models or cell cultures. Therefore, clinical trials are conducted, in which probands are administered vitamin D to investigate the influence on cancer prevention and mortality.
Getting an overview and to be able to summarise all these individual studies, so-called meta-analyses are performed. These are systematic, representative, and objective statistical evaluations comparing several studies. The report focuses on meta-analyses of vitamin D supplements and their impact on cancer incidences and mortality.

Currently, there is no clear evidence supporting the hypothesis that vitamin D supplements prevent cancer. However, there is indication that vitamin D supplementation may reduce mortality in cancer patients. This was demonstrated especially in patients with colorectal cancer. Such results should be treated with caution, as studies often examine only certain population groups, e.g., those from specific geographic regions, gender, or age groups. Also, the number of participants and the number of studies used are often insufficient. In addition, the concentration of supplemented vitamin D varies from study to study, making comparison difficult. Furthermore, it is challenging to separate cancer-promoting factors such as alcohol, smoking or genetic predispositions from the preventive effects of vitamin D.

Therefore, from a scientific point of view, it is risky to promote the myth that "vitamin D protects against cancer". The statements are unproven and require far more study data than is available today.

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Issue 3
Elisa Peters, Emily Fichter, Leonard Kurzweg, Charlotte Kricke