Can vitamin D from sun prevent cancer?

Vitamin D is rightfully called the ‘sunshine vitamin’ since it is the precursor of the potent steroidal hormone calcitriol. Most of our foods contain little vitamin D, and hence regular supplements or adequate sun exposure to maintain adequate levels are needed (FDA daily requirement is 800 IU/day) . Replenishment of vitamin D is especially needed in the elderly and children to maintain adequate stores for bone health and immunity. But can basking in the sun and taking in vitamin D prevent cancer?

What is the relation between Vitamin D and Cancer?

A few decades ago, scientists noticed a pattern: people in sunnier parts of the world had lower rates of cancer and fewer deaths from the disease than places that got less sun.

To explain the difference, researchers started studying the effects of vitamin D. Experimental evidence has shown that calcitriol can exhibit anticancer actions through different signalling pathways, including inhibition of proliferation, induction of cell death and differentiation, as well as suppression of metastasis and angiogenesis in various cancers. Vitamin D was found to have profound effects not only on cancer cells, but also the tumour microenvironment, the support structure wherein cancer cells thrive and multiply.

What kind of cancers can be prevented with regular sunbathing?

Epidemiologic and observational data around Vitamin D prevents cancer risk suggest an inverse relationship for most cancers including breast, colorectal, leukemia and lymphoma, and prostate, although for each malignancy there also exist studies that have failed to demonstrate such an inverse relationship.
A lot of research has happened in colorectal cancer, where there has been some proof of lower incidence of colon cancer in those who had adequate levels of vitamin D. Studies also have found lower odds of developing breast cancer and also later recurrences in those with normal vitamin D levels.

American Society of Oncology data was presented in June 2019, of a study from Michigan State University on 79,055 participants from 10 trials. The participants took only vitamin D supplements for at least 3 years, showing a significant link between Vitamin D. It lowered the risk of cancer related death, but did not prevent cancer.

Can taking higher levels of vitamin D prevent cancer?

In the largest-ever randomized clinical trial testing vitamin D for cancer prevention, the VITAL trial, the supplement did not reduce the risk of developing cancer. The trial included almost 26,000 patients, and was done to see if higher levels of vitamin D than recommended for bone health could prevent or reduce the chance of dying from cancer. The trial concluded that vitamin D supplements did however reduce the risk of dying from advanced disease, but in only those with normal Body Mass Index.

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There is an ongoing, larger study from Dana-Farber Institute, to evaluate if higher doses of Vitamin D prevent cancer death from advanced colorectal cancer. The study is based on the encouraging results from a similar smaller study.
So to say, there is extensive research that is ongoing to see if vitamin D supplements help prevent cancer incidence and or death, but for now the evidence is insufficient to draw definite conclusions. Adequate levels of Vitamin D are indispensable for bone health and immunity, and its role in maintaining good health cannot be more emphasized.