Vitamin D3 is a fat-soluble vitamin that acts as a prohormone. A prohormone can turn into a hormone but doesn’t have any activity of its own. It can be found in milk products and is increased in those who have been exposed to the sun.
One article (1) looked at vitamin D3 and its ability to prevent cancer. It noted that there were epidemiological studies done earlier showing that people had a decreased risk of death from cancer if they lived in a sunny climate, where vitamin D3 levels are elevated.
The article also noted that, in studies of rats and mice with cancer, taking vitamin D3 helped to decrease the risk of cancer. In human studies, the results were a bit inconsistent, meaning that some studies showed a benefit in taking vitamin D3 and others didn’t show any benefit. More research needs to be done to see if vitamin D3 can actually prevent cancer in humans.
Source: 1.Vitamin D and Cancer Prevention. http://www.cancer.gov/about-cancer/causes-prevention/risk/diet/vitamin-d-fact-sheet. Accessed 9/13/16.