Vitamin D supplement reduces cancer mortality
If you take a high-dose vitamin D supplement every day, it will not reduce your risk of cancer. But it will reduce the chance that your cancer will spread, and the chance that your disease will kill you. Researchers from Harvard University report this in JAMA Open.Study
The researchers used data collected in the VITAL study. In that project, nearly thirteen thousand test subjects took a supplement containing 2000 IU vitamin D3 every day for 5 years. That is the same as 50 micrograms of vitamin D3.

An equally large group was given a placebo.
The researchers kept track of which study participants got cancer, whether the cancer metastasized and whether the study participants died from the cancer.
Vitamin D supplementation had no effect on cancer incidence. However, vitamin D had an effect on the incidence of fatal forms of cancer and metastatic forms of cancer [advanced cancer]. In the group that received vitamin D, the risk of advanced cancer was 17 percent lower than in the placebo group.

When the researchers split their subjects according to BMI, they saw that the protective effect of vitamin D was greater in the group with a relatively low BMI.
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"This randomized clinical trial of daily high-dose vitamin D supplementation for 5 years reduced the incidence of advanced (metastatic or fatal) cancer in the overall cohort of adults without a diagnosis of cancer at baseline, with strongest risk reduction in individuals with normal weight", the researchers summarize their findings."Additional randomized trials focusing on cancer patients should be considered, as well as investigations of differential benefit by BMI. Even if vitamin D effects were modest, vitamin D supplementation at the studied levels are much less toxic and lower cost than many current cancer therapies."
JAMA Network Open. 2020;3(11):e2025850.