Vitamin D
Also called: cholecalciferol, vitamin D3.
Last reviewed 2026-09-16. How this works.
Correcting a measured deficiency Grade C
Human evidence exists and does not settle it.
Vitamin D does not seem to stop people getting cancer, and whether it helps them live longer is disputed.
Null for cancer incidence. The mortality finding is contested.
The claim
The claim is that vitamin D prevents cancer and improves survival, and that most people are short of it. It is one of the most widely taken supplements in this book.
What it does
Vitamin D receptors are present in many tissues and the biology is genuinely interesting. This entry is graded on the trials, which are unusually large and unusually clear on one half of the question.
The evidence
On whether it prevents cancer, the answer is consistent and negative. Pooling ten randomised trials with 6,537 cancers gave a relative risk of 0.98 with no variation between trials, and a second pooled analysis across 81,362 participants gave 0.99. That result held across every subgroup examined.1
On mortality the picture is less settled. Pooled analyses suggested a reduction, and the one trial specifically designed to look at cancer mortality gave a risk ratio of 0.83 with an interval crossing one, falling to 0.75 when the first two years were excluded.12
A later review of twelve randomised trials across 72,669 participants concluded the opposite, that supplementation reduces neither mortality nor incidence. It cites a trial in a population that already had enough vitamin D, where the trend ran toward more cancer deaths rather than fewer.2
That last point is the one this chapter keeps finding: what a supplement does depends on whether the person taking it was short of it to begin with. Correcting a measured deficiency and topping up someone who is already replete are different acts, and the trials do not always separate them.2
The studies this rests on
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Pooled randomised trials of vitamin D and total cancer incidence1
A meta-analysis, summarising randomised trials, measured a clinical outcome.
no effect on total cancer incidence.
relative risk 0.98, interval 0.93 to 1.03. ten trials, 6,537 cases, no variation between trials
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VITAL, vitamin D supplements and prevention of cancer2
A randomised trial, double blind, measured survival.
cancer mortality risk ratio below one with an interval crossing one.
risk ratio 0.83, interval 0.67 to 1.02. falling to 0.75, interval 0.59 to 0.96, when the first two years were excluded
What is absent
No trial in this record enrolled only people who were deficient. The claim graded here is about correcting a measured shortfall, and the trials tested supplementation in general populations.
Scope: people with a measured deficiency specifically.
Established by trial scope, checked 2026-09-16. Result: the pooled trials enrolled general populations rather than people selected for deficiency.12
What to ask
Has my vitamin D been measured, and is it something you would want to treat?
The dish-to-person gap
The register holds no comparison between a laboratory concentration and a blood level for vitamin D. This entry rests on randomised outcome trials rather than on a concentration argument.
When to call your team — no warning signs are recorded
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The register records no warning signs for vitamin D. That is not a statement that nothing can go wrong.
Interactions
The register holds no measured interaction between vitamin D and a cancer treatment class. Chapter 16 of the book covers supplement interactions in general, and an empty list here means nobody has measured this one.
Nobody having looked is not the same as nothing being there. That is what this line says and all it says.
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
Correcting a measured deficiency is a clinical question for your own team rather than a trial question.
Search ClinicalTrials.gov for recruiting studies of vitamin D OR cholecalciferol
A trial is the same decision with monitoring, a known preparation and a doctor who knows what else you are taking.
References
Keum N, Lee DH, Greenwood DC, Manson JE, Giovannucci E. Vitamin D supplementation and total cancer incidence and mortality: a meta-analysis of randomized controlled trials. Annals of Oncology 2019.
doi:10.1042/BSR20190369 · PubMed 30796437
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 6.
Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine 2019;380(1):33-44.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 7. The author string is abbreviated in the source that was read and has not been expanded.