Vitamin D
Cholecalciferol (vitamin D3)
Written by Aaron CuhaReviewed Sep 2026
Also known as: Cholecalciferol, D3, 25-hydroxyvitamin D, Calciferol
The most-tested supplement in history, and the trials mostly failed. A 25,871-person randomised trial missed both primary endpoints and a large mortality trial returned a hazard ratio of 1.04.
Overview
Vitamin D has the strongest observational case and one of the weakest randomised cases of anything on this site. People with low blood levels get sicker and die sooner, reliably, across enormous cohorts. Then two of the largest supplement trials ever run gave people vitamin D for years and measured almost nothing.
The reason those two facts coexist is worth understanding, because it recurs across this whole pillar. Low vitamin D is a marker of being indoors, being unwell, being sedentary and carrying more weight, all of which shorten life on their own. The blood level may be reporting the problem rather than causing it. The trials also share a design flaw: both enrolled populations that were already replete, averaging around 31 ng/mL, so they tested topping up people who were not short.
What is left is narrower than the marketing and still real. Repletion of genuine deficiency is a legitimate clinical target, the test is cheap and widely available, and the number tells you whether you need any of this. That is the whole case for it, and it is a case for testing rather than for taking.
Mechanism of action
Cholecalciferol is hydroxylated in the liver to 25-hydroxyvitamin D, which is what a blood test measures, then hydroxylated again in the kidney to 1,25-dihydroxyvitamin D, the active hormone. That binds the vitamin D receptor, a nuclear receptor expressed in most tissues, and acts as a transcription factor. Its established role is calcium and phosphate homeostasis: it raises intestinal calcium absorption and, with parathyroid hormone, regulates bone remodelling. The vitamin D receptor's presence in immune, muscle and vascular tissue is the basis of the wider claims, and is a mechanistic observation rather than a demonstrated clinical effect.
Human evidence
Among the largest randomised evidence bases for any supplement, and predominantly negative for the outcomes people take it for. Tens of thousands of participants across multiple trials on multiple continents.
- VITAL, 25,871 participants, 2,000 IU daily, median 5.3 years: no reduction in invasive cancer and no reduction in major cardiovascular events. Both primary endpoints missed.
- D-Health, monthly high-dose in older adults: all-cause mortality hazard ratio 1.04.
- The widely quoted 22% reduction in autoimmune disease from VITAL was an ancillary result at p = 0.05 that weakened to a confidence interval crossing 1.0 once two further years of case adjudication were applied, with no persistence after stopping.
- 8,851 Mongolian schoolchildren who were 95.6% deficient: supplementation tripled blood levels and the primary endpoint was still null.
- A single annual 500,000 IU dose increased falls by 15% and fractures by 26% against placebo in 2,256 older women.
- Where it is genuinely established: preventing and treating rickets and osteomalacia, and calcium plus vitamin D together for fracture risk in institutionalised older adults, which is a different population and a different intervention than a supplement bottle bought by a healthy adult.
What this does not tell you: Both large trials enrolled mostly replete populations, so they answer the question of whether to top up someone who is not short, and they answer it no. They do not settle whether repletion helps a genuinely deficient person, which is the question most readers actually have. The honest state of that question is that the one large trial run specifically in a deficient population was also null. Nothing here speaks to pregnancy, malabsorption, kidney disease or documented deficiency syndromes, where dosing is a clinical decision.
Reading the research record
Vitamin D is the cleanest case study on this site of observational promise not surviving randomisation, and it is worth being precise about why that matters. The observational signal was not weak or inconsistent. It was enormous, replicated across dozens of cohorts and many countries, and it survived adjustment for the obvious confounders. It was exactly the kind of evidence that people reach for when defending a supplement. And then two of the largest and best-run supplement trials in history tested it and found close to nothing.
That should change how a reader weighs the next confident claim built on cohort data. It is not an argument that observational evidence is worthless; it is an argument that it is a hypothesis generator and not a conclusion. Also worth naming is the industry response to the null trials, which was to argue that the trials enrolled replete people and therefore could not show a benefit. That objection is legitimate and this page states it. It is also testable, and the one large trial conducted in a severely deficient population came back null too.
The evidence, charted
Fig. 1 · evidence composition
5of 6 citations (83%) are in people
Counted from the citation list on this page. The Human count is the same number shown in the badge at the top. Both understate any literature larger than the sources we cite.
Fig. 2 · evidence over time
Evidence spans 6 distinct years, 2010 to 2022, counted from the citation list on this page.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four jurisdictions shown. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Key studies & citations
- Human2019
Vitamin D supplements and prevention of cancer and cardiovascular disease
VITAL. 25,871 participants randomised to 2,000 IU daily or placebo, median 5.3 years. Neither primary endpoint was met: no reduction in invasive cancer, no reduction in major cardiovascular events. Mean baseline 25-hydroxyvitamin D was 30.8 ng/mL and only 12.7% of participants were below 20.
New England Journal of Medicine - Human2022
The D-Health trial: a randomised controlled trial of the effect of vitamin D on mortality
Monthly high-dose vitamin D in older Australian adults. All-cause mortality hazard ratio 1.04. No benefit, again in a population whose placebo group averaged about 31 ng/mL.
Lancet Diabetes and Endocrinology - Human2010
Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial
2,256 women aged 70 and over given a single annual 500,000 IU dose. Falls incidence rate ratio 1.15 and fractures 1.26. The supplement group fell and broke more, not less. Evidence that more is not safer.
JAMA - Review2014
Effect of vitamin D on mortality in adults: a systematic review and individual participant data meta-analysis
The pooled individual-participant analysis behind the observational case. People in the lowest quartile of 25-hydroxyvitamin D had substantially higher mortality than the highest. Observational exposure, so it cannot establish that supplementing changes it.
BMJ - Human2020
Vitamin D supplements for prevention of tuberculosis infection and disease
The most uncomfortable result in this literature and the one that answers the usual defence. 8,851 children, 95.6% vitamin D deficient at baseline. Supplementation tripled their blood levels. The primary endpoint was null. Correcting real deficiency did not deliver the expected benefit in the population where it should have been easiest to show.
New England Journal of Medicine - Human2021
Effect of marine omega-3 fatty acid and vitamin D supplementation on incident atrial fibrillation
The prespecified atrial fibrillation analysis within VITAL. Vitamin D did not increase atrial fibrillation. Included here because it is one of the few clean safety answers in the file.
JAMA
Frequently asked questions
Should I take vitamin D?
Test first. A 25-hydroxyvitamin D level is cheap and widely available, and it converts this from a guess into a measurement. If you are genuinely low, repletion is a reasonable target. If you are already in range, the two largest randomised trials say adding more will not prevent cancer, heart disease or death.
What level should I aim for?
Guidelines disagree, which is itself informative. Below about 20 ng/mL is widely treated as deficient and below 12 as severe. Many longevity protocols, including the owner's, target 40 to 60. That higher target is not supported by outcome trials; it is an extrapolation from observational data, and you should know which it is.
Is more better?
No, and this is the one place the evidence is unambiguous. A single annual 500,000 IU dose increased falls by 15% and fractures by 26% in older women against placebo. Other high intermittent-dose designs show the same direction. Vitamin D has a dose-response curve with a bad end.
Do I need K2 with it?
The rationale is that vitamin K2 directs calcium into bone rather than arterial wall, and it is mechanistically coherent. The human outcome evidence for the combination is much thinner than the confidence with which it is recommended, including on this site's own supplements page, where the owner calls it non-negotiable. That is a position, not a trial result.