Vitamin D dosage and evidence
Written by Aaron CuhaReviewed Sep 2026
The short answer
The most-tested supplement there is, and the big tests mostly came back empty. In a study of 25,871 adults, a daily vitamin D pill did not prevent cancer or heart disease. A second large study found it did not help people live longer.
Evidence grade for Vitamin D: Strong. The rule: at least two finished studies in people, and at least 1,000 people who took it. The sum on this page: 2 completed trials, 25,871 people given it. A grade counts how much has been done to people, not whether it works.
- People given it
- 25,871
- Human studies
- 2
- Found nothing
- 1
Dose
No human dose established
Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose.
2 completed trials, 25,871 people given it
4 effects reported, 2 unwanted, 2 wanted, by people using it, not a trial
Goal pages: Ageing and longevity, Immune system and inflammation, Bone and joints
In plain English
In plain English, from Aaron
This is how I would explain it to a friend. The evidence, with the numbers, is further down the page.
Vitamin D is the sunshine vitamin. Your body makes it when skin sees light. People take pills of it to prevent cancer, heart disease and early death. Those are the claims the big trials tested.
What people take it for
- Preventing cancer and heart disease.
- Living longer.
- Fixing a low number on a blood test.
What studies in people showed
The watching data looked great. People with low blood levels get sicker and die sooner. Then the trials ran. VITAL put 25871 adults on 2000 IU a day or a dummy for more than five years. It did not prevent cancer or major heart events. D-Health asked if a monthly high dose helped older adults live longer. Deaths came out about the same, a ratio of 1.04. A yearly 500000 IU dose in 2256 older women made falls and fractures worse. One large trial in 8851 children who were almost all low still missed its main goal. Test first. If you are already in range, more has not been shown to help.
What people report
Reports, not study results
The good
- The blood test is cheap and tells you if you need any of this.
- Repletion of a real shortage is still a fair clinical target.
The bad
- Huge yearly doses can harm. That is not a theory. It was measured.
- A lot of people take it for years without ever testing.
Where these come from: The trial counts are from the papers on this page. The untested daily habit is from clinic and forum reports.
My bottom line
Test. If you are low, fix it. If you are not, the two largest trials say more will not save you.
An opinion, not a finding. I am a coach, not a doctor.
What it has been measured to do
Measured in people
Goals Vitamin D has been tested for in people, biggest studies first. We count only what the studies on this page say, so the real numbers may be higher.
- Cancer and tumours25,871 people · 1 study
- Living longerno headcount stated · 1 study · 1 found nothing
What people using it report
Real people telling you what happened to them. Nobody checked it, and the people who post are not a fair sample, so it cannot prove anything works. It is still the best guide on this page to what people actually do with Vitamin D, what they expect, and what goes wrong. The full account, including the negative reports, is below.
32 of the 36 indexed goals have no study of any kind behind Vitamin D
No study in people, animals or cells on this page reports a result for these. That does not prove Vitamin D does nothing for them. It does mean anything you have heard about Vitamin D and one of these did not come from a study cited here.
When this earns a spot
It matters when 25-OH D is actually low. It does not matter just because you read that everyone is low. VITAL and D-Health tested topping up people who were already around 31 ng/mL, and missed their main goals. Big doses taken once in a while made older women fall and break more bones. Kidney stones, high calcium, and diseases that form small clumps of inflamed tissue (granulomatous disease) are reasons to stop and see a doctor.
Aaron's list, tier 1
Bloodwork that belongs beside the bottle
If the number is very low or the dose is being pushed high, ask about calcium and PTH (a calcium-control hormone).
Test again after about 90 days. If that blood number did not move, stop.
- ng/mL and nmol/L differ by a factor of 2.5.
- The test for the active form, 1,25-dihydroxy, is the wrong first test for this question.
Reading the number
The owner's target: 40 to 60 ng/mL. Labs often call under 20 too low. VITAL's lesson is about people already in range: pushing a good number higher did not prevent disease. If ninety days did not move it, stop chasing the dose on your own.
What these tests mean: 25-hydroxy vitamin D.
What people typically order
Ask for 25-hydroxy vitamin D. Adding K2 is the owner's habit, not something a study requires. Order the blood test through a doctor or a partner on /bloodwork/partners.
Show full evidence
What is Vitamin D?
The most-tested supplement there is, and the big tests mostly came back empty. In a study of 25,871 adults, a daily vitamin D pill did not prevent cancer or heart disease. A second large study found it did not help people live longer.
Vitamin D is a strange case. People with low blood levels get sicker and die sooner. That shows up again and again in huge studies that just watched people over time. But two of the biggest supplement studies ever run gave people vitamin D for years, and it did almost nothing.
How can both be true? Low vitamin D often goes with staying indoors, being unwell, not moving much and carrying extra weight. Each of those shortens life on its own. So the low number may be a sign of the problem, not the cause. The two big studies also shared a flaw. Most people in them already had decent levels, around 31 ng/mL on average. They tested topping up people who were not short.
What is left is smaller than the marketing, and still real. Fixing a true shortage is a fair goal. The blood test is cheap and easy to get. It tells you whether you need any of this. That is the whole case: test first, then decide.
How does Vitamin D work?
Your liver turns vitamin D into a form called 25-hydroxyvitamin D. That is what a blood test measures. Your kidneys then turn it into the active hormone. The hormone switches genes on and off in most tissues of the body. Its proven job is managing calcium and phosphate. It helps your gut absorb calcium, and it helps keep bone built and repaired. The same receptor also shows up in immune cells, muscle and blood vessels. That is where the bigger claims come from. But finding a receptor in a tissue is not the same as showing a pill helps that tissue.
What have studies in people found about Vitamin D?
One of the biggest sets of human studies for any supplement, from tens of thousands of people in several countries. For the reasons people take it, the results were mostly empty.
- The VITAL study gave 25,871 adults 2,000 IU a day or a dummy pill for about 5.3 years. It did not lower cancer. It did not lower heart attacks or strokes. It missed both of its main goals.
- The D-Health study gave older Australians a big dose once a month. About the same share died in each group. If anything, slightly more died on vitamin D (a ratio of 1.04), an amount too small to tell from chance.
- You may have heard VITAL cut autoimmune disease by 22%. That was a side result, not a main goal, and it was right at the line where chance could explain it. After two more years of checking cases, it faded further. It also did not last after people stopped.
- In 8,851 schoolchildren in Mongolia, 95.6% started out short of vitamin D. The pills tripled their blood levels. The main thing the study measured, protection from tuberculosis, still did not improve.
- One huge yearly dose of 500,000 IU made things worse. In 2,256 women aged 70 and up, falls rose by 15% and broken bones by 26%, compared with a dummy pill.
- Where it truly works: preventing and treating rickets and soft bones caused by shortage (osteomalacia). Calcium plus vitamin D together also helps against broken bones in older adults living in care homes. That is a different group and a different treatment from a healthy adult buying a bottle.
What this does not tell you: Both big studies mostly signed up people who were not short of vitamin D. So they answer one question: should someone with a normal level top up? Their answer is no. They do not settle whether fixing a real shortage helps, which is the question most readers have. The one large study done in truly short children also found no benefit. None of this covers pregnancy, gut problems that block absorption, kidney disease or a diagnosed shortage. In those cases, dosing is a call for your doctor.
What do people who took Vitamin D report?
This is what real people say happened to them, good and bad. Nobody measured it. A report cannot tell a real effect from the lift people get just from expecting it to work, from a diet or training change made at the same time, or from what was really in the vial. We still put it near the top. It is the next best thing to a study in people: what happens when real people use it, including what goes wrong.
- Amounts described run from about 2,000 to 10,000 IU daily, with prescription weekly amounts around 50,000 IU also mentioned, and a repeated observation that people need very different amounts to reach the same blood level. Some describe supplementing only in winter.
- The blood level target is the central and unresolved disagreement. Some people aim at around 50 ng/mL or higher, others argue the mortality data favours lower, and a persistent minority argue that a low reading is a marker of poor health rather than something worth treating in itself.
- Pairing with K2, magnesium and sometimes vitamin A is close to universal advice in these threads, with the stated worry being calcium ending up in soft tissue rather than bone.
- Constipation is the side effect described in most detail, by people who report it persisting across many brands, amounts and timings and not responding to fibre, water or magnesium, while others at the same amounts report nothing at all. Digestive upset at higher amounts, hypercalcaemia at very high blood levels, and questions about insomnia after injections also appear.
- Reports of no subjective change at any amount are common and are made by people who kept supplementing anyway on the basis of the blood number. Several note that they changed other things at the same time and cannot separate them.
- Scepticism about the literature is loud and comes from both directions. One group argues most trials fail because they never measure the blood level achieved or account for cofactors, and another uses the newer large trials showing no mortality benefit to argue the whole practice is misplaced.
Sources: Vitamin D threads on ExcelMale covering high dose use, reported side effects and disagreement over target blood levels, found by site-restricted search and read directly, September 2026. Nothing above is attributed to any specific post and no post is reproduced.
The state of the research
Vitamin D is the clearest case on this site of a promise that fell apart when it was tested. The early signal was not weak. Dozens of studies in many countries watched people over time. People with low levels did worse, and that held up after the obvious other causes were accounted for. That is exactly the kind of evidence people use to defend a supplement. Then two of the biggest, best-run supplement studies ever done tested it directly. They found close to nothing.
That should change how you weigh the next bold claim built on watching people. Watching people is not worthless. It gives you a good guess to test, not an answer. The supplement industry pushed back on the empty results. It said the studies signed up people who were not short, so they could not show a benefit. That is a fair point, and this page says so. It can also be tested. The one large study in people who were badly short came back empty too.
The evidence, in charts
Fig. 1a · how many people
36,978people, across 3 studies in people cited here
| Study | People | Share |
|---|---|---|
| New England Journal of Medicine 2019 | 25,871 | 70% |
| New England Journal of Medicine 2020 | 8,851 | 24% |
| JAMA 2010 | 2,256 | 6% |
One study holds 70% of these people, so the total rests more on one study than its size suggests. Counts are the number of people each study on this page says it signed up. That includes studies that only watched people and gave nobody the compound. 2 more studies in people give no headcount and are not counted here. Studies still signing people up are left out, and only studies cited on this page are counted.
Fig. 1b · evidence mix
5 of 6 sources here are studies in people. The rest is not.
- Study in people · done in people583%
- Review of studies · sums up other studies117%
Counted from the list of sources on this page. It counts only what we cite, so there may be more studies out there. A review counts once, however many studies it sums up.
Fig. 2 · evidence over time
The studies cover 6 different years, 2010 to 2022, counted from the list of sources 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 countries shown. This is what the law says, not a verdict on the science. See Legal status below for the full text and any notes.
Fig. 4 · how much does what
Nobody has tested different amounts in people
This chart would show how the effect changes as the amount goes up. For this compound, nobody has measured that in people. So we leave it empty. We will not borrow an animal chart and pretend it works the same in people.
Fig. 5 · what the molecule is
- Kind of compound
- Supplement
- Molecule weight
- 384.6 Da (cholecalciferol). A secosteroid, not a peptide.
- Half-life
- Stated in words, not a numbersee the exact wording below
- Chain length
- None on file
Half-life as stated on file: Circulating 25-hydroxyvitamin D has a half-life of roughly two to three weeks, which is why a level is reasonably stable and why loading doses persist. The active 1,25-dihydroxy form turns over in hours.
No amino acid sequence is on file for Vitamin D, which is expected: a supplement is not built from amino acids.
The studies we cite
- Study in people2019Vitamin D supplements and prevention of cancer and cardiovascular disease
Vitamin D was tested properly, at scale, and it did not prevent cancer or heart disease. Nearly 26,000 adults were split by chance to take a daily vitamin D pill or a dummy pill. They were followed for more than five years. Neither of the two questions the trial was built to answer came out ahead. Most of these people already had decent vitamin D levels. Only about one in eight started out low. So this does not settle what happens when someone is truly short of it.
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.
- Study in people2022The D-Health trial: a randomised controlled trial of the effect of vitamin D on mortality
A second large test asked whether vitamin D helps people live longer, and it did not. Older Australian adults took a big dose once a month, or a dummy pill, for years. The same share died in each group, of any cause. If anything the treated group fared very slightly worse, by an amount too small to separate from chance. As in the trial above, the people getting the dummy already had reasonably good vitamin D levels. Nobody has tested a population that starts out genuinely deficient.
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.
- Study in people2010Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial
A single huge yearly dose of vitamin D made things worse, not better. Over two thousand women aged 70 and above were given one very large dose each year, or a dummy. The women on vitamin D fell more often and broke more bones than the women on the dummy. This is the clearest sign that more vitamin D is not automatically safer. It was one enormous annual dose, which is not how most people take it. It does not tell you what a normal daily amount does.
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.
- Study in people2020Vitamin D supplements for prevention of tuberculosis infection and disease
This is the result that answers the usual excuse, and it is a hard one. Nearly nine thousand children were studied in a place where almost all of them were short of vitamin D. Supplements tripled their blood levels, so the pills clearly worked on the blood test. The thing the trial was built to answer still came out empty. Fixing a real shortage did not protect them from tuberculosis. This was the population where a benefit should have been easiest to see, and it was not there.
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.
- Study in people2021Effect of marine omega-3 fatty acid and vitamin D supplementation on incident atrial fibrillation
Vitamin D did not cause an irregular heartbeat, which is a rare clean safety answer here. This analysis was planned in advance inside the big vitamin D trial, so it is not a fishing expedition after the fact. Atrial fibrillation, a fast and irregular heart rhythm, happened no more often on vitamin D than on the dummy pill. That is worth knowing on its own. It says nothing about whether vitamin D helps with anything. It only says this particular harm did not show up.
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.
- Review of studies2014Effect of vitamin D on mortality in adults: a systematic review and individual participant data meta-analysis
People with the lowest vitamin D levels died more often than people with the highest. This review pooled data on each person from studies that just watched people over time. Nobody was given anything. So it cannot show that taking a pill would change that.
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.
Common questions
Has Vitamin D been tested in people?
Yes. 25,871 people given it across 2 completed trials cited here, 1 of which found nothing. The most-tested supplement there is, and the big tests mostly came back empty. In a study of 25,871 adults, a daily vitamin D pill did not prevent cancer or heart disease. A second large study found it did not help people live longer.
What is the Vitamin D dosage?
No human dose established. Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose. 2 completed trials, 25,871 people given it
Is Vitamin D FDA-approved?
Sold as a dietary supplement in the United States, with prescription forms of ergocalciferol and calcitriol available separately. No supplement form is approved to prevent or treat any disease.
Should I take vitamin D?
Test first. The blood test for vitamin D (25-hydroxyvitamin D) is cheap and easy to get. It turns a guess into a number. If you are truly low, bringing it up is a fair goal. If you are already in range, the two biggest studies say taking more will not prevent cancer, heart disease or death.
What level should I aim for?
The guidelines do not agree, and that tells you something. Below about 20 ng/mL is widely called low, and below 12 is called very low. Many longevity plans aim for 40 to 60, including the owner's. No study has shown that higher target prevents anything. It is a guess drawn from studies that just watched people. You should know which one you are following.
Is more better?
No. This is the one place the evidence is clear. One huge yearly dose of 500,000 IU raised falls by 15% and broken bones by 26% in older women, compared with a dummy pill. Other studies using big, spaced-out doses point the same way. With vitamin D, too much has a real downside.
Do I need K2 with it?
The idea is that vitamin K2 steers calcium into bone and away from artery walls. That makes sense on paper. But the evidence in people for taking the two together is much thinner than the confidence behind the advice. That includes this site's own supplements page, where the owner calls it non-negotiable. That is his position, not a study result.