Investigations › Supplements
Investigation No. 12 · Supplements
“More vitamin D is better”
Twenty-five thousand people took 2000 IU a day for five years. Cancer and major cardiovascular events came out the same as placebo.
At a glance
- Verdict
- Mostly false
- Certainty in the evidence
- Moderate, starting at High and moved by 1 domain
- Evidence base
- VITAL randomized trial, median 5.3 years · 1 study read · 25,871 people
- Last reviewed
- 11 Sept 2026
- Next review due
- Sept 2027
How people actually say it
- “Everyone’s deficient, so more has to help.”the inference the trial was built to test
- “My level was low so I doubled the dose.”where a reasonable correction becomes an assumption
Why this verdict and not the next one
Mostly false rather than risky, because the trial found no benefit rather than a harm. The risky verdict is reserved for claims with a documented harm attached, and at trial doses this one does not have that.
Where the claim came from
From a real observation that got read backwards. Low vitamin D levels track with a long list of poor outcomes in observational data, and the inference was that raising the level would improve them. That is a confounding problem wearing a treatment plan: people who are ill, indoors and inactive have lower levels for reasons that have nothing to do with what a supplement can fix.
What the studies did
VITAL randomized 25,871 US adults, men over 50 and women over 55, including 5,106 Black participants, to vitamin D3 at 2000 IU a day or placebo, with a parallel omega-3 comparison. Over a median 5.3 years, cancer was diagnosed in 793 on vitamin D and 824 on placebo. Major cardiovascular events occurred in 396 against 409. Both confidence intervals sit across 1.
Why these papers and not others
VITAL was chosen over the many observational vitamin D studies, because the observational literature is where the confounding lives: people who are ill and indoors have lower levels for reasons a supplement cannot fix.
The full search and appraisal protocol, including what gets excluded and why, is set out in the method.
The evidence, study by study
| Study | Design | n | What it found | What it cannot say |
|---|---|---|---|---|
| Manson 2019, VITAL | Randomized, placebo-controlled, two-by-two factorial | 25,871 | Vitamin D3 2000 IU daily against placebo. Invasive cancer: hazard ratio 0.96, 95 percent CI 0.88 to 1.06. Major cardiovascular events: hazard ratio 0.97, 95 percent CI 0.85 to 1.12. Neither primary endpoint moved. | Anything about higher doses, about correcting a documented deficiency, or about outcomes it did not measure |
How the certainty grade was reached
Certainty starts at a level set by the study design, then moves down for every domain that fails a written threshold and up for every domain that clears one. The arithmetic is shown rather than asserted.
What the evidence cannot say
Speak to a person with a genuinely low measured level, which is a different question from supplementing a general population. It also tested one dose. A trial finding no benefit at 2000 IU is not a trial finding no benefit at any dose, and it is not a safety study for the much higher doses sold online.
One person, worked through
Somebody has a level come back at the low end, is told to supplement, and reasons that if some is good, more is better, so takes five times the dose. The trial says nothing about whether that helps. The dose-response literature says the upper end carries a hypercalcemia risk. Neither of those is a reason to stop correcting a real deficiency.
Whether this applies to you
A verdict is a reading of a population. Whether it describes you depends on the things below, and any one of them can change what the right move is for you.
- A documented deficiency is a different question and this entry does not answer it
- The dose you are taking decides whether any of this evidence applies at all
- Whether anybody measured your level since the whole argument turns on a number most people have never seen
- Above roughly 100 ng/mL is where the hypercalcemia concern sits, and that is a different literature again
What would change this entry
Trials at higher doses with hard endpoints, and trials restricted to people with measured deficiency, which is the group the general-population result cannot describe.
Read this before you act on it
This is an appraisal of published evidence, not medical or nutrition advice, and it is not a substitute for care from a clinician who knows your history, your medications and your labs. Nothing here is a diagnosis, a prescription or a treatment plan, and reading it does not create a clinician and patient relationship.
Do not start, stop or change a medication, a supplement or a diet on the strength of this page. If you are pregnant, breastfeeding, managing kidney or liver disease, diabetes, an eating disorder or any chronic condition, the general reading above may not describe you at all. Talk to your physician or a registered dietitian first.
The verdict above was true to the evidence on 11 Sept 2026. Evidence moves. Check the review date.
References
- Manson JE, Cook NR, Lee IM, Christen W, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine. 2019 Jan 3;380(1):33–44. doi:10.1056/NEJMoa1809944. PMID 30415629.
