The short version
- In VITAL, 25,871 adults took 2,000 IU of vitamin D3 daily or placebo for a median of 5.3 years, with no significant effect on cancer (hazard ratio 0.96), major cardiovascular events (0.97) or all-cause mortality (0.99).
- The same trial found no significant effect of 2,000 IU daily on total fractures (hazard ratio 0.98), nonvertebral fractures (0.97) or hip fractures (1.01) across 1,991 confirmed fractures.
- In 311 healthy adults aged 55 to 70 given 400, 4,000 or 10,000 IU of vitamin D3 daily for three years, radial bone mineral density fell 1.2%, 2.4% and 3.5% respectively, with no gain in bone strength.
- A single annual 500,000 IU dose in 2,256 women aged 70 and over produced 171 fractures versus 135 on placebo and a fall rate of 83.4 versus 72.7 per 100 person-years, with the excess concentrated in the three months after dosing.
- Vitamin D is absorbed better with a fat-containing meal, and accumulates across a multivitamin, a calcium supplement and a standalone capsule, so total daily intake is worth adding up.
Between about 2005 and 2015 the standard advice on vitamin D moved in one direction: upward. Deficiency was common, the vitamin was cheap and safe, and the observational data linking low levels to almost every disease anyone measured was enormous. Doses on shelves climbed from 400 IU to 1,000, then 2,000, then 5,000 and 10,000.
Then the randomised trials arrived. They are worth reading in order, because they answer three separate questions and only one of the answers is the one people expected.
Question one: does supplementing healthy adults prevent anything?
VITAL randomised 25,871 US adults — men 50 and over, women 55 and over, including 5,106 Black participants — to 2,000 IU of vitamin D3 daily or placebo, and followed them for a median of 5.3 years. Participants were not recruited for being deficient.
Cancer was diagnosed in 793 people on vitamin D and 824 on placebo, a hazard ratio of 0.96 (95% CI 0.88 to 1.06, P = 0.47). Major cardiovascular events occurred in 396 versus 409, hazard ratio 0.97 (0.85 to 1.12, P = 0.69). All-cause mortality: hazard ratio 0.99. No excess hypercalcemia was seen either — the dose was safe, and it did not do what a generation of observational studies had implied it would.
The fracture question got its own answer from the same trial. Across 1,991 confirmed incident fractures, vitamin D3 had no significant effect on total fractures (hazard ratio 0.98), nonvertebral fractures (0.97) or hip fractures (1.01). The effect did not vary by baseline 25-hydroxyvitamin D level. For generally healthy adults who are not deficient, that is about as clean a null as this field produces.
Question two: would a bigger dose have worked?
This is where it gets uncomfortable. A three-year trial in Calgary randomised 311 healthy adults aged 55 to 70, none with osteoporosis, to 400, 4,000 or 10,000 IU of vitamin D3 daily. The dose did what you would expect to blood levels. It did the opposite of what you would expect to bone.
Radial volumetric bone mineral density fell by a mean of 1.2% in the 400 IU group, 2.4% at 4,000 IU and 3.5% at 10,000 IU. Tibial density showed the same pattern, significantly lower than the 400 IU group only at 10,000 IU. There were no significant differences in bone strength at either site. The authors were careful, and so are we: these findings do not support a benefit of high-dose vitamin D for bone health, and further research would be needed to establish whether it is actively harmful.
The blunter result came from Australia. A trial gave 2,256 community-dwelling women aged 70 and over a single annual 500,000 IU dose of cholecalciferol or placebo each autumn or winter. The vitamin D group had 171 fractures against 135 on placebo — an incidence rate ratio of 1.26 (95% CI 1.00 to 1.59, P = 0.047) — and fell more often, 83.4 versus 72.7 falls per 100 person-years (rate ratio 1.15, 1.02 to 1.30, P = 0.03). A post hoc analysis found the excess concentrated in the first three months after each dose.
One very large intermittent dose is not the same intervention as a modest daily one. But the direction of that result is the reason "just take more, it is water under the bridge" quietly disappeared from serious advice.
Question three: so who should take it, and how much?
The trials above tested prevention in people who were mostly replete. They did not test treatment of deficiency, and they do not argue against it. Correcting a documented low 25-hydroxyvitamin D level is a different intervention with a different rationale, and it belongs with your clinician and a repeat test rather than with a blog.
What the evidence does support is an unglamorous shape: a modest daily dose, taken with a meal that contains fat, in people with limited sun exposure or a measured shortfall — and no reason to climb.
Two things worth knowing before you buy
Vitamin D shows up in more products than people track. A multivitamin, a calcium supplement, a fortified drink and a standalone D3 capsule can stack to a number nobody chose. Add up what you are actually taking before you add anything.
And be sceptical of the add-ons. The K2 with D3 pairing is sold as a way to make high-dose vitamin D safer for arteries; the mechanism is real and the outcome evidence is thinner than the marketing. The same applies to the assumption that vitamin D obliges you to add calcium supplements, which carry their own trade-offs. As always, whether a bottle contains what it says is a separate question from whether the dose is right, and that one comes down to third-party testing.
What we'd actually tell you
If you have never had it measured, measure it. If it is low, treat that with your clinician and re-test. If it is normal and you take a maintenance dose through a dark winter, keep it modest — the evidence for going higher ran the experiment and came back with lower bone density and more fractures, not fewer.
Good questions
How much vitamin D should I take?
For a healthy adult with no measured deficiency, modest is the defensible answer, and the case for climbing above the usual maintenance range is weak. In VITAL, 2,000 IU daily for a median of 5.3 years produced no significant effect on cancer, cardiovascular events, mortality or fractures. If you have a documented low level, dosing that is a decision for your clinician with a repeat test attached.
Is 10,000 IU of vitamin D a day safe?
The best available trial at that dose found bone density went down, not up. Over three years, radial bone mineral density fell 3.5% at 10,000 IU versus 1.2% at 400 IU, with no improvement in bone strength. The authors stopped short of calling it harmful and so do we, but there is no measured benefit on the other side of that trade.
Should I take a big monthly or annual dose instead of a daily one?
There is a specific reason to be cautious. A trial of a single annual 500,000 IU dose in older women found more falls and more fractures than placebo, concentrated in the three months after each dose. Very large intermittent dosing is not equivalent to the same total taken daily, and the evidence has not been kind to it.
Do I need to take vitamin K2 with my vitamin D?
Not on current outcome evidence. The mechanism behind the pairing is real, but the trials testing whether K2 changes hard outcomes have largely not delivered. If you are taking K2 specifically to make a high vitamin D dose safer, the more useful move is to reconsider the high dose.
Does it matter what time of day I take vitamin D?
Time of day matters less than what you take it with. Vitamin D is fat-soluble and absorption improves with a meal containing fat, so pairing it with your largest meal is the practical rule. Consistency matters more than the clock.
Can I just get vitamin D from the sun instead?
For some people at some latitudes in some seasons, yes. Skin synthesis depends on latitude, season, time of day, skin tone, age and sunscreen use, which is exactly why a blood test is more informative than a rule of thumb. Sun exposure carries its own risks, so this is not a straight swap and is worth discussing with your clinician.
Sources
- Manson JE, Cook NR, Lee IM, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med, 2019. View study
- LeBoff MS, Chou SH, Ratliff KA, et al. Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. N Engl J Med, 2022. View study
- Burt LA, Billington EO, Rose MS, et al. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength: A Randomized Clinical Trial. JAMA, 2019. View study
- Sanders KM, Stuart AL, Williamson EJ, et al. Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. JAMA, 2010. View study
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