For about a decade, the case for vitamin D had a tidy narrative: half the population is insufficient, deficiency correlates with nearly every chronic disease anyone has measured, and supplementation should be widespread. Then the trials reported. The biggest of them — the VITAL trial, with 25,871 randomized adults followed for a median of 5.3 years — found that 2,000 IU per day of vitamin D3 did not reduce invasive cancer or major cardiovascular events versus placebo.1 The vitamin D story is now a more interesting one: a hormone with real and specific uses, not a panacea.
What VITAL and its peers actually showed
VITAL was not a small study. Its negative result on the primary endpoints reshaped how cautious nutrition researchers talk about population-wide supplementation. But the secondary analyses are where the residual signal lives. Cancer mortality — not incidence — trended lower in the vitamin D arm in pre-specified subgroups, particularly with longer follow-up. A separate VITAL analysis suggested a modest reduction in autoimmune disease incidence over five years.
The D2d trial tested 4,000 IU per day in 2,423 adults with prediabetes for the prevention of progression to type 2 diabetes. The intent-to-treat result was negative. But in the subgroup of participants who started with low 25(OH)D levels, supplementation did reduce progression.2 The repeating lesson across these trials: vitamin D supplementation appears to help the people who are actually deficient, and does very little for those who are not.
A 2022 ancillary analysis of VITAL on incident fractures was also negative in this generally replete US cohort, and large meta-analyses of high-dose annual bolus dosing have suggested possible harm.3 The Institute of Medicine’s modest recommendations — 600–800 IU per day for most adults — look better now than they did in 2010 when many clinicians argued for far higher targets.
Where the evidence is strongest
The Martineau individual-participant meta-analysis of 25 trials and 11,321 participants found that daily or weekly vitamin D supplementation reduced acute respiratory infections, with the largest effect in people who started with serum 25(OH)D below 25 nmol/L (about 10 ng/mL).4 The effect was small in the replete and meaningful in the deficient — the same pattern again.
For bone, the strongest case remains in older adults with documented insufficiency, where vitamin D combined with calcium reduces fracture risk. In well-nourished younger adults with serum 25(OH)D above 30 ng/mL, adding more vitamin D does not appear to add much.
How to think about your own level
The relevant blood test is 25-hydroxyvitamin D, or 25(OH)D. The conventional ranges are:
- Below 20 ng/mL: deficient
- 20–30 ng/mL: insufficient
- 30–50 ng/mL: sufficient
- Above 50 ng/mL: ample
Anyone who works indoors, has darker skin, lives above roughly 40° latitude, or covers up outdoors is at elevated risk of insufficiency. The cheapest move is to test, decide whether you have a problem, and then decide what to do — rather than blanket-dosing.
How to use it
For maintenance in adults without documented deficiency, the published guidance supports 1,000–2,000 IU of D3 per day taken with a fat-containing meal.5 For correcting deficiency, a common protocol is 5,000 IU per day for 8–12 weeks followed by a retest. The IOM’s Upper Limit for adults is 4,000 IU per day; chronic intake above that without monitoring increases the risk of hypercalcemia.
Avoid mega-bolus regimens (50,000 IU monthly or 500,000 IU annually). Trials of these schedules have found an increased fall risk and no clear advantage over daily dosing.
What it is not
Vitamin D is not a treatment for cardiovascular disease, cancer, or COVID-19. The trial evidence is consistent that population-wide supplementation does not move those needles in mostly-replete adults. It is also not a substitute for sunlight in any practical sense — sun exposure produces vitamin D plus a cascade of other UV-mediated effects (and risks) that a capsule does not replicate.
The honest summary after VITAL: test if you have a reason to, supplement modestly if you are below the sufficiency threshold, and resist the urge to push your number to the top of the reference range. Vitamin D is a hormone with a useful but circumscribed clinical role, not the master regulator the early enthusiasm suggested.