Biomarker
Vitamin D
A hormone your skin makes from sunlight — and modern, indoor, high-latitude life suppresses it. The deficiency is silent and the fix is cheap. The catch: the benefit lives almost entirely in correcting a genuine deficiency, not in pushing an already-normal number higher.
Last updated · every claim cited to a primary source
Read your result — and get your dose
Enter your last blood test. Most people arrive here holding a number in the 20s and wanting to know two things: is it a problem, and how much should I take.
Enter your number above to see where you stand and the dose to get to target.
What the number actually is
The test to ask for is 25-hydroxyvitamin D (25(OH)D) — the storage form, and the right marker of status. Not 1,25-dihydroxyvitamin D, which some labs push but which can read normal while your stores are empty.
US labs report in ng/mL; most of the rest of the world uses nmol/L, which is 2.5× larger. A “75” from a UK or Canadian lab (75 nmol/L) is the same as 30 ng/mL — sufficient, not deficient. Mixing up the units is the single most common way people misread their own result.
The one lever that works: D3
Supplemental vitamin D3 (cholecalciferol) taken daily, with food. D3 raises 25(OH)D more reliably than D2.4Sunlight works biologically but can’t be dosed — latitude, season, skin tone, glass, and sunscreen make it unpredictable, and north of roughly Madrid most people make almost none in winter. Food barely moves the number.
Roughly 4–7 ng/mL per 1,000 IU/day, with bigger gains when you start deeply deficient and smaller ones at higher body weight.3 (The popular “1,000 IU adds 10 ng/mL” rule overshoots for most adults — the calculator above uses the lower, evidence-based rates.)
What about K2 and magnesium?
Magnesium is a genuine cofactor in vitamin D metabolism, and small trials suggest co-supplementing nudges 25(OH)D up; the effect is real but modest.9 Vitamin K2is the louder claim online (“or the calcium goes to your arteries”) — plausible in theory, but a randomized trial of high-dose K2 plus D found no effect on arterial calcification.10 Optional, not mandatory.
“50 mcg” or “2,000 IU” — is that the same dose?
Since the 2020 label change, US bottles lead with mcg, while doctors, prescriptions, and every study still talk in IU. They’re the same scale (1 mcg = 40 IU), but the mismatch makes people second-guess their dose. Convert it and see how yours sits against the 4,000 IU/day adult upper limit.
Enter the number on the label and pick its unit. 1 mcg = 40 IU.
How high should you aim? The honest answer
Three camps disagree. The old Endocrine Society line was “sufficient” at 30 ng/mL; the functional-medicine internet pushes 50–80; US dietary bodies call 20 adequate for bone health. In 2024 the Endocrine Society walked its own number back— it concluded the trials don’t support naming a higher target and stopped endorsing 30 as a goal.1
Our position: fix a real deficiency, settle in the 30s, and stop. There is no outcome evidence that 60 beats 40, and the upside of chasing it is imaginary. Toxicity is genuinely hard to reach — it shows up above ~150 ng/mL, from sustained very high dosing11— but “hard to poison yourself” is not a reason to aim high.
What raising it actually does
The uncomfortable part. The big randomized trials mostly enrolled people who were already replete (VITAL’s average was ~31 ng/mL) — which is exactly why so many came back null. Graded against the trial evidence:
Raises your 25(OH)D level
The one thing it reliably does. Dose-response confirmed across dozens of RCTs.3
Prevents rickets / osteomalacia
The original, unambiguous reason vitamin D matters — in genuine deficiency.
Lowers autoimmune-disease risk
VITAL: 22% fewer confirmed cases over 5 years (HR 0.78, just reached P=0.05). The strongest positive signal in the literature — but a secondary endpoint, not yet replicated.7
Reduces cancer mortality
Daily-dosing subgroup of a 14-trial meta-analysis: ~12% lower (HR 0.88). Driven largely by VITAL; not a primary endpoint.8
Prevents fractures / falls
Benefit shows up only in deficient elderly taking D3 with calcium. In healthy, replete adults VITAL found nothing.6
Prevents cancer (incidence)
Null in VITAL's primary result. A possible signal in lean individuals, not established.5
Prevents heart disease
Null across VITAL, ViDA, and DO-HEALTH. No confirmed pathway.5
Grades: A replicated on hard outcomes · B one good trial, needs confirmation · C mixed or surrogate-only · D tested and null.
Should you even test?
A reasonable question — and the 2024 guideline actually advises against routine screening in healthy adults, because no threshold cleanly predicts who benefits.1Testing earns its keep when you have real risk factors: northern latitude, darker skin, little sun exposure, older age, higher body weight, or a gut condition that limits absorption. If that’s you, test, dose, and retest. If it isn’t, a modest 1,000–2,000 IU/day through winter is a defensible move without ever drawing blood.
Either way, close the loop: retest about 10 weeks after starting — levels need 8–12 weeks to settle — and check again in late winter, when they bottom out.
Common questions
References
- 1.Endocrine Society 2024 clinical practice guideline — vitamin D for prevention of disease
- 2.NIH Office of Dietary Supplements — Vitamin D fact sheet for health professionals
- 3.Dose-response of vitamin D supplementation, meta-analysis of European adults (2023)
- 4.Tripkovic et al., D2 vs D3 and serum 25(OH)D — systematic review & meta-analysis, AJCN 2012
- 5.Manson et al., VITAL — vitamin D and prevention of cancer and CVD, NEJM 2019
- 6.LeBoff et al., VITAL — supplemental vitamin D and incident fractures, NEJM 2022
- 7.Hahn et al., VITAL — vitamin D and prevention of autoimmune disease, BMJ 2022
- 8.Vitamin D supplementation and cancer mortality — meta-analysis of 14 RCTs (2023)
- 9.Magnesium co-supplementation and 25(OH)D — meta-analysis (2025)
- 10.MK-7 (K2) + D and aortic valve calcification — randomized trial, Circulation 2022
- 11.Vitamin D toxicity — StatPearls, NIH Bookshelf