Vitamin D is one of the most misunderstood molecules in consumer health, and it is misunderstood in both directions. It is oversold as a fix for almost everything, and simultaneously under-treated in the small group of people who are genuinely, clinically deficient.
Despite the name, it is not really a vitamin. It is a prohormone your skin synthesizes from UVB light and your liver converts to 25-hydroxyvitamin D, the storage form your blood test measures.1 What follows is what your number actually means, what changed in the guidelines in 2024, and what the evidence says supplementation does and does not achieve.
How common deficiency really is
The "one billion people are deficient" figure gets repeated without a threshold attached, which is what makes it meaningless. The threshold decides everything.
A 2023 pooled analysis in Frontiers in Nutrition covering 7.9 million participants across population-based studies from 2000 to 2022 gives the clearest picture:2
| Serum 25(OH)D | Global prevalence below this level |
|---|---|
| Under 12 ng/mL (30 nmol/L) | 15.7% |
| Under 20 ng/mL (50 nmol/L) | 47.9% |
| Under 30 ng/mL (75 nmol/L) | 76.6% |
So "roughly 80% are deficient" is true only if you define deficiency at 30 ng/mL, which most official bodies do not. Under the IOM threshold, it is closer to 16%.1
The United States looks better than the global average. NHANES data cited by the NIH Office of Dietary Supplements put about 5% of Americans at risk of deficiency and 18% at risk of inadequacy.1 Rates climb with higher latitude, darker skin pigmentation, obesity, indoor work, and age.21
What your number means
| 25(OH)D (ng/mL) | IOM / NIH framing | Endocrine Society (2011) framing |
|---|---|---|
| Under 12 | Deficient — rickets and osteomalacia risk | Deficient |
| 12 to 20 | Inadequate for bone health | Deficient |
| 20 to 29 | Sufficient | Insufficient |
| 30 to 50 | Sufficient | Sufficient |
| Above 50 | Potential adverse effects, especially above 60 | Sufficient (the range runs to 100) |
| Above 150 | Toxicity range | Above the 30–100 sufficiency range |
The disagreement in that table is not sloppiness. The IOM asked a population question — what intake covers the bone health needs of 97.5% of people — and answered 20 ng/mL.31 The Endocrine Society asked a clinical question about patients already flagged as at risk and answered 30 ng/mL.4 Neither number was ever validated against hard outcomes in a trial, which is the crux of the whole debate.5 Our piece on 30 ng/mL specifically works through what sits in that contested band, and the broader gap between lab-normal and optimal is covered in optimal versus normal ranges.
What changed in 2024
In June 2024 the Endocrine Society published a new guideline that reversed much of its own 2011 position.6 Its main conclusions:
- Do not routinely screen 25(OH)D in healthy adults, including adults with obesity or darker skin. The stated reason is blunt: levels that produce outcome-specific benefits "have not been established in clinical trials."6
- No target serum level is specified for disease prevention, and the Society no longer endorses its 2011 definitions of "sufficiency" (30 ng/mL or above) and "insufficiency" (between 20 and 30 ng/mL).5
- Empiric supplementation without testing is suggested for four groups: children and adolescents aged 1 to 18, adults 75 and older, people who are pregnant, and adults with high-risk prediabetes.
- Adults aged 50 to 74 and non-pregnant adults under 50 should take the RDA and otherwise do nothing special.
- For adults 50 and older who need vitamin D, daily lower doses are preferred over infrequent high doses.
The USPSTF reached a compatible conclusion in 2021, issuing an "I" statement: insufficient evidence to weigh the benefits and harms of screening asymptomatic adults.7
This is the real meaning of the headline on this article. Your doctor probably will not test you, because current guidance tells them not to. That is a defensible position for population-level policy. It is less satisfying if you are an individual who wants to know where you stand.
What supplementation actually does
Here the evidence is genuinely mixed, and it is worth being precise about which claims hold.
Strong. Correcting true deficiency prevents and treats rickets in children and osteomalacia in adults. This is not contested.1
Null in large trials. VITAL randomized 25,871 adults to 2,000 IU of vitamin D3 daily or placebo for a median 5.3 years. It found no reduction in invasive cancer and no reduction in major cardiovascular events.8 A 2022 NEJM analysis of the same cohort found no reduction in total, non-vertebral, or hip fractures in generally healthy adults who were not selected for low vitamin D.9
Modest and borderline. The VITAL autoimmune substudy, published in the BMJ in 2022, found 123 confirmed autoimmune disease cases in the vitamin D arm versus 155 on placebo — an adjusted hazard ratio of 0.78 (95% CI 0.61 to 0.99, P=0.05).10 A real signal, but one sitting exactly on the significance boundary.
The clearest positive. A 2023 individual-participant meta-analysis in Annals of Internal Medicine pooled three randomized trials in people with prediabetes and found vitamin D reduced progression to type 2 diabetes by 15% (HR 0.85, 0.75 to 0.96), a 3-year absolute risk reduction of 3.3%. The same paper reported a much larger effect (76% reduction) among participants who maintained 25(OH)D at or above 50 ng/mL — but that comparison is between people who achieved a level, not between randomized groups, so it cannot be read as proof that chasing 50 ng/mL causes the benefit.11
Harm at the top end. More is not better. A randomized trial of a single annual 500,000 IU dose in older women increased falls and fractures.12 This is why the 2024 guideline explicitly prefers daily low doses.6
The fair summary: vitamin D is a cheap, safe correction for a real deficiency and a poor general-purpose preventive drug.
Raising a genuinely low level
If your level is low and you and your clinician decide to treat it:
- Use D3 (cholecalciferol), which raises and maintains 25(OH)D more effectively than D2.131
- Dose to your body size. In the 0 to 1,000 IU range, each 1,000 IU per day raises 25(OH)D by about 12 nmol/L (roughly 5 ng/mL). A 2014 PLOS ONE analysis found that response varies by weight: 13.1 nmol/L per 1,000 IU in normal-weight adults, 11.5 in overweight, and 8.6 in adults with obesity. To reach 100 nmol/L (40 ng/mL) took roughly 2,080 IU daily at normal weight versus 5,473 IU with obesity.14
- Respect the ceiling. The RDA is 600 IU daily for adults to age 70 and 800 IU after, with a tolerable upper intake level of 4,000 IU per day. Doses above that belong under clinical supervision.1
- Take it with a fat-containing meal. Absorption is meaningfully better.15
- Retest at three months, not three weeks. 25(OH)D has a half-life of about two weeks (15 days), so a new steady state takes about three months to establish.1
Deficiencies also cluster, which is why it is worth checking B12 at the same time — the two overlap more often than most people expect. If you are about to build a supplement routine, get baseline bloods first.
K2, magnesium, and sunlight
Vitamin K2. The theory is elegant: D3 increases calcium absorption, K2 activates matrix Gla protein, which helps keep calcium out of artery walls.16 The human outcome data are weaker than the theory. A randomized trial of K2 plus D did not show the slowing of aortic valve calcification or coronary artery calcium progression that the mechanism predicts.17 K2 is inexpensive and, unless you take warfarin, safe, so taking it is reasonable — but "always pair D3 with K2" overstates what has actually been demonstrated.16 We cover the D3 and K2 evidence in detail here.
Magnesium. Magnesium is a cofactor for the enzymes that hydroxylate vitamin D in the liver and kidney, so poor magnesium status can blunt your response to supplementation.1819 If you are supplementing and your level will not move, this is worth checking. Different magnesium forms differ in absorption, though the evidence comes from small studies.20
Sunlight. Roughly 5 to 30 minutes of midday sun on the face, arms and legs, at least twice a week, is the heuristic some researchers suggest.1 But it is heavily conditional: at Boston's latitude (42°N) and farther north, winter sunlight produces essentially no vitamin D regardless of exposure time, and darker skin requires considerably longer.2122 Sunscreen is a smaller factor than commonly claimed, because real-world application is thinner than test conditions.231
Vitamin D status also interacts with inflammatory signalling, which is one reason low D and elevated CRP often turn up on the same panel.24
When testing is genuinely worth it
Ask for a 25(OH)D test if you have malabsorption (celiac, Crohn's, prior bariatric surgery), chronic kidney or liver disease, osteoporosis or a fragility fracture, long-term corticosteroid or antiepileptic use, dark skin with limited sun exposure at high latitude (a 2011 criterion the 2024 guideline no longer supports), or unexplained bone pain with proximal muscle weakness.46 It is also a reasonable inclusion in a broader longevity blood panel if you are already testing, and our guide to how often to get blood work covers sensible intervals.
How Merios helps
Merios extracts 25(OH)D from your uploaded results and tracks it across seasons, which matters more than any single reading — most people's levels swing substantially between February and August. It sets your level against calcium, PTH where available, magnesium and B12, so a low number is read in context rather than treated as an isolated flag.
Track your vitamin D with Merios
This article is for informational purposes only and does not constitute medical advice. Discuss vitamin D testing and supplementation with your physician, particularly if you have kidney disease, sarcoidosis, or take medications that affect calcium.