Vitamin D (25-OH): where guidelines and the internet disagree most
Low levels are linked with almost everything in observational studies. Large trials of supplements mostly found nothing. Both facts belong on the same page.
The standard vitamin D test measures 25-hydroxyvitamin D, written 25(OH)D, the form the liver makes from vitamin D taken in or made in the skin. It has a half-life of a few weeks, so it reflects stores rather than yesterday's sun. It is reported in ng/mL in the US or nmol/L elsewhere; multiply ng/mL by 2.5 to get nmol/L.
The guideline numbers
| 25(OH)D | nmol/L | Interpretation (US National Academies, via NIH ODS) |
|---|---|---|
| Under 12 ng/mL | Under 30 | Associated with deficiency, rickets and osteomalacia |
| 12 to under 20 ng/mL | 30 to under 50 | Generally inadequate for bone health in some people |
| 20 ng/mL or more | 50 or more | Adequate for bone health in most people |
| Over 50 ng/mL | Over 125 | Linked with potential adverse effects |
The Endocrine Society's 2011 guideline used 30 ng/mL as the line for sufficiency, and many labs still print that. Its 2024 replacement went the other way: it suggests against routine 25(OH)D testing in healthy adults, including people with obesity, and did not endorse a target level for disease prevention, because trials have not shown that aiming for a number improves outcomes. It does suggest empirical supplementation for some groups, including children and adolescents, adults aged 75 and over, pregnancy, and high-risk prediabetes. "Optimal" ranges of 50 to 80 ng/mL seen online are opinion.
What raises and lowers it
- Raises: sun exposure on bare skin, supplements, oily fish, fortified foods.
- Lowers: winter at higher latitudes, darker skin, covering skin, older age, obesity (vitamin D is stored in fat), malabsorption, some gut surgery, liver and kidney disease, and some anti-seizure drugs.
- Seasons matter: late-summer and late-winter results in the same person can differ a lot.
What it predicts, and what the trials found
In observational studies, low 25(OH)D is associated with falls, fractures, infections, diabetes, heart disease, cancer and death. The problem is that ill, inactive and older people go outside less and have lower levels, so the arrow can point either way.
The VITAL trial (NEJM, 2019) tested it directly. 25,871 US adults, men 50 and over and women 55 and over, were randomised to vitamin D3 2,000 IU a day or placebo for a median 5.3 years. Invasive cancer: hazard ratio 0.96 (95% CI 0.88 to 1.06). Major cardiovascular events: 0.97 (0.85 to 1.12). Neither was significant. Most participants were not deficient at the start, which is the main caveat, and it is one reason the 2024 guideline focuses on groups rather than on numbers.
On GLP-1 drugs and weight loss
The pivotal GLP-1 trials do not report vitamin D. People with obesity tend to have lower levels, and smaller food intake reduces dietary sources further, so some prescribers check it at baseline, particularly after gut surgery. There is no trial-based target specific to GLP-1 treatment. The micronutrients page covers intake.
How to test
- No fasting needed.
- Same season for comparisons, or at least note the month.
- After a change in dose, wait about three months before retesting, since 25(OH)D takes weeks to settle.
- Assays differ. Immunoassays and mass spectrometry can give different values for the same sample, so keep the lab constant.
- Do not order the 1,25-dihydroxy form for routine status. It is a different test for specific conditions.
One more reason to keep the series simple: supplements change 25(OH)D slowly, and the rise per unit of dose is smaller in people with higher body weight and in those starting from a higher level. Two results taken a season apart, on different assays, after a change in supplement, can look like a trend when they are not. Note the month, the lab and the daily dose next to each result, and judge the direction over a year rather than a single pair.
Tracking it in Peptide AI
In Peptide AI, vitamin D (25-OH) sits in the metabolic panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Supplements logged in the supplement scanner appear on the same timeline as the result. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.
Questions people ask
What is a normal vitamin D level?
The US National Academies consider 20 ng/mL (50 nmol/L) or more adequate for bone health in most people, under 12 ng/mL (30 nmol/L) deficient, and over 50 ng/mL (125 nmol/L) potentially too high.
Should I get my vitamin D tested?
The Endocrine Society 2024 guideline suggests against routine testing in healthy adults, because trials have not shown that targeting a level improves outcomes. Testing makes sense for specific conditions, which your clinician can judge.
Is 30 ng/mL the target?
It was the Endocrine Society sufficiency line in 2011 and many labs still print it. The 2024 guideline did not endorse a specific target for disease prevention.
Did vitamin D supplements prevent cancer or heart disease in VITAL?
No. In 25,871 adults over a median 5.3 years, 2,000 IU a day did not significantly reduce invasive cancer (HR 0.96) or major cardiovascular events (HR 0.97).
How do I convert vitamin D ng/mL to nmol/L?
Multiply ng/mL by 2.5. So 20 ng/mL is 50 nmol/L and 30 ng/mL is 75 nmol/L.
Sources
- Vitamin D fact sheet for health professionals, NIH Office of Dietary Supplements
- Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline, Demay et al., Journal of Clinical Endocrinology and Metabolism, 2024
- Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline, Holick et al., Journal of Clinical Endocrinology and Metabolism, 2011
- Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL), Manson et al., New England Journal of Medicine, 2019
