Marker · Ranges for learning, not a diagnosis
25-hydroxy vitamin D
Also called 25-OH D, calcidiol, vitamin D blood test.
The storage form of vitamin D made by the liver, and the test used to judge whether someone is short. It is not the active hormone form, 1,25-dihydroxy vitamin D.
Read this first
- Coach, not doctor. Aaron Cuha is a coach, not a doctor. No medical credential is claimed for him, or for these pages. A number on a blood test is something to talk over with a doctor, not something to diagnose yourself with. Nothing here is a prescription, a plan to follow, or a reason to start or stop a medicine. Educational use only. This database summarizes published research and is not medical advice. Where a dose appears it is a record of what a trial or clinic ran, or what people describe doing, with its source attached. Nothing here is a recommendation for personal use.
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Why a longevity audience asks
In huge studies that followed people over time, low levels went with dying sooner. But the biggest studies that gave people vitamin D gave it to people who already had enough, and it did not reach their main goals. The useful move is to measure, not to assume every adult needs a bottle.
How a coach reads it
The owner's published target is 40 to 60 ng/mL. Labs often mark under 20 ng/mL as too low, and 20 to 30 as not quite enough. Those are ranges to discuss, not a diagnosis to give yourself. To turn nmol/L into ng/mL, divide by 2.5.
Units on this page: ng/mL, nmol/L
When it is low
Often means little sun, more time indoors, more body fat, or not enough in your diet. How to bring it back up is a talk to have with a doctor. The big prevention studies were not built to answer that question.
When it is high
Too high is a real problem, especially with big doses taken now and then. One 500,000 IU dose a year led to more falls and broken bones in older women. More is not safer.
Easy ways to misread it
- The VITAL and D-Health studies took people who averaged about 31 ng/mL. So finding no benefit there tells you nothing about people who are truly low.
- If the dose is being pushed high, calcium and PTH (a hormone that controls calcium) should be checked too.
- Vitamin D3 on a label is not the same thing as the blood test.
What to ask your doctor
Ask for 25-hydroxy vitamin D. If the number is very low or the dose is high, ask whether calcium and PTH should be checked too.
Related supplements
Related when-playbooks
Sources on this page
- Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease.
VITAL. 25,871 people, most of whom already had enough vitamin D. It found no benefit on either of its two main goals.
2019PMID 30415629 - The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality.
No difference in deaths (a risk score of 1.04, where 1 means no difference). The dummy pill group already had enough vitamin D.
2022PMID 35026158 - Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial.
One 500,000 IU dose a year raised the rate of falls (1.15) and broken bones (1.26), where 1 means no difference.
2010PMID 20460620