The form of vitamin D measured in your blood test, in a small tablet.
6 questions to ask when reading the label and your test result.
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Vitamin D labels put units such as IU, µg and %NRV side by side. A lab report shows ng/mL or nmol/L. This guide goes through the six questions to ask when reading the label and your test result, in order. Under each question you will find what to look for and how that information appears on Uth-FastD™. The guide does not rank products and does not suggest a personal amount.
Vitamin D is the name of a group. Supplements most often use D3 (cholecalciferol). D2 (ergocalciferol) and calcidiol (calcifediol, 25(OH)D3) are also forms in this group.
D3 is the form made in the skin with the UVB in sunlight, and the form found in animal foods. In supplements it usually comes from sheep's wool fat (lanolin), and in vegan products from lichen. D2 comes from plants and fungi. The D3 that enters the body is first converted in the liver into 25(OH)D3, that is, calcidiol (calcifediol). This is the form measured in a blood test. Calcidiol is the form in which the liver conversion step is already complete. It is then converted into the active form in the kidneys. We explain this path step by step on the science of vitamin D page.
1 µg (microgram) of vitamin D is 40 IU. To go from IU to µg, divide by 40. To go from µg to IU, multiply by 40.
Because vitamin D is used in very small amounts, you will see two units on labels. µg (microgram, sometimes written “mcg”) is a unit of weight, one millionth of a gram. IU (International Unit) is a unit defined by the biological activity of the vitamin. For vitamin D, the relationship between the two is fixed: 1 µg is 40 IU.[6] In Türkiye, supplement labels give the amount in µg, often with the IU value next to it. People often search for “vitamin D 1000 mg”, but vitamin D is not measured in milligrams. 1000 IU is 25 µg, or 0.025 mg.
NRV is the nutrient reference value, and for vitamin D it is 5 µg. The “%NRV” on the label shows the amount in one serving as a share of this value.
On supplement labels, a “%NRV” column sits next to the nutrition table. Under the Turkish Food Codex, the nutrient reference value for vitamin D is 5 µg (200 IU).[3] This is a labelling measure, not an amount recommended for everyone. That is why %NRV can look high: a serving with 25 µg (1000 IU) equals 500% of the NRV. The percentage is calculated only against the reference used on the label and does not show your personal need. Personal need varies with age, time spent in the sun and diet.
For a supplement, the measure is the recommended daily serving on the label. According to EFSA, the adequate intake for adults is 15 µg (600 IU) and the safe upper limit is 100 µg (4000 IU).
EFSA has set the adequate daily intake for adults and children over 1 year at 15 µg (600 IU). This value was calculated for conditions in which the skin makes little vitamin D.[1] The highest amount not expected to pose a health risk when taken every day over a long period, the upper limit, is 100 µg (4000 IU) a day for adults and 11–17-year-olds, and 50 µg (2000 IU) for 1–10-year-olds.[2] The upper limit includes the total from supplements, fortified foods and other foods. High doses such as 50,000 IU (1250 µg) are used as medicines, under a physician's supervision. You can review your own needs with your physician, based on your blood test result.
A particular time of day is not required. Products containing cholecalciferol (D3) are best taken after eating, with a meal that contains fat. Products containing calcidiol have no such requirement.
“Should vitamin D be taken in the morning or the evening?” is a common question. There is no strong evidence that the time of day changes the result. D3 is a fat-soluble vitamin. That is why supplements containing D3 are usually taken with a meal that contains fat. In one study, the same amount of D3 was absorbed in greater amounts when taken with a meal containing fat than with a fat-free meal.[4] Calcidiol, on the other hand, is a more polar molecule than D3, which means it dissolves more easily in water, and its absorption from the gut depends less on fat and bile.[10] So products containing calcidiol do not need to be taken with a fatty meal. A practical approach is to pair the supplement with the same meal every day. That makes it harder to forget. When choosing between drops, tablets, capsules and sprays, you can look at how easy the dose is to measure and which one you find easier. The dose for each is written on the label.
In a blood test, the vitamin D level is measured as 25(OH)D. The result is given in ng/mL or nmol/L. Multiply the ng/mL value by 2.5 to get nmol/L.
On a lab report, this value appears under names such as “25-OH vitamin D”, “25(OH)D” or “vitamin D (25-hydroxy)”. Laboratories give the result in one of two units: 20 ng/mL corresponds to 50 nmol/L. “What should my vitamin D level be?” has no single answer for everyone. In its 2011 report, the US Institute of Medicine (IOM) took 20 ng/mL (50 nmol/L) as the level that meets the needs of the large majority of the population.[5] The Endocrine Society's 2024 guideline does not recommend routine testing in healthy people and does not set a specific target value.[7] So review your result against the reference range on your report, together with your physician. You can find measurement data from Türkiye on the science of vitamin D page.
The factor is approximate (the exact value is 2.496). The calculator does not interpret the result. It only converts the unit.
The main source of vitamin D is the sun. The skin makes D3 with the UVB in sunlight. The amount it makes depends on the season, the time of day, your latitude, skin colour, age and sunscreen use.[6] In food, vitamin D is found in only a few sources. 85 g of cooked salmon contains about 14.2 µg (570 IU) of vitamin D, and one large egg about 1.1 µg (44 IU).[6] Fruit and vegetables contain almost no vitamin D. Food supplements are not a substitute for a varied diet.
Source: EFSA NDA Panel[8] and the Turkish Food Codex Regulation on Nutrition and Health Claims[9]. The claims belong to the nutrient (vitamin D), not to the product.
This list is a short version of the six questions above. Look at the box in your hand and your lab report, and tick each item you can find the answer to. You can ask the manufacturer or your pharmacist for anything you cannot find. The list does not score a product. It only checks whether the information is on the label or in the report.
This list does not recommend a product and is not a health assessment. Your ticks are not saved and are cleared when you close the page.
Look at your sun, eating and supplement routine in a few questions. This is not a health assessment and does not diagnose anything. Leave your email address at the end of the test: when Uth-FastD™ goes on sale, your 20% launch code is sent to your email. The test is currently in Turkish only.
Take the Vitamin D TestStart the TestD3 is one form of vitamin D. Vitamin D is the name of a group that covers D2, D3 and the forms the body makes from them. Supplements most often use D3 (cholecalciferol). The form measured in a blood test is 25(OH)D, that is, calcidiol.
1000 IU is 25 µg (mcg). For vitamin D, 1 µg equals 40 IU. To convert IU to µg, divide by 40. To convert µg to IU, multiply by 40. For example, 2000 IU is 50 µg and 400 IU is 10 µg.
There is no strong evidence that a particular time of day changes the result. What matters is the form. Cholecalciferol (D3) dissolves in fat, so products containing D3 are best taken after eating, with a meal that contains fat. Calcidiol dissolves more easily in water and its absorption depends less on fat (Quesada-Gomez and Bouillon, 2018), so products containing calcidiol do not need a fatty meal. Follow the instructions on the label and take the supplement in the same routine every day.
There is no single target value for everyone. A blood test measures 25(OH)D and gives the result in ng/mL or nmol/L (ng/mL × 2.5 = nmol/L). The IOM's 2011 report takes 20 ng/mL (50 nmol/L) as its reference. The Endocrine Society's 2024 guideline does not set a specific target value. Review your result against the reference range on your report and with your physician.
Few foods contain vitamin D: oily fish such as salmon, mackerel and sardines, egg yolk, and mushrooms exposed to UV light. Fruit and vegetables contain almost none. The main source is D3 made in the skin with the UVB in sunlight.
For both, what matters is the amount on the label and regular use. With drops, the dose is given in drops or ml, and you need to read the amount per drop on the label. With tablets, the dose is fixed. A practical approach is to use whichever you find easier, and use it regularly.
Calcidiol (calcifediol, 25(OH)D3) is the form D3 is converted into in the liver. It is the form measured as your vitamin D level in a blood test. This form, in which the liver conversion step is already complete, is also used in supplements as calcidiol monohydrate.
The six questions help you read a vitamin D label and a lab report. You can find the path of vitamin D in the body, and where calcidiol sits on that path, on the science page. The small tablet that contains calcidiol is on the Uth-FastD™ page.
The sources reflect official scientific assessments of vitamin D, regulatory texts and the general scientific literature. They do not carry any product-specific clinical outcome claim.
If you would like to read more widely, our other guides are on the Longevity Insights page.
*Health claims apply when the nutrient is consumed in sufficient amounts. This page is for information only. It is not a product recommendation or a health assessment. Food supplements are not medicines, and food supplements are not a substitute for a varied diet. If you are pregnant or breastfeeding, take medication or have a chronic condition, consult your physician. Do not exceed the recommended daily dose.
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