How many units of vitamin D a day:
600, 2,000 or 4,000, and why the figure is a winter one
The adequate intake set by EFSA for an adult is 15 micrograms a day, that is, 600 IU. What none of the top Google results tells you is that the figure was calculated for a situation in which your skin produces nothing. In other words, exactly for winter.
The figure you are looking for is 600 IU,
and it is a winter figure
If you have searched for how many units of vitamin D you need a day, you have already found ten different answers. 400. 600. 1,000. 2,000. 4,000 if you are deficient. The figures differ by a factor of ten, and none of the articles on the first page says where they come from.
There is a European reference figure, and it is easy to quote exactly. In 2016, EFSA set an adequate intake of 15 micrograms a day for adults, for children from the age of one, and during pregnancy and breastfeeding, and of 10 micrograms a day for infants aged 7 to 11 months. In international units, that means 600 IU and 400 IU respectively.
So far, it sounds like what you have already read. The difference is in the footnote, and it is a footnote that changes everything: the values are set, the EFSA table says, “under conditions of assumed minimal cutaneous vitamin D synthesis”. The opinion states plainly that, in the presence of cutaneous synthesis, the requirement from the diet is lower or may even be zero.
So the 600 IU is not an annual average, and it is not a summer figure cautiously scaled down. It is the figure for a situation in which the sun contributes nothing at all. For someone living in Romania, that situation has a name: from autumn to spring.
600 IU is not a small dose. It is the figure calculated precisely for the season in which your skin no longer produces anything. The right question is not “how much extra should I take in winter”, but “where do I get those 600 IU now, when the sun no longer provides them”.
Three units on the same box:
IU, micrograms and a confusing percentage
Any box of vitamin D in Romania usually states three different things about the same amount. The amount in international units, the amount in micrograms and a percentage of the reference value. All three are correct. It is just that the third does not mean what it seems to.
The first two convert into each other with a fixed factor, stated in the EFSA opinion on vitamin D itself: one microgram equals 40 international units, and one international unit equals 0.025 micrograms. So 15 micrograms are 600 IU, 50 micrograms are 2,000 IU, and 100 micrograms are 4,000 IU. No approximations, just a simple multiplication.
The third is another story. The percentage on the label refers to the nutrient reference value in Annex XIII of Regulation 1169/2011, and for vitamin D that value is 5 micrograms, that is, 200 IU. A single figure, with no age-specific versions, set for labelling, not for nutrition. It is three times lower than the adequate intake set by EFSA.
The practical consequence: a product that delivers exactly the adequate intake for an adult, that is, 600 IU, will state 300% of the NRV on its label. And one that delivers exactly the adult ceiling, that is, 4,000 IU, will state 2,000%. The percentage climbs dramatically and tells you absolutely nothing about whether the dose suits you.
| The box says | In micrograms | Percentage of the NRV | Compared with the adequate intake of 600 IU |
|---|---|---|---|
| 400 IU | 10 µg | 200% | two thirds |
| 600 IU | 15 µg | 300% | exactly that |
| 1,000 IU | 25 µg | 500% | 1.7 times more |
| 2,000 IU | 50 µg | 1,000% | 3.3 times more |
| 4,000 IU | 100 µg | 2,000% | 6.7 times more |
The table above is all the maths you need to read any box on the shelf. The rest of the article is about what you do with the figure once you have translated it.
The label translator:
move the slider to the figure on your box
Take the box out of the drawer and find the figure in IU. Then move the slider to it. You will see the same amount written in all three ways, plus where it sits against the two reference points that matter: the adequate intake and the ceiling.
Adequate intake and ceiling,
by age, in both units
Two different figures, easy to confuse. The adequate intake tells you how much you need. The ceiling, that is, the tolerable upper intake level, tells you how much your body can tolerate in the long term from all sources combined. Both are set by EFSA and both are public.
| Age | Adequate intake | Ceiling | Ceiling in IU |
|---|---|---|---|
| 0 to 6 months | not set as an AI | 25 µg | 1,000 IU |
| 7 to 11 months | 10 µg, that is, 400 IU | 35 µg | 1,400 IU |
| 1 to 10 years | 15 µg, that is, 600 IU | 50 µg | 2,000 IU |
| 11 to 17 years | 15 µg, that is, 600 IU | 100 µg | 4,000 IU |
| Adults | 15 µg, that is, 600 IU | 100 µg | 4,000 IU |
| Pregnancy and breastfeeding | 15 µg, that is, 600 IU | 100 µg | 4,000 IU |
Look at the second column. The adequate intake is identical from the age of one upwards: child, teenager, adult, pregnant woman, all 15 micrograms. The ceiling, on the other hand, doubles at age 11. That is why an adult product does not become suitable for a child if you split it, and we have written about this at length in the article on why the adult dose does not split in two.
Why winter is different:
it is a question of angle, not clouds
The usual explanation is that in winter you spend more time indoors and it is cloudy. True, but secondary. The real problem is geometry.
Cutaneous vitamin D synthesis needs ultraviolet B radiation. In winter, at high latitudes, the sun rises so little above the horizon that its rays pass through a much thicker layer of atmosphere, and the B component is lost along the way. You can spend a whole day outdoors in January under a clear sky: your skin produces nothing, because the wavelength needed no longer reaches the ground.
The classic figures on this come from a 1988 study by Webb, Kline and Holick, cited among others in an official document of the European Commission’s scientific committees. Its conclusion, in the document’s words: in Boston, at 42 degrees north, no previtamin D3 was produced at all during the winter months, and in Edmonton, at 52 degrees, the dead period was even longer. In Los Angeles and Puerto Rico, synthesis took place all year round.
Now look at the map. Bucharest is at roughly 44 degrees north, Cluj at almost 47, Suceava above 47. Romania sits between Boston and Edmonton, closer to Boston but north of it. It is not a perfect comparison, because altitude, pollution, skin pigmentation and clothing matter too. But it is enough to explain why, here, the month in which the EFSA figure becomes relevant is not a matter of opinion.
What this means for what you take: not that your requirement doubles in winter, but that its source changes. In summer, part of it comes from your skin. In winter, that part disappears and what is left is your plate and, possibly, the box.
What 1,000, 2,000 and 4,000 IU
mean in practice
These are the three figures you see most often on the shelf in Romania. Tap each one.
1,000 IU, that is, 25 micrograms
Above the adequate intake of 600 IU and at a quarter of an adult’s ceiling. It is the amount that leaves the most room for everything else: fortified foods, a multivitamin taken alongside it, summer sun exposure. On the label it appears as 500% of the NRV, which sounds like a lot and is not.
2,000 IU, that is, 50 micrograms
Half of an adult’s ceiling. The same figure, however, is the entire ceiling for a child aged 1 to 10, which explains why an adult product is not shared with a little one. On the label, 1,000% of the NRV.
4,000 IU, that is, 100 micrograms
Exactly the upper limit for an adult, not a routine dose. Important: the ceiling refers to everything that goes into your body, so a 4,000 IU product on its own takes up all the available room. Any other source taken alongside it puts you over. On the label, 2,000% of the NRV.
If you already have a 25(OH)D test result and are trying to understand what it says, those figures are read differently from the ones on the box. We have written separately about what the value on your lab report means, including why laboratories in Romania print a reference range that differs from the European threshold.
Five things
almost everyone believes
Tap each one to see the verdict.
Vitamin D is fat-soluble and is stored in the body, which is precisely why EFSA set an upper limit: 100 micrograms a day for adults. The limit is not a target to climb towards; it is a threshold you do not cross. Above it there is no additional benefit, only risk.
Water-soluble vitamins are excreted relatively easily. Fat-soluble ones, including D and A, are not. The fact that there is an upper limit set at European level, with separate bands for infants and children, is itself proof that the dose matters. The word “natural” changes nothing about pharmacology.
The EFSA opinion says that, in the presence of cutaneous synthesis, the requirement from the diet is lower or may even be zero. So the idea is not wrong, but it depends on how much time you actually spend outdoors, at what time of day, and with how much skin uncovered. Someone who works in an office and goes out at 7 pm is not in a biological summer; they are in an extended winter.
100% of the NRV means 5 micrograms, that is, 200 IU, which is a third of the adequate intake set by EFSA. The reference value on the label is a tool for uniform labelling, not a nutritional recommendation. It is useful for comparing two products with each other, not for deciding whether you are taking enough.
Regimens with large doses given infrequently do exist in medical practice, and they are prescribed, with monitoring. They are not the same thing as a supplement you buy yourself, and the daily ceiling set by EFSA is designed for ordinary intake, not for correction regimens. If someone suggests a loading dose to you, ask who is supervising it.
When the figure
is not something you pick off the internet
Everything you have read so far is a population reference point. These figures answer the question “what is reasonable in general”, not the question “what do I need”.
Ask your doctor first if: you are pregnant or breastfeeding; you are choosing something for a child, at any age; you have kidney disease, liver disease, sarcoidosis or another condition that affects calcium metabolism; you take thiazide diuretics, digoxin or other long-term medicines; you already have a 25(OH)D test with a value well below or above the range. In all of these situations, the dose is not estimated from an article, not even ours.
And one more simple rule: the ceiling refers to the total. If you take a vitamin D supplement and, alongside it, a multivitamin, a D3 and K2 product and fish oil, the figures add up. It is worth checking beforehand, especially if the products were bought at different times. We have written about what can and cannot be taken together in the supplement timing guide.
What we do
and why our box says 4,000
Our vitamin D3 gummies deliver 4,000 IU in the daily dose, that is, 100 micrograms. You already know what this figure means, because we wrote it above: it is exactly the upper limit set by EFSA for an adult.
It is not a maintenance dose, and we do not present it as one. It is an amount that makes sense for an adult who is not taking any other source of vitamin D alongside it and who has discussed this with their doctor, especially if they plan to take it for months on end. A single gummy is 2,000 IU, which for many people is a more reasonable starting point, and it is completely fine to use the product that way.
What we do not say: that 4,000 IU is “the dose doctors recommend”, that it is a therapeutic dose, or that more is better. Vitamin D has an authorised health claim in the European Union, it contributes to the normal function of the immune system and to the maintenance of normal muscle function, and the claim can only be used if the product contains a significant amount. That is all. It does not say it prevents colds and it does not say it treats anything.
If you would first like to understand where you are starting from, rather than what you are buying, begin with the autumn guide to what a label is allowed to say and with the article on the daily dose.
Comments (0)
There are no comments for this article. Be the first one to leave a message!