Vitamin D3: how much you need,
when to start and how to tell
In Romania, between October and March, the sun sits too low for your skin to produce vitamin D, however long you spend outdoors. Below: what dose makes sense, what 4,000 IU actually means, what the blood test shows and where the limit you shouldn't go past lies.
Why the season starts now,
not in December
Vitamin D is the only nutrient your body makes by itself, but with an odd condition: it needs ultraviolet B radiation, and UVB only reaches the ground when the sun is high enough in the sky. It doesn't matter how clear the sky is, or how long you sit by the window. What matters is the angle.
Romania lies between the 44th and 48th parallels, roughly the same latitude as northern Italy and southern France. Here, the sun's angle drops below the useful threshold somewhere in early October and only comes back around March. Throughout that period, your skin produces very little or nothing at all, however much you walk around outside.
Add a few everyday things that cut into the good months too: glass blocks UVB, so nothing happens by the window; clothes cover almost everything from September onwards; sunscreen reduces synthesis, which is no reason to give it up; and if you work indoors, the peak hours of the day go by without you.
Your body's vitamin D reserve runs down gradually, not all at once. If you start in January, you're already starting from a low level. The end of summer is the logical time to look at this, not the middle of winter.
How much vitamin D3 a day
and what 4,000 IU means
The first confusion comes from the units. Vitamin D is measured either in micrograms or in international units, and the conversion is simple: 1 µg = 40 IU. From there, the figures that matter:
| Benchmark | In micrograms | In IU | What it is |
|---|---|---|---|
| Adequate intake, adults | 15 µg | 600 IU | The reference value set by EFSA for the general population. |
| The nutrient reference value used on labels | 5 µg | 200 IU | The figure used to calculate the percentages on packaging in the EU. |
| Tolerable upper intake level, adults | 100 µg | 4,000 IU | The threshold EFSA advises against exceeding, from all sources combined. |
So a product with 4,000 IU per serving isn't “ten times better” than one with 400. It's a product that takes you right up to the upper limit considered safe for adults, and that asks two things of you: don't combine it with other sources of vitamin D without your doctor knowing, and don't give it to children, whose limits are much lower.
What dose makes sense for you depends on where you're starting from, how much sun you get, your age, your weight and whatever else you take. It isn't a figure an article can give you. It's a figure best decided after a blood test.
How to tell
where you stand
The test is called 25-hydroxyvitamin D, or 25(OH)D for short. It's a simple blood test, available without a referral at any private laboratory, and it generally costs about as much as lunch out. It measures the storage form of the vitamin, meaning your real reserve, not what you ate yesterday.
- When. The most informative moment is at the end of winter, around February or March, when you're at your yearly low. A measurement at the end of summer shows you your peak.
- What you see. The result comes in nanomoles per litre or in nanograms per millilitre. Interpretation thresholds differ between laboratories and between guidelines, and there is genuine debate among specialists around them.
- Who reads it. Interpreting it and deciding on a dose are your doctor's job, not a table on the internet's. Take the result to them, especially if it's far from the reference range.
If you choose not to get tested, which is what most people do, then at least treat the figure on the box with respect and don't stack sources.
How to take it
so it counts
Vitamin D is fat-soluble. It's absorbed significantly better if it reaches your stomach together with something fatty, whether that's eggs, yoghurt, cheese, avocado or olive oil.
Your body works more predictably with a small amount every day than with a huge dose once a month. And a daily routine is easier to keep than one you have to remember.
D3 or D2
Cholecalciferol, or vitamin D3, is the form your skin produces too. Ergocalciferol, D2, comes from fungal sources. The two aren't equivalent, and D3 is the preferred form in most products precisely because it's the one your body uses more efficiently. If you're vegan, look for D3 derived from lichen.
What it pairs with
Vitamin K2 and magnesium often appear alongside D3, and that's a topic worth understanding on its own, not summed up in two lines. We've written about it here: why vitamin D3 and K2 are taken together. And if you want to see what shouldn't be taken at the same time as what, we also have the complete supplement timetable.
What the label
is allowed to say
Unlike many fashionable herbs, vitamin D really does have authorised health claims in the European Union. There are four, and they are worded exactly like this:
- Vitamin D contributes to the normal function of the immune system.
- Vitamin D contributes to the maintenance of normal bones and teeth.
- Vitamin D contributes to the maintenance of normal muscle function.
- Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus.
The key word in all four is “normal”. Vitamin D supports the normal functioning of these systems; it doesn't improve them beyond normal and it doesn't treat anything. Any product that promises you more than that is selling you a claim it has no right to make.
If you want to see how this rule applies to the whole autumn shelf, not just vitamin D, we've put together a separate guide on which vitamins to take in autumn, starting from blood tests and authorised claims.
Where the limit is
and who needs to be careful
Vitamin D is stored in the body, unlike water-soluble vitamins, which the body flushes out. That's why “more” doesn't mean “better”, and the upper limit isn't a formality.
Do not exceed the recommended dose on the packaging, and do not combine several sources of vitamin D without your doctor's approval. A product with 4,000 IU per serving is intended for adults and already reaches the tolerable upper intake level set by EFSA. Ask your doctor before use if you are pregnant or breastfeeding, if you have a kidney or liver condition, a disease affecting calcium levels or sarcoidosis, or if you take thiazide diuretics, heart medication or any long-term treatment. Supplements are not medicines and do not replace treatment or a varied diet.
If you also want vitamin C and zinc in the same cold-season routine, the combined option is Vitamin C & Zinc & D3, with a lower dose of D.
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