Vitamin D is the one nutrient where the geography of where you live does more work than the contents of your fridge, and it is worth understanding why before deciding whether to do anything about it.
The number, and the assumption inside it
EFSA sets an adequate intake of 15 micrograms (600 IU) per day for adults. The footnote attached to that figure is the important part: it applies under conditions of assumed minimal cutaneous vitamin D synthesis. In the presence of sunlight on skin, EFSA says plainly, the dietary requirement is lower or may be zero.
So the 15 µg is not a statement that food should supply all your vitamin D. It is a statement about the worst case — and in northern Europe the worst case lasts about half the year.
Why winter sun does nothing
Vitamin D synthesis needs UVB, and UVB reaches the ground only when the sun is high enough in the sky. Above roughly 40° north — which is most of Europe: everything north of Madrid, Naples and Istanbul — the sun spends the winter months too low for meaningful UVB, regardless of how bright the day looks or how long you stand in it. A clear, cold, blinding January afternoon produces essentially none.
This is also why the usual advice sounds contradictory between countries. Spain and the UK are answering different questions.
What food actually provides
The list of genuinely rich dietary sources is short:
- Oily fish — salmon, mackerel, sardines, herring. The reliable one.
- Egg yolks — modest, but they count.
- Fortified foods — in much of Europe, some margarines, breakfast cereals and plant drinks. Where fortification is voluntary, coverage is patchy.
- UV-treated mushrooms — real, and rarely sold.
Reaching 15 µg a day from food alone, every day, means eating oily fish several times a week or relying on fortification that your country may not do. For a large share of the population that simply does not happen, which is why national bodies — the NHS among them — advise considering a daily supplement through autumn and winter, and year-round for people who cover up outdoors or have darker skin.
The advice exists because the default is a shortfall, not because the nutrient is unusually powerful.
The thing vitamin D is for
Bone. Vitamin D governs calcium and phosphate absorption, and the consequences of a serious deficiency — rickets in children, osteomalacia in adults — are skeletal. That is the evidence base the reference value rests on.
The broader claims that circulate — immunity, mood, respiratory infections, cancer — range from plausible-but-unsettled to actively disappointing when tested in large trials. The honest summary is that correcting a deficiency is clearly worth doing, and that pushing intake far above adequacy in someone who is already replete has not shown the benefits the supplement aisle implies.
More is not the strategy
Vitamin D is fat-soluble and stored, which means overdoing it is possible in a way it is not for vitamin C. EFSA has set tolerable upper intake levels for exactly that reason, and very high-dose products sit far above the intake the reference value describes. A 10–25 µg daily supplement in winter is the ordinary, boring, evidence-shaped move. A 250 µg one is a decision to make with a doctor, if at all.
If you want to know where you actually stand rather than reason from latitude, a blood test for 25-hydroxyvitamin D is the measurement, and a GP is the person to interpret it.
