Vitamin D and the Nordic winter
If you live in Norway, this is the one supplement story that's genuinely about where you live. From roughly October to March, this far north, your skin makes almost no vitamin D — no matter how much you get outside.
Why the winter sun doesn't cut it
Vitamin D is made in the skin from UVB light. But when the sun sits low, UVB has to travel a longer path through the atmosphere and gets absorbed before it reaches you. Classic research showed essentially no vitamin D synthesis from November to February even at 42°N (Boston) — and Norway sits at 58–71°N. It doesn't help that UVB doesn't pass through glass, and that clouds and clothing block it too.
What the authorities recommend
Norwegian health authorities (Helsedirektoratet) recommend 10 µg (micrograms) per day for most children and adults, rising to 20 µg per day for adults over 75 and for anyone with little or no sun exposure — which, in a Nordic winter, is most of us.
The effects that are actually approved
Vitamin D has a solid set of EU-authorised health claims. Among them:
- Vitamin D contributes to the normal function of the immune system.
- Vitamin D contributes to the maintenance of normal bones and normal muscle function.
- Vitamin D contributes to normal absorption/utilisation of calcium.
These are about keeping normal function — not "boosting" anything beyond normal.
The honest caveats
The clearest benefit is correcting a deficiency, and the bone/muscle evidence is the strongest. Broader claims — general immunity, mood — are biologically plausible but the trial evidence is mixed, and often null in people who already have enough. More is not better: there's an upper limit of 100 µg/day, and there's no benefit to megadosing above your needs.
The takeaway
For most people in the Nordics, a modest 10–20 µg/day through the dark season is a sensible, well-supported habit — and one of the few that's specifically justified by our latitude.
Sources
- Helsedirektoratet — recommended intakes of vitamins and minerals.
- EU Reg. 432/2012 — authorised health claims for vitamin D.
- NIH Office of Dietary Supplements — Vitamin D fact sheet (thresholds, upper limit).
- Webb & Holick — latitude, season and vitamin D synthesis.