Vitamin D in Europe: why your levels fall between October and April

By Linus Persson 2026-07-27

Above roughly 40 degrees north, the winter sun never climbs high enough for your skin to make vitamin D at all. That line runs through Madrid and Naples, which means almost every European city spends part of the year in a synthesis blackout — six months of it in Stockholm, Copenhagen and Edinburgh. Here is what that does, and what to do about it.

The short version

Latitude, not weather. Below a certain solar angle, UVB does not reach the ground at all.
Your reserve runs down over winter. Levels typically bottom out in February and March.
Take it with a meal that contains fat. Vitamin D is fat-soluble, like A, E and K.
D3 and K2 belong together. One raises calcium absorption, the other directs where it goes.

The latitude problem

Vitamin D synthesis needs UVB, and UVB is filtered out by the atmosphere when the sun sits low. The threshold is roughly 45 degrees of solar elevation; below that, you can stand outside all day in January and produce nothing. Cloud cover, glass, sunscreen and darker skin all reduce synthesis further, but none of them is the main factor at European latitudes in winter. Geometry is.

What your body does with the reserve

Vitamin D is stored in fat tissue, so summer exposure carries you some distance into the autumn. How far depends on how much you built up and how much fat mass you have to distribute it across. In most European cohorts, blood levels peak in late August and reach their lowest point in February or March — which is also when the reasons people care about vitamin D, from immune function to mood, are under most pressure.

Who runs low first

Anyone who works indoors, which is most people. Anyone with darker skin living at northern latitudes — more melanin means more UVB is needed for the same synthesis. Older adults, whose skin converts less efficiently. People who cover up for cultural or medical reasons, and anyone who is genuinely careful about sun exposure. If you belong to two of these groups, the winter shortfall is not a marginal question.

D3, K2 and why they travel together

Vitamin D increases how much calcium you absorb from food. Vitamin K2 activates the proteins that decide where that calcium is deposited — towards bone, rather than soft tissue. Taking D3 alone at higher doses without adequate K2 is a gap worth closing, which is why the better formulas pair them. Both are fat-soluble, so both want a meal with some fat in it.

How much, and how to know

National recommendations across Europe generally sit between 10 and 20 micrograms (400–800 IU) daily for adults, with higher amounts common in supplement formulas. The honest answer is that the right dose depends on your starting level, and the only way to know that is a blood test — 25-hydroxyvitamin D, available through most GPs and every private lab. Testing once at the end of winter tells you more than any general guideline.

Products for the dark half of the year

Vitamin D Supreme pairs D3 with both K1 and K2, Nanoemulsified D3-K2 is the liquid version if capsules are a problem, and Solgar Cod Liver Oil is the traditional route, with vitamin A alongside. See immune support for the rest.

Summary

For half the European year, sunlight is not a source of vitamin D no matter how much time you spend outside. Supplement through the dark months, take it with fat, pair D3 with K2, and test once at the end of winter so you are working from your own number rather than an average.