Why your skin can't make enough of it here
Vitamin D isn't really absorbed from food the way most nutrients are. It's made in your skin, triggered by a specific band of UVB light from the sun. That system works beautifully in summer. The problem is what happens the rest of the year.
Across almost all of Europe, from the Netherlands up to Scandinavia and even much of the Mediterranean, the sun sits too low in the sky from October to March. Its UVB rays simply don't reach the ground with enough strength for your skin to make vitamin D. It doesn't matter how many walks you take or how clear the sky looks. This stretch is sometimes called the "vitamin D winter", and for half the year, your body is simply unable to produce any, regardless of sun exposure.
In the darkest winter months, deficiency rates in northern Europe climb even higher. Diet alone rarely closes the gap either: oily fish and egg yolks contain vitamin D, but not nearly enough to replace six months of missing sunlight.
The signals your body sends
Low vitamin D tends to creep in quietly. Most people attribute the signs to winter in general, rather than to a nutrient their body has simply stopped receiving.
Physical
- Persistent fatigue
- Aching bones or joints
- Muscle weakness
- Frequent colds or infections
- Slow-healing wounds
- Hair thinning
Mind & mood
- Low winter mood
- Brain fog
- Trouble concentrating
- Disrupted sleep
- Irritability
"Vitamin D isn't really a vitamin. It behaves like a hormone, and much of Europe is running on a deficit it can't tan its way out of."
Six reasons levels run low
- Latitude. Living north of southern Spain or Sicily means winter sun simply isn't strong enough, no matter how much time you spend outside.
- Indoor life. Offices, cars and screens mean most people get a fraction of the midday sun exposure their skin evolved for.
- Skin tone. Melanin acts as a natural filter, so darker skin can need several times longer in the sun to produce the same amount as lighter skin.
- Age. Skin becomes less efficient at converting sunlight into vitamin D as you get older, even with identical sun exposure.
- Body fat. Vitamin D is fat-soluble and can get stored away in fat tissue instead of circulating where it's needed.
- Sunscreen and cloud cover. Both block the UVB rays required, which is protective for your skin but leaves this particular gap unfilled.
A simple rule of thumb: if your shadow is longer than you are tall, the sun is too low for your skin to make vitamin D, whatever the season. It's why Scandinavian households have taken a daily spoonful of cod liver oil through winter for generations, long before anyone had a word for vitamin D.
Mushrooms can do something similar to your skin. Leave them gill-side up in sunlight for an hour or so before cooking, and the UV light converts a compound in their skin into vitamin D, the same reaction that happens in you.
- Choose an oil-based liquid supplement. Vitamin D is fat-soluble, so it needs fat to be absorbed properly, which makes drops in an oil base the most effective form, well ahead of a dry tablet.
- Take it alongside vitamin K2. K2 helps direct the calcium that vitamin D allows you to absorb toward your bones, rather than your arteries.
- Take your dose in the morning. Vitamin D can be mildly energising for some people, and taking it late in the day has been linked to more disrupted sleep.
What actually helps
In northern Europe, this isn't a nutrient you can reliably top up through lifestyle alone. A few starting points:
- Ask for a 25(OH)D blood test. This is the specific marker for vitamin D status, and it's the only way to know your actual level rather than guessing from symptoms.
- Eat foods that naturally contain it, too. Cod liver oil is the richest source by far, followed by oily fish such as salmon, mackerel, herring and sardines. Egg yolks and UV-exposed mushrooms add smaller amounts, and some plant milks and cereals are fortified with it as well.
- Supplement through autumn and winter, at minimum. Many people in northern Europe benefit from year-round supplementation, since even a sunny summer rarely builds enough reserve to last until spring.
- Pick the right form and timing. Oil-based drops taken with vitamin K2, in the morning, cover absorption, balance and energy in one small habit.
- Reassess with the seasons. Needs shift with sun exposure, skin tone, age and body composition, so what works in July may not be enough by January.
This article is for educational purposes and does not replace medical advice. If you recognise these symptoms, speak to your doctor and ask about a 25(OH)D test.