Nutrition & Supplements · Micronutrients

Vitamin D in winter: do you really need supplements?

4 min read 2026-06-01 YUTHI Redaktion

From October to March the sun in our latitudes is too low for the skin to produce vitamin D. What this means, who is affected and when supplements make sense.

Why your skin doesn't produce vitamin D in winter

Vitamin D is produced in the skin when UV-B radiation hits a precursor molecule. What matters is the angle of the sun: when it sits lower than about forty-five degrees above the horizon, the atmosphere filters out almost all of the UV-B component. North of the fortieth parallel, this is the case from October to March.

O'Neill and colleagues measured this availability across all of Europe in 2016. Their finding: in Central Europe, the body's own production is practically zero in winter — regardless of how much time a person spends outdoors. What was stored in fat and muscle tissue during the summer lasts anywhere from a few weeks to a few months, depending on your starting point.

How widespread deficiency actually is

Cashman and colleagues analysed data from over fifty-five thousand people across fourteen European countries in 2016. Thirteen percent had levels below 30 nanomoles per litre — the threshold at which deficiency is said to begin. In the winter half of the year, this share rose to eighteen percent.

But the numbers say little about any individual. More important than the population average is your own situation: someone who spends a lot of time outdoors, has fair skin, and is at a healthy weight goes into winter with better reserves than someone with a desk job, darker skin, and little sun exposure. A blood test costs little and beats guessing.

Who should pay particular attention

Several groups are at significantly higher risk: people aged around seventy and above, because the skin produces less vitamin D and many spend less time outdoors. People with darker skin, because melanin shields UV-B — they need several times more sun exposure. People with significant excess weight, because the fat-soluble vitamin becomes sequestered in fatty tissue.

This also includes pregnant and breastfeeding women, infants, people with chronic inflammatory bowel disease or after gastric surgery, and anyone who avoids the sun for medical reasons. Those taking anti-epileptic drugs or certain corticosteroids also have increased requirements.

Supplementing sensibly — without overdoing it

The European Food Safety Authority recommends an intake of fifteen micrograms per day, which equals six hundred international units. The safe upper limit for adults is one hundred micrograms, or four thousand units. In practice, typical winter supplements fall in the range of eight hundred to two thousand units per day.

High-dose boluses are not recommended. Bischoff-Ferrari and colleagues showed in 2016 that monthly high doses of sixty thousand units led to more falls in older adults than a moderate dose — more is demonstrably not better here. Take vitamin D with a meal containing fat, as this improves absorption.

What a test can and can't tell you

What's measured is 25-hydroxy-vitamin D in the blood. The best time to test is late winter — February or March — when levels are at their lowest. A value above 50 nanomoles per litre is considered sufficient; below 30 is considered deficient.

What vitamin D cannot do: prevent colds, protect against cancer, or cure depression. Large randomised trials such as VITAL with over twenty-five thousand participants found no evidence for this. What is well established is its role in bone health and muscle function. For symptoms or pre-existing conditions, the appropriate dose should be discussed with a doctor.

How the YUTHI app helps you

You log blood test results in the app with a date and can see your vitamin D trend over the years — for this value in particular, the seasonal pattern is very revealing. Many people only notice when they see the chart how far apart their levels in August and March actually are.

Vito and Vita remind you to check your levels in late winter and put the result in context, rather than just showing it as a number. And they keep track of what dose you're taking — so that at your next doctor's appointment, both pieces of information are on the table together.

Continue in the app

That was the snapshot. The app turns it into daily support.

Measure every day instead of calculating once: Vito or Vita puts your numbers in context, builds you a weekly plan and speaks up when something stands out.

Soon on theApp Store Soon onGoogle Play

Launching at the end of 2026 — iPhone first, Android to follow. Let me know when it’s out ↓