Vitamin D in winter: How much sun and how much vitamin D do you need?
In summer, your body produces most of its vitamin D itself – with the help of the sun. From October to March, however, the sun in Germany is not strong enough for this. This article shows how much sun you need in summer, which dose can make sense in winter and why your blood level is the most important guide.
Why vitamin D runs low in winter
Most vitamin D is produced in the skin when it is exposed to UVB radiation. Through food, on the other hand, you only take in small amounts, even if you regularly eat fortified foods.
How much vitamin D the skin produces depends on latitude, season and time of day, weather, clothing, skin type and sunscreen. In Germany, the sun is too low from October to March. UVB radiation is then not strong enough to produce sufficient vitamin D – no matter how long you spend outdoors.
The body can store vitamin D from the summer. For many people, however, these stores do not last the whole winter. That is why blood levels often drop considerably towards the end of winter.
How long should you spend in the sun?
From around April to September, current knowledge suggests it is enough to expose your face, hands and arms to the sun several times a week for about 5 to 25 minutes, uncovered and without sunscreen. If you spend a little time outdoors every day, you usually manage this without even thinking about it.
- Lighter skin tends to need shorter times, darker skin longer ones.
- UVB radiation is strongest around midday. Less time is enough then.
- As a rule of thumb: about half the time after which you would get sunburnt without protection. For skin type II and a high UV index, this works out at about 12 minutes.
- Always avoid sunburn. For longer periods outdoors, sunscreen is a must.
Unfortunately, these times do not help in winter. Then it comes down to intake through a supplement.
How much vitamin D makes sense in winter?
For cases in which the body produces no vitamin D itself, the German Nutrition Society (DGE) gives an estimated value of 20 micrograms per day. This corresponds to 800 international units (IU).
In practice, 1,000 IU a day is already enough for many people to achieve good vitamin D status over the winter.
However, there are also many cases in which 2,000 to 5,000 IU a day are necessary. This mainly affects people who get little or limited sun exposure – for example people who work indoors a lot, older people, people with darker skin or severe overweight, and people with conditions that impair absorption in the gut.
Important: The European Food Safety Authority (EFSA) has set a tolerable upper intake level of 4,000 IU (100 micrograms) per day for adults. You should only take doses above this based on a blood test and after consulting your doctor.
Why the dose should be checked individually
How well vitamin D is absorbed and used differs from person to person. The same dose can lead to very different blood levels in two people. A blood test is therefore the most reliable way to find the right dose.
The value measured is 25-hydroxyvitamin D, or 25(OH)D for short. It shows how well your stores are filled. A good time is the end of winter, around February or March. After changing your dose, wait about three months before the next measurement so that the level can settle.
A practical example
You take 1,000 IU a day throughout the winter. In February, a blood test shows that your level is still not in the good range. Then you can make a note to increase the dose to 2,000 IU next winter and to check your level again after a few months. This way, you find the dose that suits your body step by step.
Tip: Vitamin D is fat-soluble. It is best taken with a meal that contains some fat. For a vegan option, there is vitamin D3 made from lichen.
Which vitamin D blood level is good?
The following values refer to 25-hydroxyvitamin D in the blood. Labs report the result either in ng/ml or in nmol/l. To convert: 1 ng/ml equals 2.5 nmol/l.
| Rating | ng/ml | nmol/l | What it means |
|---|---|---|---|
| Deficiency (critical) | below 12 | below 30 | Increased risk of bone disease. Have it checked by a doctor and replenish in a targeted way. |
| Suboptimal | 12–20 | 30–50 | Not necessarily a deficiency, but intake should be improved. |
| Adequate | 20–30 | 50–75 | According to BfR and DGE, sufficient for bone health. |
| Good target range | 30–50 | 75–125 | Often aimed for in practice so that you are still well supplied at the end of winter. |
| Elevated | 50–100 | 125–250 | No additional benefit has been shown. Review the dose and reduce it if necessary. |
| Markedly elevated | above 100 | above 250 | Reduce the dose and see a doctor. From about 150 ng/ml (375 nmol/l), the risk of toxicity with too much calcium in the blood increases. |
Not all professional societies and labs use the same cut-offs. Some labs already classify values below 30 ng/ml as insufficient, others only values below 20 ng/ml. So always discuss your result in the context of your personal situation.
Vitamin D together with vitamin K2?
Vitamin D is often sold together with vitamin K2. Among other things, vitamin K2 helps incorporate calcium into the bones. However, the evidence does not show that the combination is strictly necessary for good vitamin D status.
Nevertheless, I recommend taking vitamin D together with K2. Vitamin K2 is mainly found in fermented foods such as natto, which rarely end up on our plates. Many people's intake is therefore not ideal. K2 can thus be a useful supplement in its own right.
EFSA has not set an upper intake level for vitamin K, as no harmful effects from high intake are known so far. Based on current knowledge, an overdose is not a concern.
Important: If you take anticoagulants of the vitamin K antagonist type, for example phenprocoumon (Marcumar) or warfarin, only take vitamin K after consulting your doctor. Vitamin K can weaken the effect of these medicines.
Conclusion
From October to March, your body produces hardly any vitamin D from the sun in Germany. In summer, a few minutes of sun several times a week are usually enough. In winter, 1,000 IU a day is enough for many people, while others need 2,000 IU or more. Your blood level is the most reliable way to find out which dose suits you. Adjust the dose from winter to winter based on the results. Vitamin K2 is not a must, but it can be a useful addition.