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September 27, 2026
Lena Sämann
Vitamin D deficiency often goes unnoticed because there are no clear-cut symptoms. Find out who is particularly at risk, when a blood test might be useful — and who may benefit from taking vitamin D supplements in fall.
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In summer, even a short time outdoors is often enough for the body to produce vitamin D. But as autumn arrives, the skin receives less and less UV-B radiation. In Germany, this so-called "vitamin D winter" begins around October. Does that automatically mean we should start taking vitamin D supplements?
Vitamin D occupies a unique place among vitamins. While most vitamins must come from food, the body can largely produce vitamin D on its own — but the skin needs UV-B radiation to do so.
In Germany, UV-B radiation is generally too weak from around October to March for the skin to produce adequate vitamin D. The reason is that the sun sits so low in the sky that too little of the necessary UV-B radiation reaches the Earth's surface. Even a long walk on a sunny winter's day makes barely any difference.
That said, a vitamin D deficiency does not automatically set in come October. The body can store vitamin D in fatty and muscle tissue, drawing on reserves built up over the summer months.
How effectively this works varies from person to person. Those who rarely spend time outdoors, or whose skin receives little sunlight, face a higher risk of inadequate levels. The skin's ability to produce vitamin D also declines with age.
People with darker skin require more UV-B radiation to produce the same amount of vitamin D, due to their higher melanin content. Those with certain chronic conditions may also be more prone to inadequate supply. Low vitamin D levels are likewise frequently measured in people with obesity.
This is exactly where the challenge lies: a vitamin D deficiency often cannot be identified by any clear-cut symptoms.
Fatigue, exhaustion, and muscle pain are frequently linked to low vitamin D levels. However, these complaints can have many other causes and are therefore not reliable indicators for self-diagnosing a deficiency.
A severe and prolonged vitamin D deficiency, however, can have serious consequences for and muscles. Vitamin D plays a key role in calcium and phosphate metabolism and is therefore essential for bone mineralization. In adults, a severe deficiency can lead to osteomalacia, a condition in which the bones soften. Muscle weakness can also occur.
A blood test typically measures the concentration of 25-hydroxyvitamin D — or 25(OH)D for short. This is the most important laboratory value for assessing a person's vitamin D status.
Routine vitamin D screening is not recommended for healthy individuals, however. Testing may be worthwhile when there is an increased risk of deficiency, or when doctors suspect a deficiency based on symptoms or a particular medical condition.
It is also worth paying attention to how results are interpreted: different professional associations sometimes apply different threshold values. With regard to bone health, the German Nutrition Society refers to a 25(OH)D concentration of at least 50 nmol/l, or 20 ng/ml.
Diet can cover only a comparatively small share of the body's vitamin D needs, as very few foods naturally contain significant amounts.
The best sources are fatty sea fish such as herring, salmon, and mackerel. Smaller amounts are found in egg yolk and certain mushrooms, for example. Foods fortified with vitamin D can also contribute to overall supply.
During the months with little sunlight, such foods can make a useful contribution. Even so, it is difficult to fully compensate through a normal diet alone for what the body would otherwise produce in the skin.
The fact that the skin produces almost no vitamin D in winter does not automatically mean that everyone needs a dietary supplement from October onwards.
For adults, the estimated requirement in the absence of endogenous vitamin D production is 20 micrograms per day, equivalent to 800 International Units (IU). Whether a supplement is appropriate depends, among other things, on how well vitamin D levels were maintained during the sunnier months and whether an increased risk of deficiency exists.
The "more is better" principle does not apply here. Vitamin D is fat-soluble and can accumulate in the body when intake is consistently too high. In severe cases, this can cause blood calcium levels to rise sharply and, among other consequences, damage the kidneys. High-dose supplements should therefore not be taken on one's own initiative without a medical reason.
In autumn especially, vitamin D attracts attention partly because of its association with the immune system. Vitamin D does indeed support normal immune function. Whether taking additional supplements can also prevent colds is, however, a separate question.
A updated meta-analysis published in 2025 evaluated data from more than 60,000 people. Overall, vitamin D supplementation showed no statistically significant reduction in acute respiratory infections. Earlier analyses had still pointed to a small protective effect.
This underlines an important distinction: maintaining an adequate vitamin D supply matters for the body. Taking extra high doses, however, does not automatically translate into additional health benefits.
Those who spent regular time outdoors during the sunnier months can initially draw on their stored vitamin D through autumn and winter. Vitamin D-rich foods can provide an additional contribution.
Those with an increased risk of deficiency, on the other hand, should not reach for high-dose supplements based solely on non-specific symptoms. In such cases, a doctor can assess whether measuring vitamin D levels and targeted supplementation are advisable.
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