Strictly speaking, vitamin D is a hormone that the body produces itself in the skin under the influence of UV-B radiation. The storage form, 25-OH vitamin D3, is measured because it reliably reflects vitamin D status over several weeks. Vitamin D regulates calcium absorption from the intestines and also affects the muscles and immune system.
Why is this value measured?
In Germany, sunlight from October to March is insufficient to produce enough vitamin D. Deficiencies are therefore widespread and usually go unnoticed because the symptoms – fatigue, muscle weakness, and susceptibility to infections – are nonspecific.
Too high
Very high levels practically only result from overdosed supplements and can cause harm through elevated calcium levels. An overdose from sunlight alone is not possible.
Too low
Low levels promote bone loss, muscle weakness, and susceptibility to infections. At-risk groups include older people, people with dark skin, shift workers, and anyone who spends little time outdoors.
Frequently asked questions
When should vitamin D levels be measured?
The most informative time to measure it is at the end of winter, in February or March, when levels are at their lowest.
How long does it take for the level to improve?
With regular supplementation, levels rise over approximately eight to twelve weeks. A follow-up measurement is advisable no earlier than three months later.
Is summer sunlight enough?
For people with fair skin, 10 to 20 minutes of midday sun on the face and forearms is often enough in summer. In winter, the sun is too low in Germany for significant vitamin D production.
