Strictly speaking, vitamin D is a hormone that the body produces in the skin when exposed to 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 gut and also affects muscles and the immune system.
Why is this level 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.
High level
Very high levels are practically only caused by taking excessive doses of supplements and can be harmful by raising calcium levels. It is not possible to overdose through sunlight alone.
Low level
Low levels increase the risk of bone loss, muscle weakness, and susceptibility to infections. At-risk groups include older adults, people with dark skin, shift workers, and anyone who spends little time outdoors.
Frequently asked questions
When should vitamin D levels be tested?
A test is most informative at the end of winter, in February or March, when stores are at their lowest.
How long does it take for levels to improve?
With regular use, levels rise over about eight to twelve weeks. A follow-up test is advisable no sooner than three months later.
Is summer sun 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 production in the body.
