Medical Encyclopaedia › Vitamin D (short definition)
In brief: Vitamin D is a fat-soluble vitamin that the body largely produces itself when the skin is exposed to UVB radiation. Above all, it regulates calcium and phosphate balance and is therefore important for healthy bones.
Functions
Active vitamin D (calcitriol) promotes the absorption of calcium and phosphate from the gut and their incorporation into bone. It also acts on muscles and the immune system. Many other effects, for example on the heart, cancer or infections, are being researched; large studies have so far mostly been unable to show any additional benefit from supplements in people with adequate levels.
Deficiency and risk groups
In Central Europe, the sun is hardly sufficient for the body’s own production from about October to March. People at risk are older people, people who spend little time outdoors or cover their skin, people with darker skin, infants, and people with certain bowel, liver or kidney diseases. Severe deficiency can lead to rickets in children and osteomalacia in adults and increase the risk of osteoporosis.
Measurement and requirements
Vitamin D status is determined by measuring 25-hydroxyvitamin D in the blood. Vitamin D prophylaxis is recommended for infants in their first year of life. For adults, professional bodies such as the German Nutrition Society (DGE) give an estimated value of 20 micrograms (800 IU) per day if the body’s own production is lacking. Whether and at what dose supplementation makes sense should be clarified individually.
Beware of overdosing
As vitamin D is stored in fatty tissue, excessive intake over a long period can lead to raised calcium in the blood with nausea, cardiac arrhythmias and kidney damage. High-dose preparations should therefore not be taken without consulting a doctor.
Related terms in the encyclopaedia
Calcium, Phosphate, Osteoporosis, Osteomalacia, Rickets, Hypercalcaemia, Food supplements, UV radiation
Further reading on Medizin Aktuell
Sources
- Deutsche Gesellschaft für Ernährung (DGE): Referenzwerte für die Nährstoffzufuhr – Vitamin D
- Robert Koch-Institut (RKI): Antworten auf häufig gestellte Fragen zu Vitamin D
- Bundesinstitut für Risikobewertung (BfR): Fragen und Antworten zu Vitamin D
Note: This article is for general information and does not replace advice, diagnosis or treatment from a doctor or pharmacist. Last updated: October 2026.