In short, vitamin D is a fat-soluble prohormone that the body makes itself when it gets enough sunlight. In Central Europe, UVB radiation is generally insufficient to meet requirements, especially from October to March. Vitamin D supplements are available in D2 and D3 forms, as well as in drops, capsules, or tablets, with dosages ranging from 800 IU to 20,000 IU. The German Nutrition Society (DGE) recommends an intake of 800 IU (20 µg) per day if the body is unable to produce it naturally.