The fat-soluble vitamin D takes over many tasks in our organism. For example, it strengthens the bones and has an influence on muscle strength. Vitamin D is converted by synthesis from 7-dehydrocholesterol in the body. This requires sufficient dehydrocholesterol in the body and sunlight - the UVB component - in the skin. People usually do not consume enough vitamin D because they are mostly indoors and protect themselves from the sun when the sun is shining. However, direct sunlight is essential for the photochemical reaction in the body that produces vitamin D3. Only about 5 to 20 percent are absorbed from food.