Vitamin D is actually not a vitamin at all, but a prohormone, because the body, more precisely the skin, can produce it itself from cholesterol through UVB radiation (i.e. sunlight). That is why it is also called the sun vitamin. Previtamin D3 is then produced, which is converted into vitamin D3 (cholecalciferol) by further processes. If the sun is lower than 45 degrees in the sky, vitamin D is no longer produced. This is the case in the morning and evening, but also during the winter months, so that the intensity of the UV radiation is no longer sufficient for vitamin D production. So a balanced diet doesn't really help to prevent a deficit. Even foods with a relatively high vitamin D content (such as dairy products, egg yolks, oily fish and liver) contain quite a small amount of the vitamin compared to the amounts we need.