Our need for vitamin D can be met in two different ways: Either vitamin D is produced in the skin under the influence of UV-B rays from precursors, or it is ingested with food. The skin is the only organ in humans capable of producing vitamin D. According to studies, under our living conditions, about 80–95% of the required vitamin D must be formed in the skin with the involvement of UV-B radiation. This is because only a few foods (including fatty fish such as salmon, mackerel and herring, fish oils, cod liver oil, offal, eggs, mushrooms and to a limited extent also dairy products) contain relevant amounts of vitamin D. The low vitamin D content of most foods practically does not allow us to achieve sufficient vitamin D supply through dietary adjustment alone under our living conditions. As a rule, under our dietary habits (“western diet”), only about 5–20% of the need can be covered by intake with food.