According to studies, our skin can produce more than 20,000 IU of vitamin D on a sunny day. In healthy individuals, even after extensive sunbathing, there is no risk of an oversupply of the biologically active vitamin D metabolite 1,25(OH)2D3. This is because as soon as more 1,25(OH)2D3 than desired is produced in the body, its new formation is throttled. UV-B radiation is essential for the formation of vitamin D in the skin. Due to the shallow angle of incidence of the sun’s rays, not enough UV-B radiation reaches the earth’s surface in Central Europe during the winter months (October-March) after the long journey through the atmosphere to ensure sufficient vitamin D production (“Vitamin D Winter”). The amount of the vitamin D precursor 7-dehydrocholesterol (7-DHC) in the skin decreases with age. It is estimated that—partly due to the reduced 7-DHC content—the skin’s ability to produce vitamin D3 decreases by about a factor of 3 in old age.