The main source of vitamin D in humans is the conversion of 7-dehydrocholesterol (7-DHC) in the epidermis to previtamin D3, which isomerizes to form vitamin D3. Seasonal and local variations of ultraviolet (UV) radiation, increasing indoor activities, but also public awareness of the hazardous effects of sun exposure that lead to sun avoidance strategies or the use of sunscreens
limit the endogenous synthesis of vitamin D3. As a consequence, the need for vitamin D from food sources is on the rise.