Sun Exposure and Vitamin D.

Vitamin D is generally accepted in its importance on the regulation of calcium homeostasis and bone metabolism. Moreover, further health effects due to vitamin D are under discussion. In its effect, vitamin D is more like a hormone. In the classic view, a vitamin is an essential nutrient, which cannot be synthesized independently in the body. Besides nutrition, vitamin D will be produced in the body itself. The skin contains the provitamin D3 7-dehydrocholesterol, a precursor of vitamin D. Provitamin D3 will be photoconverted to previtamin D3 by UVB radiation that penetrates the skin superficially. In this way, the vitamin D metabolism will be started independent of the nutrition. In everyday life, this photosynthesis will be carried out due to the solar UVB radiation penetrating the uncovered skin. In the same spectral waveband range of UVB radiation, which causes the beneficial health effect of starting the vitamin D metabolism, the UVB radiation causes simultaneously acute and chronic harmful health effects as UV erythema (sunburn), skin aging and skin cancer. There is no vitamin D production in the skin without simultaneous DNA damage in the skin. Against this background, risks and benefits have to be balanced carefully.