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. In everyday life, this photosynthesis will be carried out due to the solar UVB radiation penetrating the uncovered skin. In this way, the vitamin D metabolism will be started independent of the nutrition. Provitamin D3 will be photoconverted to previtamin D3 by UVB radiation that penetrates the skin superficially. The skin contains the provitamin D3 7-dehydrocholesterol, a precursor of vitamin D. Besides nutrition, vitamin D will be produced in the body itself.
In the classic view, a vitamin is an essential nutrient, which cannot be synthesized independently in the body. In its effect, vitamin D is more like a hormone. Moreover, further health effects due to vitamin D are under discussion. Vitamin D is generally accepted in its importance on the regulation of calcium homeostasis and bone metabolism.
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