Frontiers in Bioscience-Scholar (FBS) is published by IMR Press from Volume 13 Issue 1 (2021). Previous articles were published by another publisher on a subscription basis, and they are hosted by IMR Press on imrpress.com as a courtesy and upon agreement with Frontiers in Bioscience.
Vitamin D3 is a key regulator of vertebrates homeostasis. It is synthesized from the precursor 7- dehydrocholesterol upon UVB exposure in the skin and then hydrolyzed in the liver in position 25, to be finally converted into its active form, 1,25-dihydroxyvitamin D (1,25(OH)2D or calcitriol), in the kidneys. The biological activity of this molecule depends on its binding to the nuclear receptor VDR, which binds VDRE once complexed with RXR-alpha. Despite being present in different types of food, the best way to assume it at physiological levels remains the exposure to UVB radiation at certain hours of the day and at particular angles of the Earth's crust. There is plenty of evidence that altered levels of vitamin D3 are associated with pathological conditions, such as osteoporosis, cancer, immunological and infectious diseases. In this review, we discuss vitamin D3 metabolism, its role in several diseases and the link between vitamin D3 and immune cells.