ABSTRACT
The primary source of vitamin D is from the action of ultraviolet (UV) radiation on 7-dehydrocholesterol in skin. As noted in Chapter 1, high-energy UVB wavelengths (290-315 nm) convert 7-dehydrocholesterol to previtamin D, which then isomerizes to vitamin D at body temperature. Upon entering the circulation, it is transported by its binding protein and converted by 25-hydroxylase mainly in the liver to 25-dihydroxyvitamin D, the major circulating form of vitamin D. It is subsequently converted to the active hormone 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) principally in the kidney by the action of 1α-hydroxylase (Malloy et al. 1999; Reichrath 2007; Holick 2001, 2011). The local conversion of 7-dehydrocholesterol to the active hormone has also been demonstrated in the epidermis (Bikle et al. 1986; Lehmann et al. 1999, 2000, 2001). Continued irradiation of skin converts previtamin D and vitamin D into what are known as overirradiation products (Holick et al. 1981), which prevent excessive vitamin D from being produced by UV exposure, but have no other known function.
