The photosynthesis of vitamin D evolved over 750 million years ago; the phytoplankton coccolithophor Emeliani huxleii is an early example. Vitamin D played a critical role in the maintenance of a calcified skeleton in vertebrates as they left their calcium-rich ocean environment for land over 350 million years ago.
Vitamin D can only be synthesized via a photochemical process so early vertebrates that ventured onto land either had to ingest foods that contained vitamin D or had to be exposed to sunlight to photosynthesize vitamin D in their skin to satisfy their body's vitamin D requirement.[12]
[edit] Production in the skin
In the epidermal strata of the skin, production is greatest in the stratum basale (colored red in the illustration) and stratum spinosum (colored light brown).
Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet light (UVB) at wavelengths between 270 and 300 nm, with peak synthesis occurring between 295 and 297 nm.[13] These wavelengths are present in sunlight when the UV index is greater than three and also in the light emitted by the UV lamps in tanning beds. Tanning lamps produce ultraviolet primarily in the UVA spectrum, but typically produce 4% to 10% of the total UV emissions as UVB. At this solar elevation, which occurs daily within the tropics, daily during the spring and summer seasons in temperate regions, and almost never within the arctic circles, vitamin D3 can be made in the skin. Depending on the intensity of UVB rays and the minutes of exposure, an equilibrium can develop in the skin, and vitamin D degrades as fast as it is generated.[14]
The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer, thinner epidermis. Thick epidermis in the soles and palms consists of five strata; from outer to inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Vitamin D is produced in the two innermost strata, the stratum basale and stratum spinosum.
In some animals, the presence of fur or feathers blocks the UV rays from reaching the skin. In birds and fur-bearing mammals, vitamin D is generated from the oily secretions of the skin deposited onto the fur and obtained orally during grooming.[19]
In 1923, it was established that when 7-dehydrocholesterol is irradiated with light, a form of a fat-soluble vitamin is produced. Alfred Fabian Hess showed "light equals vitamin D."[20] Adolf Windaus, at the University of Göttingen in Germany, received the Nobel Prize in Chemistry in 1928, for his work on the constitution of sterols and their connection with vitamins.[21] In the 1930s he clarified further the chemical structure of vitamin D.[citation needed]
Vitamin D is formed just under the surface of the skin, if its exposed to UV rays, but there are co factors involved to insure optimal production and absorption. The oil on the surface of the skin along with bacteria, kind of form a protective layer of cholesterol rich co factors that are necessary for optimal vitamin D production. If you constantly wash away the layer of protective oil and bacteria on the surface of the skin then whatever vitamin D that does form just under the surface will be reduced, or leach out and not be fully adsorbed.
