Production of Vitamin D in the Skin

During exposure to sunlight, the ultraviolet B photons with energies between 290 and 315 nm are absorbed by provitamin D3 (7-dehydrocholesterol) in the skin. This absorption results in a photolysis of the B-ring of provitamin D3 resulting in the formation of previtamin D3 (Figure 3). However, since previtamin D3 is thermodynamically unstable, it quickly undergoes an isomerization (rearrangement) of its triple bond system to form vitamin D3. This isomerization process is enhanced in skin cells because the previtamin D3 is synthesized in the cell membrane, which restricts its movement thereby accelerating the transformation of previtamin D3 to vitamin D3. Once vitamin D3 is formed in the skin cell membrane, it is no longer restricted in its movement and freely translocates into the extracellular space to find its way into the dermal capillary bloodstream where it is bound to a specific vitamin D-binding protein (Figure 3).

An increase in skin pigmentation and zenith angle of the sun (change in latitude, season, and time of day) and the topical application of a sunscreen can markedly diminish or even prevent the production of vitamin D3 in the skin. Over the age of ^65 years, there is a three- to fourfold decline in the synthetic capacity of the skin to produce vitamin D3. Excessive exposure to sunlight cannot cause vitamin D3 intoxication because once previta-min D3 and vitamin D3 are made in the skin, excessive quantities are rapidly destroyed by sunlight (Figure 3).

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