Influential factors on sun-induced vitamin D synthesis

Ultraviolet B radiation is absorbed in the epidermis by 7-dehydrocholesterol, giving rise to previtamin D3. This seco-steroid is then converted into Vitamin D3, exiting the skin, and entering systemic circulation. Once it reaches the liver it undergoes hydroxylation at carbon 25 by 25-hydroxylase, one of various cytochrome P-450 enzymes. The resulting compound is calcifediol – 25-hydroxyvitamin D3 (25OHD3).

After being discharged into circulation, 25OHD3 will travel to various organs and tissues, where it will be converted, by the action of the enzyme 25-hydroxyvitamin D 1-α-hydroxylase (CYP27B1) into calcitriol – 1α,25-dihydroxyvitamin D3. This active form of vitamin D will serve as a ligand for the Vitamin D receptor expressed in several cells, thus regulating numerous physiological functions.

Despite its important role, vitamin D insufficiency is widespread according to multiple studies. This can be explained by a wide range of factors affecting either vitamin D synthesis, intake, or metabolism. This lecture emphasizes factors compromising the cutaneous synthesis of vitamin D, such as the solar zenith angle (which in turn is influenced by latitude, season and hour of the day), meteorological conditions, air pollution, skin type, aging, and sunscreen use.