Sunlight Regulates the Cutaneous Production of Vitamin D3by Causing Its Photodegradation*

Abstract

Exposure to sunlight initiates the formation of vitamin D₃ in skin as the UV B radiation in the solar spectrum causes the photoconversion of 7-dehydrocholesterol to previtamin D₃. A heat-induced isomerization then converts previtamin D₃ to vitamin D₃ over a period of days. A number of irradiation products of vitamin D₃ are known to form upon irradiation with high intensity UV radiation, but the effect of subsequent exposures to sunlight on the vitamin D₃ formed in skin is not known. To investigate this phenomenon, human skin containing vitamin D₃ was exposed to sunlight in Boston. A model system of [³H]vitamin D₃ in methanol was also used to study the effects of sunlight on vitamin D₃ throughout the year. Vitamin D₃ proved to be exquisitely sensitive to sunlight, and once formed in the skin, exposure to sunlight resulted in its rapid photodegradation to a variety of photoproducts, including 5,6-transvitamin D₃, suprasterol I, and suprasterol II.