Undetectable vitamin D(3) in equine skin irradiated with ultraviolet light

Vitamin D requirements for most animals are expected to be fulfilled through daily exposure of the skin to solar ultraviolet B radiation. The synthesis of vitamin D(3) in skin depends on different factors including melanin pigmentation, the amount of UVB radiation reaching the skin, type of clothing/hair coat, latitude and altitude, season, and time of day. Alternatively vitamin D(2) may be obtained from UVB irradiated pasture species. Recent studies have shown that in unsupplemented grazing horses 25-hydroxyvitamin D(2) is the predominant form of vitamin D in plasma, and that 25OHD(3) is undetectable suggesting horses may rely on diet to obtain vitamin D. In order to mimic the natural environment of skin to sunlight exposure, five equine and two ovine devitalized skin samples were irradiated with 5 J/cm(2) of UVB light followed by measurement of 7-dehydrocholesterol (7-DHC) and vitamin D(3) concentrations using reverse-phase high pressure liquid chromatography (HPLC). HPLC revealed the presence of 7-DHC in the skin of both horses and sheep. Vitamin D(3) was undetectable in both ovine and equine skin prior to irradiation, but after irradiation with UVB light, ovine skin showed an increase in vitamin D(3) concentration (mean 0.16 ± 0.07 µg/g), whereas vitamin D(3) was undetectable in equine skin. These results provide additional evidence that horses make negligible quantities of vitamin D(3) in their skin after exposure to UVB light and may therefore rely on their diet as a primary source of vitamin D.