Undetectable vitamin D3 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 D₃ 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 D₂ may be obtained from UVB irradiated pasture species. Recent studies have shown that in unsupplemented grazing horses 25-hydroxyvitamin D₂ is the predominant form of vitamin D in plasma, and that 25OHD₃ 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/cm² of UVB light followed by measurement of 7-dehydrocholesterol (7-DHC) and vitamin D₃ 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 D₃ was undetectable in both ovine and equine skin prior to irradiation, but after irradiation with UVB light, ovine skin showed an increase in vitamin D₃ concentration (mean 0.16 ± 0.07 µg/g), whereas vitamin D₃ was undetectable in equine skin. These results provide additional evidence that horses make negligible quantities of vitamin D₃ in their skin after exposure to UVB light and may therefore rely on their diet as a primary source of vitamin D.