Previously undescribed vitamin D C-3 epimer occurs in substantial amounts in the blood of cats.

Objectives The aim of this report is to describe the identification of a novel vitamin D metabolite, a C-3, alpha-epimer of 25-hydroxycholecalciferol (3-epi-25(OH)D), in serum and plasma extracts of cat blood and compare its abundance in cat, dog and rat serum to 25-hydroxycholecalciferol (25(OH)D), a conventional marker of vitamin D status. Methods Serum 25(OH)D and 3-epi-25(OH)D concentrations were measured in healthy cohorts of cats (n = 8), dogs (n = 8) and rats (n = 17) using validated reverse and normal-phase high-performance liquid chromatography methods. The methods were verified using liquid chromatography tandem mass spectrophotometry. Dietary intake and dietary concentrations of vitamin D were also measured for evaluation of species differences and effect of dietary change on vitamin D metabolite concentrations. Differences between cat serum and plasma metabolite concentrations were determined. Results Detectable concentrations of 3-epi-25(OH)D were observed in all cats and rats. No 3-epi-25(OH)D was detected in dogs, where our limit of detection was 5 ng/ml. There were significant differences ( P <0.05) in serum concentrations of 25(OH)D and 3-epi-25(OH)D among species, with cats having the greatest concentrations of both metabolites. Serum and plasma results were not significantly different. A diet change, which resulted in an increase in vitamin D intake among the cats, affected serum concentration with an increase ( P = 0.004) in 3-epi-25(OH)D but no significant change in 25(OH)D. Conclusions and relevance Serum and plasma of cats contain 3-epi-25(OH)D in varied and extraordinary concentrations, much greater than in rats and certainly than that of dogs, a species for which the metabolite was not detected. Importantly, this finding indicates a C-3 epimerization pathway is quantitatively significant for vitamin D metabolism in domestic cats, making 3-epi-25(OH)D assays essential for the evaluation of vitamin D status in cats and positioning the cat as a novel model for study of this pathway.