Vitamin A in the urine of carnivores

Vitamin A levels (retinol equivalents) in the urine of canines were between 423 ng/ml (dog) and 6304 ng/ml (silver fox). Neither vitamin A nor vitamin E was found in the urine of herbivores, omnivorous and rodents. No vitamin A but low levels of vitamin E were detected in cats. Vitamin A in the urine was present as retinol and retinyl esters (basically retinyl palmitate/oleate). The total excretion of vitamin A represented 15 to 63% of the daily uptake in dogs, while less than 4% of vitamin E was excreted. Results after precipitation and ultracentrifugation indicate that similar carrier proteins may exist for retinol, retinyl esters and alpha-tocopherol in the urine. The biological significance of this phenomenon is discussed with regard to the high concentrations of retinyl esters in the blood plasma of carnivores bound to lipoproteins.