Effect of feeding on vitamin A concentrations in blood plasma of dogs.
The effects of different concentrations of vitamin A in food as well as different food compositions on serum vitamin A levels in dogs were investigated. Contrary to current knowledge for all other species, vitamin A levels in dogs were clearly affected by the daily vitamin A supply as well as the type of food. This behavior, similar to that of vitamin E, can be explained by the fact that dogs and other carnivores transport most of their vitamin A in blood as retinyl esters (mainly retinyl stearate) bound to lipoproteins. The differences in the retinyl ester pattern between vitamin A in liver and in blood indicates that the mobilization of vitamin A from the liver involves selective mechanism. The possible importance of nonspecific, lipoprotein-bound vitamin A esters in the blood of dogs is discussed.