Effect of Dietary Vitamin A, Breed, and Kidney Function on the Onset of Urinary Vitamin A Excretion and the Vitamin A Transport Complex in Growing Dogs

Vitamin A (retinol and retinyl esters) is present in substantial amounts in the urine of healthy adult dogs, but the mechanisms affecting its excretion are poorly understood. To assess whether the onset of urinary vitamin A excretion is dependent on dietary vitamin A intake, breed, and/or parameters of kidney function, 48 puppies from two breeds, the Labrador Retriever and the Miniature Schnauzer were included. Dogs were given four diets with increasing vitamin A content (5000; 12,500; 75,000 and 100,000 IU vitamin A/1000 kcal metabolizable energy). Plasma and urine samples were collected and the onset of a continuous vitamin A excretion in urine was determined for each dog. Vitamin A concentration was determined by HPLC. Concentrations of creatinine, urea, protein and albumin were determined by colorimetric assays. Retinol-binding protein 4 (RBP4) and Tamm-Horsfall protein (THP) were measured by semiquantitative Western blot. Plasma vitamin A concentrations increased with increased dietary vitamin A intake (p < 0.001), but the onset of urinary vitamin A excretion (p = 0.158) and the amount of vitamin A excreted at this time point were not different between the feeding groups (p = 0.128) or breed (p = 0.279). Concentrations of creatinine, urea, protein and albumin in plasma and protein in urine were not different between groups. There were no differences in the concentrations of RBP4 in plasma (p = 0.309), RBP4 in urine (p = 0.345) and THP in urine (p = 0.384). The onset of vitamin A excretion in the urine of dogs seems to be not directly associated with the plasma vitamin A concentration and, thus not simply explained by dietary vitamin A intake alone. The vitamin A excretion is not breed specific and not dependent on kidney function or the occurrence of transport proteins in plasma (RBP4) or urine (THP). Thus, the onset and the amount of vitamin A excreted are determined by factors that remain to be elucidated.