USE OF A MODIFIED RELATIVE DOSE RESPONSE TEST OR VITAMIN A FRACTIONATION TEST FOR DETERMINING VITAMIN A STATUS FROM SERUM IN THE HORSE AND RABBIT

Abstract

Methods for assessing vitamin A status in the horse and rabbit were developed and evaluated using a modified Relative Dose Response Test (%RDR = A₄-A₀/A₄ X 100, where A₄ and A₀ represent four hours post-feeding and fasting serum total vitamin A levels, respectively) for horses and a Vitamin A Fractionation (VAF) Test monitoring serum levels of vitamin A palmitate, vitamin A acetate and retinol for horses and rabbits. In Experiment 1 (RDR test), 5 horses per treatment group were fed 0 (deficient), 10,000 (control) or 80,000 (excess) I.U. vitamin A palmitate daily, for 30 days. RDR Test was positive (>20%) for all horses receiving diets deficient in vitamin A and negative «20%) for all horses receiving control or excess diets. In Experiment 2 (VAF test), rabbits were fed varying dietary levels of vitamin A palmitate (ranging from 0 to 58000 I.U./kg feed) for up to 87 days. Percentages of retinol and vitamin A palmitate were reflective of vitamin A status. An approaching vita'min A deficiency or toxicity is indicated when percentages of vitamin A palmitate and retinol are more than 1 SD from the means observed for control rabbits (6.2±1.8 and 92.9±3.5, respectively). If a deficiency is approaching then percentage of vitamin A palmitate will be between 21% and 73% and percentage retinol between 8% and 21%. If toxicity is approaching then percentage vitamin A palmitate and retinol will be greater than 21% and less than 73%. Rabbit is normal if percentages are maintained within the ± SD of the mean. Experiment 3 (VAF test) was conducted using the same horses and conditions as in Experiment 1. After 30 days on treatment, percentages of retinol and vitamin A palmitate were significantly lower and higher (P<.05) than controls, for deficient and excess horses, respectively. The percentages of vitamin A palmitate and retinol in deficient horses were intermediate between values observed in horses from the other two treatment groups. If percentage retinol is between 45% and 65% and percentage vitamin A palmitate is between 31% and 45% the horse is approaching deficiency. If the percentage retinol is less than 45% and and vitamin A palmitate is greater than 45%, then the horse is probably approaching toxicity. Results suggests that both RDR and VAF tests can be used to determine vitamin A status before appearance of overt signs of deficiency occur, however only the VAF test is suitable for detecting toxicity.