Vitamin A Activity of Corn Carotenes for Swine

Abstract

Three trials, using 204 weanling pigs, were conducted to estimate the vitamin A potency of corn carotenes for the pig. The pigs were depleted of their vitamin A stores by feeding a wheat- or milo-based diet until serum vitamin A levels approached or fell below 10 μg/100 ml. After depletion the pigs were assigned at random to one of the following seven repletion diets: 1) basal diet; 2) basal diet supplemented with 3 levels of carotene from corn or corn gluten meal, or both, with carotene concentrations from 1.04 to 10.3 mg/kg of diet; 3) basal diet supplemented with 3 levels of all-trans-retinyl palmitate, with retinyl palmitate concentrations from 73 to 654 μg/kg diet. Repletion diets were fed until the pigs reached market weight. Serum vitamin A determinations were made initially and periodically throughout the trials. Liver vitamin A stores were monitored by collecting livers from three or four pigs from each of the three trials at the start and end of depletion, and from all pigs at the end of repletion. The carotenes in corn and corn gluten meal were separated by column chromatography, identified as to structural and stereoisomeric form, and quantitated spectrophotometrically. Estimates of vitamin A potency were based on total liver vitamin A stores which the carotenes supported during repletion. Estimating equations were calculated by regressing the logarithm of total vitamin stores in milligrams/liver on total milligrams of carotene or retinyl palmitate consumed during repletion. Conversion efficiencies were calculated by dividing the regression coefficients for carotene by the regression coefficients for retinyl palmitate. At higher levels of carotene intake, when corn gluten meal was included in the repletion diets, 1 mg of carotene had a vitamin A potency of 123 to 174 IU. When corn was the only source of carotenes and the concentration was more typical of corn-soy swine diets, 1 mg of carotene had a vitamin A potency of 261 IU. It would appear that the NRC relationship, between 1 mg of dietary carotene and 500 IU of vitamin A activity, exaggerates the usefulness of corn carotenes for swine when liver vitamin A storage is used as the criterion.