TOLERANCE OF YOUNG-PIGS FOR DIETARY VITAMIN-A AND BETA-CAROTENE, WITHSPECIAL REFERENCE TO THE IMMUNE-RESPONSE

Guidelines need to be drawn up for the fortification of diets with vit amin A, because of its known toxicity. In exp. 1, 35 pigs aged 5 wk an d weighing about 10 kg were placed in individual cages and allocated t o seven dietary treatments for 28 d. In treatments 1-6 the levels of v itamin A or beta-carotene represented 1, 10 or 100 times the National Research Council (1988) requirement for vitamin A, and in treatment 7 vitamin A and beta-carotene were combined 50:50 to provide a total rep resenting 100 times the vitamin A requirement. The vitamin A biopotenc y of beta-carotene was assumed to be 200 IU mg(-1). There was no signi ficant effect of vitamin A or beta-carotene on growth performance, pla sma Escherichia coli antibody, or on lymphocyte proliferation. Serum r etinol at 14 and 28 d increased as the level of dietary vitamin A incr eased but the effect was not significant (P = 0.241 and 0.073, respect ively). beta-carotene save higher serum retinol at 14 and 28 d than vi tamin A, but the effect was not significant (P = 0.232 and 0.059, resp ectively). Liver retinol increased with increasing level of dietary vi tamin A or beta-carotene (P < 0.001). In exp. 2, 30 pigs aged 5 wk and of 10 kg initial weight were penned in groups of three, injected i.m. and 3 wk later with 2 mL of a commercial TGE vaccine, and were fed fo r 5 wk on diets containing graded levels of vitamin A representing 0, 1, 10, 100 and 200 times the requirement. Growth performance was not s ignificantly affected by treatment, nor was the morphology of tissues integral to the immune system. Anti-TGE titre increased markedly after the second injection of vaccine, but was not affected at any stage by treatment. Serum retinol increased significantly (P < 0.002) and seru m tocopherol was reduced significantly (P < 0.001) with increasing die tary vitamin A. The data fitted a cubic model. Liver retinol was incre ased significantly (P < 0.001) and liver tocopherol was reduced signif icantly (P < 0.001) with increasing dietary vitamin A. The data fitted a cubic model. These results show that increasing dietary vitamin A r aises both the circulating and storage levels of retinol, and that the response may be predicted from the equations derived. The results als o show that increasing dietary vitamin A has a significant depressing effect on vitamin E status. Taken in conjunction with previous finding s from our laboratory, the results suggest that the tolerable dietary range of vitamin A for the young pig in the weight range 10-30 kg is u p to 10 times the requirement.