Tolerance of young pigs for dietary vitamin A and μ-carotene, with special reference to the immune response

Abstract

Guidelines need to be drawn up for the fortification of diets with vitamin A, because of its known toxicity. In exp. 1, 35 pigs aged 5 wk and weighing about 10 kg were placed in individual cages and allocated to seven dietary treatments for 28 d. In treatments 1–6 the levels of vitamin A or β-carotene represented 1, 10 or 100 times the National Research Council (1988) requirement for vitamin A, and in treatment 7 vitamin A and β-carotene were combined 50:50 to provide a total representing 100 times the vitamin A requirement. The vitamin A biopotency of β-carotene was assumed to be 200 IU mg−1. There was no significant effect of vitamin A or β-carotene on growth performance, plasma Escherichia coli antibody, or on lymphocyte proliferation. Serum retinol at 14 and 28 d increased as the level of dietary vitamin A increased but the effect was not significant (P = 0.241 and 0.073, respectively), β-carotene gave higher serum retinol at 14 and 28 d than vitamin A, but the effect was not significant (P = 0.232 and 0.059, respectively). Liver retinol increased with increasing level of dietary vitamin A or β-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 for 5 wk on diets containing graded levels of vitamin A representing 0, 1, 10, 100 and 200 times the requirement. Growth performance was not significantly 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 serum tocopherol was reduced significantly (P < 0.001) with increasing dietary vitamin A. The data fitted a cubic model. Liver retinol was increased significantly (P < 0.001) and liver tocopherol was reduced significantly (P < 0.001) with increasing dietary vitamin A. The data fitted a cubic model. These results show that increasing dietary vitamin A raises both the circulating and storage levels of retinol, and that the response may be predicted from the equations derived. The results also show that increasing dietary vitamin A has a significant depressing effect on vitamin E status. Taken in conjunction with previous findings 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 up to 10 times the requirement. Key words: imunity, pig, β-carotene, toxicity, vitamin A, vitamin E