Graduate Thesis Or Dissertation
 

Hypervitaminosis A : effects on reproduction and interactions with pyrrolizidine alkaloids

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  • In the first experiment the minimum dietary toxic levels of vitamin A were determined in pregnant rabbits over three parities. The treatments consisted of basal diet plus 10,000, 30,000, 60,000, and 90,000 IU/Kg added vitamin A. The basal diet was formulated to be free of vitamin A and beta-carotene; rice hulls were used in place of alfalfa meal as the dietary fiber source. It was found that during storage, considerable destruction of vitamin A occured, probably stimulated by lipoxidases in the rice hulls. As a result, the 10,000 IU/Kg diet became deficient and the rabbits on this diet showed vitamin A deficiency symptoms such as fetal resorptions and fetal hydrocephalus. Rabbits on the 90,000 IU/Kg diet exhibited symptoms associated with toxicity. The diets with intermediate amounts of vitamin A supported relatively normal reproduction, and the does on these diets were healthier in terms of incidence of respiratory and enteric disease. Of these two diets the 30,000 IU/Kg yielded the highest total grams of kits weaned over three parities. The liver vitamin A reserves of the female rabbits increased with increasing dietary vitamin A levels; however, the plasma concentrations did not follow the same pattern. Plasma vitamin A concentrations increased significantly (P<.05) as the dietary levels increased from 10,000 to 30,000 IU/Kg. When these levels increased from the latter to 60,000 IU/Kg, the average plasma vitamin A levels increased, but the difference was not significant (P<.05). It was found that the plasma vitamin A concentrations dropped markedly in animals fed the 90,000 IU/Kg diet and became significantly (P<.05) lower than in those fed the 60,000 IU/Kg diet but not significantly (P<.05) different from the vitamin A deficient plasma samples. It was concluded that in rabbits vitamin A at high concentrations inhibits its own secretion from the liver into the circulation. The symptoms, although associated with low plasma vitamin A levels, are attributed to hypervitaminosis A. This might explain why the symptoms of hypo- and hyper-vitaminosis A in rabbits are similar. This condition could then be termed "Secondary hypovitaminosis A" since it is not caused by low vitamin A intake and the liver contains high amounts of the vitamin.In the second experiment the interactions between Sececio pyrrolizidine alkaloids (PA) present in Senecio jacobaea or tansy ragwort (TR) and vitamin A were investigated in rats. It was found that PA present in a 5% TR diet reduced both plasma and liver vitamin A levels significantly compared to the control (P<.05). Increasing the amount of TR in the diet from 5 to 10% did not make a significant change (P<.05); suggesting that 5% dietary TR was sufficient for this type of study. It was concluded that the reduction in hepatic and plasma vitamin A was due to either higher catabolism or lower intestinal absorption rate. In the third experiment the possibility of impaired fat and fat-soluble vitamin absorption was considered. The PA cause biliary hyperplasia and bile duct obstruction, hence reducing bile secretions into the small intestine. Because of the necessity of bile acids and fat emulsification in the small intestine for vitamin A and fat absorption, PA induced liver damage could thus impair absorption of these nutrients. Dietary PAs reduced fat absorption in 5% TR fed rats (P<.05) compared to the control.
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