Abstract
It was observed that rats fed a low dietary level of retinoic acid had markedly lower plasma concentrations of alpha-tocopherol than did rats fed the same amount of retinol. In this report, the possible mechanisms by which retinoic acid alters vitamin E metabolism has been investigated. Weanling male rats were fed a complete purified diet with either retinol or retinoic acid at 4 mg/kg diet; plasma and tissues were analyzed after 2-5 weeks. The plasma alpha-tocopherol concentration in rats ingesting retinoic acid was one-half that of rats ingesting retinol, and this difference also occurred in the liver and adipose tissue. Similar effects occurred in chicks. This low dietary level of retinoic acid had no effect on plasma triglyceride concentration, as has been reported for higher intakes, and plasma cholesterol and total lipids were also unaffected. Retinoic acid did not affect the rate of decrease in endogenous alpha-tocopherol in normal rats fed a vitamin E-free diet for 3 weeks. In rats with mesenteric lymph cannulas, dietary retinoic acid caused a reduced absorption of 3H-labeled alpha-tocopherol. In chicks fed retinoic acid, plasma and liver radioactivity 2.5 hours after an oral dose of 3H-alpha-tocopherol was one-fifth that of chicks fed retinol. More oxidation of alpha-tocopherol occurred during absorption in rats fed retinoic acid than in those fed retinol, as evidenced by more alpha-tocopherylquinone in the collected lymph. We postulate that dietary retinoic acid reduces the intestinal absorption of alpha-tocopherol and may also promote its oxidation.
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