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Chapter: Introduction to Human Nutrition: The Vitamins

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Interactions of vitamin A with drugs and other nutrients

Historically, there was considerable confusion between vitamins A and D, and for many years it was not clear which acted in which system.

Interactions of vitamin A with drugs and other nutrients

Historically, there was considerable confusion between vitamins A and D, and for many years it was not clear which acted in which system. By the 1950s it was believed that the problem had been solved, with clearly defined functions of vitamin A in vision, and vitamin D in calcium homeostasis and bone develop-ment. However, both have overlapping effects on a number of systems, including bone metabolism and immune system function. It is now known that this is the result of formation of retinoid–vitamin D recep-tor heterodimers, so that in some systems both are required in appropriate amounts for normal regula-tion of gene expression.

Chlorinated hydrocarbons, as contained in agricul-tural pesticides, deplete liver retinol. Metabolites of polychlorinated biphenyls bind to the thyroxine binding site of transthyretin, and in doing so impair the binding of RBP. As a result there is free RBP-bound retinol in plasma, which is filtered at the glom-erulus and hence lost in the urine. Habitual use of barbiturates may also lead to deficiency as a result of induction of cytochrome P450, which catalyzes the catabolism of retinol.



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