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INTRODUCTION

Vitamins and herbal preparations, particularly those sold in health food stores, are considered by many to be innocuous but may have potential toxicity, especially when taken in excessive amounts over a period of time. Also, herbal preparations may contain toxic contaminants that can cause poisoning.

VITAMINS

Hypervitaminosis from the fat-soluble vitamins A, D, E, and K can produce subacute toxicity (after days to a few weeks) or chronic toxicity (after weeks to months of excessive ingestion). Of the water-soluble vitamins, niacin, pyridoxine, and ascorbate are associated with toxicity (Table 205-1).

TABLE 205-1Clinical Features of Hypervitaminosis

VITAMIN A

Dietary vitamin A is present in two forms: preformed vitamin A (retinol and retinyl palmitate [an ester] from animal sources) and provitamin A (carotenoids) found in plants. After ingestion, the ester form is hydrolyzed in the GI tract to retinol. Retinol is then absorbed into intestinal mucosal cells, where it then combines with a fatty acid to again become a retinyl ester. Carotenoids are dark-colored compounds found in plants. β-Carotene is the carotenoid most efficiently converted to retinol by duodenal mucosal cells. The recommended daily allowance (RDA) for preformed vitamin A (retinol) is 900 micrograms (3000 IU) for adolescents and adult men, and 700 micrograms (2400 IU) in ...

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