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Vitamins and herbal preparations, particularly those sold in health food stores, are considered by many to be innocuous, but several have potential as toxins when taken in excessive amounts over a period of time. Also, herbal preparations may contain toxic contaminants that can cause acute poisoning. This chapter covers the available vitamins and selected herbals; agents compounded with vitamins, such as iron, are covered in other chapters.

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Hypervitaminosis from the fat-soluble vitamins A, D, and E can produce chronic toxicity (after weeks to months of excessive ingestion) or subacute toxicity (after days to a few weeks). Of the water-soluble vitamins, niacin, pyridoxine, and ascorbate are associated with toxicity (Table 199-1).

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Table 199-1 Symptoms of Hypervitaminosis 
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Dietary vitamin A from animal sources is primarily in the form of an ester, retinyl palmitate. After ingestion, the ester form is hydrolyzed in the GI tract to retinol (vitamin A1 alcohol). Retinol is then absorbed into intestinal mucosal cells where it combines with a fatty acid to again become a retinyl ester. The retinyl ester then travels through the lymphatic system and blood stream to storage sites in the liver. Carotenoids are dark colored compounds found in plants, about 10% of which, principally β-carotene, the human body can metabolize into retinol. The liver contains approximately 95% of the vitamin A stores of the entire body.

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Vitamin A forms part of the visual pigments of the retina (rhodopsin and iodopsin), is important for the formation of mucus-secreting cells ...

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