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Vitamins and herbal preparations, particularly those sold in health food stores, are considered by many to be innocuous but may have potential toxicity when taken in excessive amounts over a period of time. Also, herbal preparations may contain toxic contaminants that can cause acute poisoning.
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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 205-1).
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Dietary vitamin A is usually found in two forms: retinyl palmitate (an ester) from animal sources or carotenoids found in plants. After ingestion, the ester form is hydrolyzed in the gastrointestinal 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. Plants containing the carotenoids have β-carotene, which is the vegeTable compound most efficiently converted to retinol. The liver contains approximately 95% of body vitamin A stores.
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Vitamin A forms part of the visual pigments ...