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et al., 1985). Furthermore, the growth, differentiation, and activation of B lymphocytes requires retinol (Blomhoff et al., 1992).

Proposed functions of provitamin A carotenoids are described in Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids (IOM, 2000).

Physiology of Absorption, Metabolism, and Excretion

Absorption and Bioconversion

Absorption of Vitamin A. Intestinal absorption of preformed vitamin A occurs following the processing of retinyl esters in the lumen of the small intestine. Within the water-miscible micelles formed from bile salts, solubilized retinyl esters as well as triglycerides are hydrolyzed to retinol and products of lipolysis by various hydrolases (Harrison, 1993). A small percentage of dietary retinoids is converted to retinoic acid in the intestinal cell. In addition, the intestine actively synthesizes retinoyl β-glucuronide that is hydrolyzed to retinoic acid by β-glucuronidases (Barua and Olson, 1989). The efficiency of absorption of preformed vitamin A is generally high, in the range of 70 to 90 percent (Sivakumar and Reddy, 1972). A specific retinol transport protein within the brush border of the enterocyte facilitates retinol uptake by the mucosal cells (Dew and Ong, 1994). At physiological concentrations, retinol absorption is carrier mediated and saturable, whereas at high pharmacological doses, the absorption of retinol is nonsaturable (Hollander and Muralidhara, 1977). As the amount of ingested preformed vitamin A increases, its absorbability remains high (Olson, 1972). Vitamin A absorption and intestinal retinol esterification are not markedly different in the elderly compared to young adults, although hepatic uptake of newly absorbed vitamin A in the form of retinyl ester is slower in the elderly (Borel et al., 1998).

Absorption and Bioconversion of Provitamin A Carotenoids. Carotenoids are also solubilized into micelles in the intestinal lumen from which they are absorbed into duodenal mucosal cells by a passive diffusion mechanism. Percent absorption of a single dose of 45 μg to 39 mg β-carotene, measured by means of isotopic methods, has been reported to range from 9 to 22 percent (Blomstrand and Werner, 1967; Goodman et al., 1966; Novotny et al., 1995). However, the absorption efficiency decreases as the amount of dietary carotenoids increases (Brubacher and Weiser, 1985; Tang et al., 2000). The relative carotene concentration in micelles can vary in response to

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