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retinyl esters are hydrolyzed; the products are associated first with lipid globules and then with bile salt-containing mixed micelles in the upper part of the small intestine. These mixed micelles contain carotenoids as well as retinol. However, absorption of retinol and carotenoids, especially ß-carotene, differs in several ways. For example, in physiological amounts, retinol is more efficiently absorbed than are most carotenoids, e.g., 70 to 90% compared to 20 to 50% (Bauernfeind, 1972; Reddy and Sivakumar, 1972). However, carotenoids present in oils are well absorbed (Rao and Rao, 1970). As the amount ingested increases, the efficiency of retinol absorption usually remains high (60 to 80%), whereas carotenoid absorption falls markedly to levels as lowas 10% or less (Bauernfeind, 1972; Olson, 1972).

Absorbed retinol is largely esterified in intestinal mucosal cells and incorporated into chylomicrons, as is the portion of absorbed ß-carotene and other biologically active carotenoids that is not cleaved in intestinal cells. Most absorbed ß-carotene normally is converted to retinol (and then to retinyl esters) in mucosal cells. The retinyl esters and carotenoids are taken up from the blood with chylomicron remnants, mainly in the liver by hepatocytes (Blomhoff et al., 1982; Goodman and Blaner, 1984). Studies in animals have shown that when liver reserves of vitamin A are adequate, much of the newly absorbed retinol is transferred from hepatocytes to stellate cells of the liver and stored as retinyl esters (Blomhoff et al., 1982, 1985). In well-nourished individuals, the storage efficiency of ingested vitamin A in the liver is more than 50% (Sauberlich et al., 1974), and the liver contains =90% of the total body stores of the vitamin (Underwood, 1984). In vitamin A-depleted rats, liver stores are reduced and the kidneys and other tissues contain an appreciable percentage (10 to 50%) of the small amount of total body reserve. In humans, carotenoids are deposited more widely, including localization in adipose tissues and adrenals; relatively small amounts are found in the liver (Raica et al., 1972).

Retinol circulates in the blood as a 1: 1:1 trimolecular complex with retinol-binding protein (RBP) and transthyretin (TTR) (Goodman, 1984a). RBP is released from the liver in combination with retinol, and the holo-RBP complex combines with TTR in the blood. Subsequently, retinol is slowly metabolized in the liver to numerous products, some of which are conjugated with glucuronic acid or taurine, and eliminated in the bile (Sporn et al., 1984). Of the total retinol metabolized, approximately 70% of the metabolic products appear in the feces and 30% are excreted in the urine. Almost all these excreted products are biologically inactive metabolites. For a more detailed review of retinol metabolism, see Goodman and Blaner (1984).