the physical state of the carotenoid (e.g., whether it is dissolved in oil or associated with plant matrix materials). A number of factors affect the bioavailability and bioconversion of carotenoids (Castenmiller and West, 1998). Carotene bioavailability can differ with different processing methods of the same foods and among different foods containing similar levels of carotenoids (Boileau et al., 1999; Hume and Krebs, 1949; Rock et al., 1998; Torronen et al., 1996; Van den Berg and van Vliet, 1998) (also see Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids [IOM, 2000]).

Absorbed β-carotene is principally converted to vitamin A by the enzyme β-carotene-15, 15′-dioxygenase within intestinal absorptive cells. The central cleavage of β-carotene by this enzyme will, in theory, result in two molecules of retinal. β-Carotene can also be cleaved eccentrically to yield β-apocarotenals that can be further degraded to retinal or retinoic acid (Krinsky et al., 1993). The predominant form of vitamin A in human lymph, whether originating from ingested vitamin A or provitamin A carotenoids, is retinyl ester (retinol esterified with long-chain fatty acids, typically palmitate and stearate) (Blomstrand and Werner, 1967; Goodman et al., 1966). Along with exogenous lipids, the newly synthesized retinyl esters and nonhydrolyzed carotenoids are transported from the intestine to the liver in chylomicrons and chylomicron remnants. Derived from dietary retinoids, retinoic acid is absorbed via the portal system bound to albumin (Blaner and Olson, 1994; Olson, 1991).

Vitamin A Activity of Provitamin A Carotenoids: Rationale for Developing Retinol Activity Equivalents. The carotene:retinol equivalency ratio (μg:μg) of a low dose (less than 2 mg) of purified β-carotene in oil is approximately 2:1 (i.e., 2 μg of β-carotene in oil yields 1 μg of retinol) (Table 4-1). This ratio was derived from the relative amount of β-carotene required to correct abnormal dark adaptation in vitamin A-deficient individuals (Hume and Krebs, 1949; Sauberlich et al., 1974). The data by Sauberlich et al. (1974) were given greater consideration because (1) the actual amount (μg) of vitamin A and β-carotene consumed was cited, (2) varied amounts of vitamin A or β-carotene were consumed by each individual, and (3) a greater sample size was employed (six versus two subjects). In addition to these studies, an earlier study by Wagner (1940) estimated a carotene:retinol equivalency ratio of 4:1; however, the method employed for measuring dark adaptation was not standardized and used an imprecise outcome measure.

Studies have been performed to compare the efficiency of absorption of β-carotene after feeding physiological amounts of β-carotene