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Low (< 21 μg/d) vitamin A diet plus a single dose of supplemental vitamin A or β-carotene were provided to subjects after depletion period

Abnormal dark adaptation was reversed with 1,300 IU (390 μg)b of vitamin A and 2,500 IU (1,500 μg)c of β-carotene; thus the retinol equivalency ratio is assumed to be 3.8:1

Low vitamin A diet (< 23 μg) plus varying doses of supplemental vitamin A (37.5–25,000 μg/d) or β-carotene (150–2,400 μg/d) were provided after the depletion period

600 μg/d retinol corrected dark adaptation; 1,200 μg/d β-carotene corrected dark adaptation; therefore the retinol equivalency ratio was concluded to be 2:1

tion. Based on this finding, approximately 7 μg of dietary β-carotene is equivalent to 1 μg of β-carotene in oil. This absorption efficiency value of 14 percent is supported by the relative ranges in β-carotene absorption reported by others using similar methods for mixed green leafy vegetables (4 percent) (de Pee et al., 1995), carrots (18 to 26 percent) (Micozzi et al., 1992; Torronen et al., 1996), broccoli (11 to 12 percent) (Micozzi et al., 1992), and spinach (5 percent) (Castenmiller et al., 1999) (Table 4-2).

Only one study has been published to assess the relative bioconversion of β-carotene from fruits versus vegetables by measuring the rise in serum retinol concentration after the provision of a diet high in vegetables, fruits, or retinol (de Pee et al., 1998). This study used methods similar to those employed by other researchers (Castenmiller et al. [1999], de Pee et al. [1995], Micozzi et al. [1992], Torronen et al. [1996], and Van het Hof et al. [1999]), and indicated that the vitamin A activity was approximately half the activity for dark, green leafy vegetables compared to equal amounts of β-carotene from orange fruits and some yellow tubers, such as pumpkin squash (de Pee et al., 1998) (Table 4-2). Because of the low content of fruits contained in the principally mixed vegetable diet of Van het Hof et al. (1999) and the low proportion of dietary β-carotene that is consumed from fruits compared to vegetables in

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