Retinoids (vitamin A derivatives) have dual functions in physiology. 11-cis-Retinal serves as the universal chromophore of the visual pigments in the eye, and the hormonal retinoids, mainly all-trans- and 9-cis-retinoic acid (RA),2 regulate the expression of target genes via activation of two classes of nuclear retinoid receptors, the retinoic acid receptors (RARs), and the retinoid X receptors (RXRs) (1). The dietary sources of retinoids are the proforms of the vitamin, mainly esterified retinol, or the ultimate precursors for retinoids, various carotenoids.
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Lidén et al. (2006) studied this question.
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