Vitamin A is available in dietary sources as either preformed vitamin A or pro-vitamin A carotenoids. Rich dietary sources of preformed vitamin A include egg yolk, liver, butter, cheese, whole milk and cod-liver oil. In many developing countries, the consumption of foods containing preformed vitamin A is limited, and
© CAB International 2002. Nutrition and Immune Function (eds P.C. Calder, C.J. Field and H.S. Gill)
pro-vitamin A carotenoids often comprise the major dietary source of vitamin A. The major pro-vitamin A carotenoids consist of a-carotene and p-carotene, found in such foods as dark green leafy vegetables, carrots, sweet potatoes, mangoes, and papayas and p-cryptoxanthin, found in oranges and tangerines.
Digested foods that contain preformed vitamin A are emulsified with bile salts and lipids in the small intestine. Retinol is esterified in the intestinal mucosa, packaged into chylomicrons and carried to the bloodstream via the lymphatic circulation. Pro-vitamin A carotenoids, such as p-carotene, may be converted to retinaldehyde through cleavage by carotenoid-15,15'-dioxyge-nase or by an asymmetrical cleavage pathway. The bioavailability of pro-vitamin A carotenoids is less than that of preformed vitamin A, due to a variety of factors, including differences in efficacy of absorption and biochemical conversion (De Pee et al., 1995; West, 2000). About 90% of the vitamin A in the body is stored in the liver as retinyl esters, and the liver has the capacity to store enough vitamin A to last for several months, with a larger storage capacity among adults than among children. Retinol is released from the liver in combination with plasma retinol-binding protein (RBP) and transthyretin (TTR). Retinol is poorly soluble in water and is carried in the blood sequestered inside the carrier proteins, RBP and TTR. Retinol seems to enter cells via specific receptors, although it is unclear whether all cells contain these receptors.
Vitamin A exerts its effects via retinoic acid and retinoid receptors, which are found in the nucleus of the cell. Retinol is converted to all-trans-retinoic acid and 9-cis-retinoic acid in the cytoplasm. Retinoic acid influences gene activation through specific receptors, which belong to the superfamily of thyroid and steroid receptors (Chambon, 1996). Retinoic acid receptors (RARs) act as transcriptional activators for many specific target genes. The RAR is expressed as several iso-forms (RAR a, p and 7), and the retinoid-x receptor (RXR) is also expressed as several isoforms (RXR a, p and 7) (Kliewer et al., 1992). All-trans-retinoic acid is a ligand for RARs whereas 9-cis-retinoic acid is a ligand for both RARs and RXRs. The DNA sequences that interact with RAR and RXR are known as retinoic acid response elements. RARs and RXRs form heterodimers, which bind to DNA and control gene expression. In addition, RXRs can also form heterodimers with the thyroid hormone receptor, vitamin D3 receptor, peroxisome proliferator activator receptors and a number of newly described 'orphan receptors'. Most retinoic acid response elements occur in the regulatory region of genes.
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