Handbook of vitamins | Zempleni J., et al.

Vitamin A (retinol) is an essential micronutrient for all vertebrates. It is required for normal vision, reproduction,

(CLINICAL NUTRITION IN HEALTH AND DISEASE) Zempleni J., et al. (eds.) - Handbook of vitamins-CRC (2007)

Vitamin A (retinol) is an essential micronutrient for all vertebrates. It is required for normal vision, reproduction, embryonic development, cell and tissue differentiation, and immune function. Many aspects of the transport and metabolism of vitamin A, as well as its functions, are well conserved among species. Dietary vitamin A is ingested in two main forms preformed vitamin A (retinyl esters and retinol) and provitamin A carotenoids (b-carotene, a-carotene, and b-cryptoxanthin) although the proportion of vitamin A obtained from each of these form varies considerably among animal species and among individual human diets. These precursors serve as substrates for the biosynthesis of two essential metabolites of vitamin A: 11-cis-retinal, required for vision, and all-trans-retinoic acid, required for cell differentiation and the regulation of gene transcription in nearly all tissues.

Research on vitamin A now spans nine decades. Over 34,000 citations to vitamin A, 7,000 to b-carotene, and 20,000 to retinoic acid can be found in the National Library of Medicine’s PubMed database [1], covering topics related to nutrition, biochemistry, molecular and cell biology, physiology, toxicology, public health, and medical therapy. Besides the naturally occurring forms of vitamin A indicated earlier, numerous structural analogs have been synthesized. Some retinoids have become widely used as therapeutic agents, particularly in the treatment of dermatological diseases and certain cancers.

In this chapter, we focus first on vitamin A from a nutritional perspective, addressing its chemical forms and properties, the nutritional equivalency of compounds that provide vitamin A activity, and current dietary recommendations. We then cover the metabolism of carotenoids and vitamin A. Finally, we provide a brief discussion of the key uses of vitamin A and retinoids in public health and medicine, referring to their benefits as well as some of the adverse effects caused by ingesting excessive amounts of this highly potent group of compounds.

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