Vitamin A is a fat-soluble nutrient, and its derivatives have a number of already established functions, as well as new functions currently being discovered. The name vitamin A is synonymous with the alcohol form of the vitamin retinol. It is oxidized into retinal and further oxidized to RA which can exist as all- trans or several cis isomers. Besides the well-established role of retinal for vision, vitamin A in the form of RA exerts its eVects via nuclear receptors that act as transcription factors for changing gene expression.
Carotenoids are a family of compounds of which some can form vitamin A. One of the carotenoids, b-carotene, is converted into retinal and eventually retinol by an intestinal central cleavage enzyme. As the activity of this enzyme is very low in humans, very small amounts are converted into vitamin A. The conversion ratio is about 12:1 (b-carotene:retinol). An appropriate animal model for the study of b-carotene metabolism in humans is the ferret. Thus, if one wants to test the effect of vitamin A in humans, then retinol or its derivatives should be used as opposed to b-carotene. Orally supplemented, purified b-carotene cannot be used in place of the dietary retinoid form for human studies because of the potential deleterious side products that are formed with excess b-carotene. When isolated b-carotene is ingested in excess by ferrets, abnormal cleavage products are formed, along with the typical intestinal central cleavage enzyme product, retinal, a vitamin A metabolite (Wang et al., 1999). The atypical acentric products that are also formed have been found to interfere with normal retinol metabolism and potentially cause a functional deficiency. In the ferret model, cigarette smoking increased the production of these abnormal metabolites and decreased retinoid signaling.
There are many functions of vitamin A. Along with vision, vitamin A is important for reproduction, immune function, growth, bone development, cellular differentiation, proliferation, and cell signaling. Vitamin A is particularly important for normal epithelial cell (surface cell) function and integrity. These epithelial cells affected by vitamin A include the intestinal cells, skin cells, urogenital cells, and lung cells. Within the last 20 years, there has been an increased interest in vitamin A, as discoveries have highlighted the importance of vitamin A in maintenance of normal lung function and prevention of injury.
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