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FIGURE 4-1 Structure of retinol and provitamin A carotenoids.

of their conjugated chain. An exception is lycopene, which has no ring structure and does not have vitamin A activity. Preformed vitamin A is found only in animal-derived food products, whereas dietary carotenoids are present primarily in oils, fruits, and vegetables.

Function

The 11-cis-retinaldehyde (retinal) form of vitamin A is required by the eye for the transduction of light into neural signals necessary for vision (Saari, 1994). The retinoic acid form is required to main-



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