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.
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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"4 Vitamin A."
Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc.
Washington, DC: The National Academies Press, 2001.
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