Information AboutVitamin A |
| CATEGORIES ABOUT VITAMIN A | |
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Vitamin A is an essential human nutrient. It exists not as a single compound, but in several forms. In foods of animal origin, the major form of vitamin A is an Alcohol ( Retinol ), but can also exist as an Aldehyde ( Retinal ), or as an acid ( Retinoic Acid ). Precursors to the vitamin (a Provitamin ) are present in foods of plant origin as some of the members of the Carotenoid family of compounds. Carolyn Berdanier. 1997. Advanced Nutrition Micronutrients. pp 22-39 All forms of Vitamin A have a Beta-ionone ring to which an Isoprenoid chain is attached. This structure is essential for vitamin activity.
DISCOVERY OF VITAMIN A The discovery of Vitamin A stemmed from research dating back to one of these substances was independently discovered by Elmer McCollum at the University Of Wisconsin-Madison , and Lafayette Mendel and Thomas Osborne at Yale University . Since "water-soluble factor B" ( Vitamin B ) had recently been discovered, the researchers chose the name "fat-soluble factor A". SOURCES Vitamin A is found naturally in many foods. Each of the following contains at least 0.15 mg of Vitamin A or beta carotene per 1.75-7 oz. (50-200 g):
REQUIRED DIETARY INTAKE Vitamin A US Dietary Reference Intake :
(Note that the limit refers to Retinoid forms of vitamin A. Carotene forms from Dietary sources are not Toxic . 2) SYMPTOMS OF DEFICIENCY Night blindness, corneal drying ( Xerosis ), triangular gray spots on eye ( Bitot's Spots ), corneal degeneration and blindness ( Xerophthalmia )3, impaired immunity, hypokeratosis (white lumps at hair follicles), softening of the cornea ( Keratomalacia ). SYMPTOMS OF OVERDOSE See Also: Hypervitaminosis A As vitamin A is fat-soluble, disposing of any excesses taken in through diet is a lot harder than with water-soluble vitamins B and C. As such, vitamin A toxicity can result. This can lead to nausea, jaundice, irritability, Anorexia (not to be confused with Anorexia Nervosa , the eating disorder), vomiting, blurry vision, headaches, muscle and abdominal pain and weakness, drowsiness and altered mentality. In chronic cases, hair loss, drying of the mucous membranes, fever, insomnia, fatigue, weight loss, bone fractures, anemia, and diarrhea can all be evident on top of the symptoms associated with less serious toxicity.http://www.emedicine.com/med/topic2382.htm BIOEFFICIENCY OF RETINOIDS AND CAROTENOIDS Vitamin A intake is often expressed in International Unit s (IU) or as retinol equivalents (RE), with 1 IU = 0.3 micrograms retinol. Because the production of retinol from provitamins by the human body is regulated by the amount of retinol available to the body, the conversions apply strictly only for Vitamin A deficient humans. The efficiency of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.NW Solomons, M Orozco. '' Alleviation of Vitamin A deficiency with palm fruit and its products ''. Asia Pac J Clin Nutr, 2003 SEE ALSO EXTERNAL LINKS REFERENCES |
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