Retinol is one of the animal-based forms of vitamin A. Retinol is essential for night-vision, strong immunity, healthy skin and normal fetal development. Retinol is fat-soluble and stored within the body long-term, which makes deficiency symptoms relatively uncommon in developed countries. However, in Third World countries, vitamin A deficiency is common, causing up to 500,000 children to go blind each year.
Forms of Vitamin A
There are two animal-based sources of vitamin A that are immediately available to the body, namely retinol and retinaldehyde. Carotenoids, such as beta-carotene, are converted to retinol and are called precursors or provitamins. Retinol is found in animal livers, meats, eggs, fatty fish and oils, whereas carotenoids are found in fruits and vegetables containing yellow, orange and dark green pigments. Retinol is converted to retinal in the liver and stored there, which helps reduce rates of deficiency. Amounts of vitamin A are measured in Retinal Equivalents, with one RE equal to 0.001 mg of retinal, 0.006 mg of beta-carotene and 3.3 IU of vitamin A. However, the rate at which beta-carotene is converted to retinol has been revised considerably, changing from a 6:1 ratio to a 21:1 ratio based on studies carried out in developing nations, as cited in "Contemporary Nutrition." Thus, it seems much more beta-carotene is needed to yield retinol, which means many more people are vitamin A deficient than previously thought.
Recommended Amounts of Retinol
The National Institutes of Health states the recommended daily amounts of retinol range from 0.3 mg for infants, to 0.9 mg for men, to 1.3 mg for lactating women. Put another way, an adult male needs 900 mcg per day, or about 3,000 IU.
Roles of Retinol
Retinol is converted to retinal and becomes essential for acute vision, especially at night. Retinol is also converted to retinoic acid and becomes essential for healthy, supple skin and normal bone and tooth development, as cited in "Vitamins: Fundamental Aspects in Nutrition and Health." Further, retinol contributes to sexual reproduction and the maintenance of mucous membranes that line the eyes, sinuses, lungs, mouth and digestive tract. Retinol is also a powerful antioxidant, which is able to eliminate harmful free-radicals that contribute to blood vessel damage and tissue deterioration.
An early symptom of retinol deficiency is impaired vision in low-light conditions, or night blindness. Retinol is necessary for the production of rhodopsin, a light-sensitive pigment required by the retina to detect movement and images in the dark. Without enough rhodopsin, the retina is not able to register nearly as many shades of gray.
Dry Eye and Total Blindness
The membranes of the eye depend on retinol for moisture and lubrication, which is necessary for blinking. Chronic retinol deficiency leads to xeropthalmia, or dry eye, which causes scarring and ulceration of the cornea and, ultimately, total blindness.
Other Deficiency Symptoms
Chronic retinol deficiency also causes reduced immune response, higher rates of infection, iron metabolism abnormalities, reduced human growth hormone production and skin conditions, such as acne, eczema, xerosis and dandruff.
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