Vitamin A

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Overview

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.[1]

All forms of Vitamin A have a Beta-ionone ring to which an isoprenoid chain is attached. This structure is essential for vitamin activity.[1]

  • retinol, the animal form of Vitamin A, is a yellow, fat-soluble, vitamin with importance in vision and bone growth.
  • other retinoids, a class of chemical compounds that are related chemically to Vitamin A, are used in medicine.[2]

Discovery of Vitamin A

The discovery of Vitamin A stemmed from research dating back to 1906, indicating that factors other than carbohydrates, proteins, and fats were necessary to keep cattle healthy.[3] By 1917 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".[3]

Sources

Vitamin A is found naturally in many foods. Each of the following contains at least 0.15 mg (which is equal to 150 micrograms (mcg). See Recommended Daily Intake below.) of Vitamin A or beta carotene per 1.75-7 oz. (50-200 g): butter, lemon, sweet potatoes, carrots, collard greens, milk, beetroot, pumpkin, spinach, beef, apple, winter squash, apricots, cantaloupe melon, mango, liver, (beef, pork, chicken, turkey, fish) eggs, broccoli, and leafy vegetables.

Recommended daily intake

Vitamin A US Dietary Reference Intake:

  • 900 micrograms for men
  • 700 for women.
  • Upper limit - 3,000 micrograms.

(Note that the limit refers to retinoid forms of vitamin A. Carotene forms from dietary sources are not toxic.[4])

Equivalencies of retinoids and carotenoids

Vitamin A intake is often expressed in international units (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 absorption of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.[5]

Substance and its chemical environment Micrograms of retinol equivalent per microgram of the substance
retinol 1
beta-carotene, dissolved in oil 1/2
beta-carotene, common dietary 1/12
alpha-carotene, common dietary 1/24
beta-cryptoxanthin, common dietary 1/24

Conversion of carotenoids into retinol relies on adequate intake of vitamin C, zinc and protein.

Symptoms of deficiency

Night blindness, corneal drying (xerosis), triangular gray spots on eye (Bitot's spots), corneal degeneration and blindness (xerophthalmia)[6], impaired immunity, hypokeratosis (white lumps at hair follicles), keratosis pilaris, softening of the cornea (keratomalacia).

Symptoms of overdose

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.[7]

See also

External links

References

  1. 1.0 1.1 Carolyn Berdanier. 1997. Advanced Nutrition Micronutrients. pp 22-39
  2. American Cancer Society: Retinoid Therapy
  3. 3.0 3.1 Wolf, George (2001-04-19). "Discovery of Vitamin A". Encyclopedia of Life Sciences. doi:10.1038/npg.els.0003419. Retrieved 2007-07-21. 
  4. "Sources of vitamin A". Retrieved 2007-08-27. 
  5. NW Solomons, M Orozco. Alleviation of Vitamin A deficiency with palm fruit and its products. Asia Pac J Clin Nutr, 2003
  6. Roncone DP (2006). "Xerophthalmia secondary to alcohol-induced malnutrition". Optometry (St. Louis, Mo.). 77 (3): 124–33. PMID 16513513. doi:10.1016/j.optm.2006.01.005. 
  7. http://www.emedicine.com/med/topic2382.htm


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