Vitamin A Introduction

Along with vitamins D, E, and K, vitamin A is classified as a fat-soluble vitamin. One half of the body’s available supply of Vitamin A is preformed, meaning that it is biologically ready for use in the body. Preformed Vitamin A occurs only in animal based foods (i.e. organ meats, eggs, and meat products). Biologically active forms of Vitamin A include retinol (alcohol and aldehyde), and retinal esters (retinal acetate and palmitate). The many fortified foods and vitamin supplements consumed by Americans today consist of the preformed Vitamin A, usually the retinal ester based form of the vitamin.

Carotenoids, precursors to vitamin A, are found in many of the most popularly consumed plant sources within the human diet. Carotenoids constitute approximately 1/2 of our daily dietary intake of Vitamin A. A key in determining the given availability of Vitamin A in a given fruit or vegetable is the color it provides. The more vivid and deeper colors of produce, (orange, yellow, and green), the higher the Vitamin A value that particular fruit or vegetable provides. Beta Carotene is the most active and predominant form in fruits and vegetables.

Selected animal sources of Vitamin A When compared to animal based foods, plant sources do not provide the highest bioavailability of Vitamin A. However, both fruit and vegetable sources remain equally important for helping to achieve the recommended daily allowances of Vitamin A.

Selected plant sources of Vitamin A

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Vitamin A is a key nutrient responsible for eye health, bone growth and remodeling, maintenance and health of epithelial cells. The characteristics of vitamin A enable it to create an effective barrier to infection, thereby boosting overall immunity as well. [2]

Vitamin A assists in the vision cycle and our ability to adapt to darkness. It also provides in the proper functioning of the tear glands of the eye, providing for the lubrication of one’s corneas. As mentioned, bone growth, reproduction, and remodeling of bone tissue are also an important roles fulfilled and sustained by Vitamin A. Therefore, having adequate amounts available in the body is a necessity. Furthermore, calcium, magnesium, and Vitamin D also provide nourishment for particular maturation processes involving vitamin A. [3]

Epithelial cells are those found in two critical areas of our bodies: 1 - the outer protective layers of our skin, 2 - the linings of all body openings. Vitamin A maintains epithelial cell health by assisting in the continual secretion of mucous membranes. [4] Body openings include; gastro-intestinal, genitor-urinary, respiratory tracts, and also the eyes. Defense against various infections, viruses, and diseases ceases when these mucous membranes dry.

Other popular uses for Vitamin intake and supplementation include, but are not limited to:

• Acne (high dosages for extended periods of time [5])
• Alzheimer's Disease
• Breast Cancer
• Burns
• Cataracts
• Crohns' Disease (lessens severity and complications [6])
• Food Poisoning
• HIV and AIDS (may help increase life expectancy for patients with vitamin A deficiencies [7])
• Intestinal Parasites
• Measles
• Osteoarthritis and Osteoporosis
• Psoriasis
• Skin Cancer
• Tuberculosis
• Warts
• Wounds

Recommended Daily Allowances (RDA) defined:

[1]

Vitamin A Toxicities

The excess ingestion and poisonings caused by Vitamin A are very rare. Supplements containing Vitamin A should be taken as directed and never in excess, unless it is recommended one’s personal physician. Toxicity has been documented in adults when ingestion is 50,000IUs or greater, for a duration of seven to ten days (or longer). However, in young children and infants, toxicity can begin with equivalents of 20,000ius or greater. This poisoning from Vitamin A (or any other supplementary vitamin) is defined as hypervitaminosis. Additionally, the over digestion of Vitamin A’s carotenes may cause yellow discolorations of skin. Symptoms of this excess in carotenoids will dissipate as one reduces his/her intake of the vitamin.

Are you Vitamin A deficient?

Deficiency state of Vitamin A will manifest as irregular eye function, and processes concerning epithelial tissue. Signs of severe deficiency include; rough skin, diarrhea, malabsorption (key nutrients, vitamins, and minerals) kidney stones, and night blindness. [8]

1. Clinical Nutrition Service, Warren Grant Magnuson Clinical Center, National Institutes of Health (NIH), NIH. Vitamin E December. 9. 2002. http://www.cc.nih.gov/ccc/supplements/

2. Hof H. Vitamin A: The ‘Anti-infective’ Vitamin? MMW Munch Med Wochenschr. Nov1976;118(46):1485-88.

3. Advani S, Wimalawansa SJ. Bones and nutrition: common sense supplementation for osteoporosis. Curr Womens Health Rep. 2003 Jun;3(3):187-92.

4. Basu TK. Vitamin A and Cancer of Epithelial Origin. J Hum Nutr. Feb1979;33(1):24-31.

5. Kligman AM, et al. Oral Vitamin A in Acne Vulgaris. Preliminary Report. Int J Dermatol. May1981;20(4):278-85.

6. Dvorak AM. Vitamin A in Crohn’s Disease. Lancet. Jun1980;1(8181):1303-04.

7. Semba RD, et al. Increased Mortality Associated with Vitamin A Deficiency during Human Immunodeficiency Virus Type 1 Infection. Arch Intern Med. Sep1993;153(18):2149-54.

8. Herbert, Victor. “Vitamins and Minerals Plus Antioxidant Supplements” Total Nutrition Ed. Victor Herbert, M.D., Genell J. Subak-Sharpe, M.S. New York: Saint Martin’s Griffin, 1995. 94-118.