Vitamin A is a fat-soluble vitamin that is necessary for the function of photoreceptors in the retina. It also helps keep the epithelium of the skin, lungs, intestine, and urinary tract healthy and protects against infections. (1) Vitamin A is also known as retinol. Vitamin A is necessary for both low-light (scotopic vision) and color vision. (2) Vitamin A is an antioxidant. (3)
Functions: In food, vitamin A typically occurs as a fat compound called retinyl palmitate. The body converts retinyl palmitate to retinol in the small intestine. The retinol functions as a storage form of the vitamin, and can be converted to its visually active form, retinal. The associated acid (retinoic acid), a metabolite that can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina for the visual cycle. (2,4)
Food Sources:While retinol, or preformed vitamin A, occurs only in foods of animal origin, fruits and vegetables that contain certain carotenoids also provide vitamin A activity. Carotenoids are plant pigments, responsible for the red, orange, and yellow color of fruits and vegetables. (4) Carotenoids are available as alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin. (2) Once consumed, carotenoids are slowly converted to vitamin A in the body. Carotenoids are best absorbed from cooked or homogenized vegetables served with some fat or oil. (1). However it has been found that vegetarian foods contain much less of vitamin A and night blindness can be common unless Vitamin A fortified food it given. (2)
Recommended daily allowance: 0-8 years = 4-8mcg/d, 9-18 years = 600-700mcg/d (Boys 14-18 years: 900mcg/d)
Deficiency: Vitamin A deficiency is estimated to affect approximately one third of children under the age of five around the world. (2) Vitamin A deficiency can occur due to poor intake of vitamin A rich foods, due to malabsorption or due to liver disease. Vitamin A deficiency impairs immunity, the vision and affects the epidermal growth. It leads to night blindness and xerosis of cornea and conjunctiva and even keratomalacia which may ultimately lead to permanent blindness. Keratinization of the skin and of the mucous membranes in the respiratory, GI, and urinary tracts can occur. Drying, scaling, and follicular thickening of the skin and respiratory infections can result. Serum vitamin A levels will be low. Normal range of serum retinol levels are 28 to 86 ?g/dL. (1) Treatment consists of vitamin A. Oral vitamin A is administered in a dose of 50,000, 1 lakh and 2 lakh international units (IU) in children aged <6 months, 6-12 months and 2 years respectively. The same dose is repeated next day and 4 weeks later. Parenteral doses are recommended in persistent vomiting and severe malabsorption.
Overdosage: Accidental ingestion of high dose of vitamin A can lead to headache and increased intracranial pressure. Long term ingestion of high dose of vitamin A (> 10 times the RDA) can lead to alopecia, visual impairment, dry itchy skin, weight loss, hypoplastic anemia and benign increased intracranial pressure. Pregnant women who require vitamin A supplementation should not given high-dose supplements due to potential teratogenic effects. Treatment consists of stopping vitamin A.
1. Vitamin A: Vitamins: Merck Manual Home edition. Available at URL: http://www.merck.com/mmhe/sec12/ch154/ch154f.html. Accessed on 25th October 2010
2. Vitamin A - Wikipedia, the free encyclopedia. Available at URL: http://en.wikipedia.org/wiki/Vitamin_a. Accessed on 25th October 2010.
3. Vitamin A - Medline Plus. Available at URL: www.nlm.nih.gov/medlineplus/vitamina.html. Accessed on 25th October 2010
4. WHFoods: Vitamin A. Available at URL: http://www.whfoods.com/genpage.php?tname=nutrient&dbid=106. Accessed on 25th October 2010
5. Vitamin E - Wikipedia, the free encyclopedia. Available at URL: http://en.wikipedia.org/wiki/Vitamin_e. Accessed on 25th October 2010
6. WHFoods: Vitamin E. Available at URL: http://www.whfoods.com/genpage.php?tname=nutrient&dbid=111. Accessed on 25th October 2010
7. Vitamin E: Vitamins: Merck Manual Professional Edition. Available at URL: http://www.merckmanuals.com/professional/sec01/ch004/ch004l.html. Accessed on 25th October 2010