Overview

Vitamin A is a group of unsaturated nutritional organic compounds, that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids, among which beta-carotene is the most important. Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule  necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.

Inadequate Intake and Deficiencies

Vitamin A deficiency can occur as either a primary or a secondary deficiency. A primary vitamin A deficiency occurs among children and adults who do not consume an adequate intake of provitamin A carotenoids from fruits and vegetables or preformed vitamin A from animal and dairy products. Early weaning from breastmilk can also increase the risk of vitamin A deficiency.

Secondary vitamin A deficiency is associated with chronic malabsorption of lipids, impaired bile production and release, and chronic exposure to oxidants, such as cigarette smoke, and chronic alcoholism. Vitamin A is a fat soluble vitamin and depends on micellar solubilization for dispersion into the small intestine, which results in poor use of vitamin A from low-fat diets. Zinc deficiency can also impair absorption, transport, and metabolism of vitamin A because it is essential for the synthesis of the vitamin A transport proteins and as the cofactor in conversion of retinol to retinal. In malnourished populations, common low intakes of vitamin A and zinc increase the severity of vitamin A deficiency and lead physiological signs and symptoms of deficiency.

Excessive Intake

Since vitamin A is fat-soluble, disposing of any excesses taken in through diet takes much longer than with water-soluble B vitamins and vitamin C. This allows for toxic levels of vitamin A to accumulate.

Excessive vitamin A consumption can lead to nausea, irritability, anorexia (reduced appetite), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain and weakness, drowsiness, and altered mental status. In chronic cases, hair loss, dry skin, 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.

Best Food Sources

Vitamin A is found naturally in many foods including carrots, sweet potatoes, cantaloupe and other orange foods, all of which get their hue from the carotene pigment.

Vitamin A Sources

 

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  Sweet Potatoes
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  Carrots
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  Spinach
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  Kale
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  Mustard Greens

 

 

References

• Fennema, Owen (2008). Fennema's Food Chemistry. CRC Press Taylor & Francis. pp. 454–455. ISBN 9780849392726.
• Tanumihardjo SA (2011). "Vitamin A: biomarkers of nutrition for development". The American Journal of Clinical Nutrition 94 (2): 658S–665S. doi:10.3945/ajcn.110.005777. PMC 3142734. PMID 21715511.
• Wolf G (2001). "The discovery of the visual function of vitamin A". The Journal of Nutrition 131 (6): 1647–1650. PMID 11385047.
• "Vitamin A". News Medical. "What is Vitamin A?". Retrieved 1 May 2012.
• Combs, Gerald F. (2008). The Vitamins: Fundamental Aspects in Nutrition and Health (3rd ed.). Burlington: Elsevier Academic Press. ISBN 978-0-12-183493-7.
• Eledrisi, Mohsen S. "Vitamin A Toxicity". eMedicine.