Vitamin A is fat-soluble, which means that it is absorbed in the presence of fat and excess amounts are stored in fatty tissues. Vitamin A can be consumed as retinol, which is plentiful in the livers of animals and fish, or as beta-carotene, which is converted to retinol in the liver and stored there. Minimal requirements of vitamin A are established by health and government agencies for all ages, although some conditions and lifestyles may require more to prevent disease. Because vitamin A is stored in the body, high consumption can lead to toxicity.
Vitamin A Functions
Vitamin A is required for the formation and maintenance of moist mucous membranes that line the eyes, nose, mouth, sinuses, lungs and gastrointestinal system. Consequently, vitamin A aids the immune system in forming a physical barrier, which traps and eliminates debris, toxins and potential pathogens. Vitamin A is also an antioxidant, which scavenges free radicals that can cause inflammatory reactions and tissue aging. Further, vitamin A is essential for the synthesis of rhodopsin, a light-sensitive retinal pigment needed to detect shades of gray in low-light conditions.
Recommended Daily Amounts
According to the National Health Institutes, the recommended daily amounts of retinol range from 300 mcg for infants, to 900 mcg for adult males, to 1,300 mcg for lactating females. Good sources of retinol include pig, beef and fish livers, eggs, some cheeses and fortified dairy products, as cited in the "American Dietetic Association Complete Food and Nutrition Guide." For example, 100 g of pig liver contains 23,000 mcg of retinol, whereas the same amount of eggs contain 200 mcg. The body can synthesize retinol from carotenoids, such as beta-carotene, but about three times as much food is required. For example, fleshy orange vegetables are rich in beta-carotene, and 100 g of carrots contains about 8,000 mcg of it, but that is the equivalent to only 2,500 of retinol once it is converted.
Potential Higher Requirements
According to "Human Biochemistry and Disease," cigarette smoke that is inhaled into the lungs causes the long, sweeping cilia that prevent debris from depositing to become dysfunctional and paralyzed. The mucous producing goblet cells and lung cilia, which are within the membranes, are stimulated by vitamin A, which can trap cigarette toxins and protect smokers from throat and lung damage. Further, beta-carotene may play an important role in the prevention of age-related macular degeneration, cataracts and other blinding disorders, according to "Professional Guide to Diseases." Thus, smokers, people with eye disorders or people requiring excellent night vision may benefit from higher doses than ordinarily recommended.
Symptoms of Deficiency
An early symptom of vitamin A deficiency is impaired vision in low-light conditions, commonly called night blindness. Night blindness would be of special concern to people working night shifts or driving in the dark. Vitamin A deficiency also leads to xeropthalmia, or dry eye, which can eventually cause corneal injury, ulcers, scarring and eventual blindness. Reduced immune function, higher rates of infection and skin disorders, such as acne, eczema and boils, are other symptoms of vitamin A deficiency.
References
- National Health Institutes: Office of Dietary Supplements: Vitamin A and Carotenoids
- "American Dietetic Association Complete Food and Nutrition Guide"; American Dietetic Association; 2006
- "Human Biochemistry and Disease"; Gerald Litwack; 2008
- "Professional Guide to Diseases: Ninth Edition"; Springhouse Publishing; 2009
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