Fundamentals of Human Nutrition/Vitamin A< Fundamentals of Human Nutrition
7.1 Vitamin AEdit
Vitamin A can come from plant or animal sources. Plant sources include colorful fruits and vegetables. Animal sources include liver and whole milk. Vitamin A is also added to foods like cereals. This vitamin is fat-soluble vitamin (like D, E and K, too).
Vitamin A regulates the growth and differentiation of many cells and tissues. It primarily affects the mucous membranes of the respiratory tract. Prolonged Vitamin A deficiency results in metaplasia of the respiratory epithelium, a pre cancer condition. Also controls the morphogenesis of many tissues during embryonic development.
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Vegetarians, young children, and alcoholics may need extra Vitamin A. If you have certain conditions, such as liver diseases, cystic fibrosis, and Crohn's disease, you might also need more vitamin A.
Vitamin A plays an essential role in the body's immune system health, bone growth, and reproduction. Without proper intake of Vitamin A, an individual would be unable to transcript cells associated with cell differentiation and this would lead to reduced resistance to disease and growth retardation. Therefore, it is vital that humans are receiving appropriate amounts of vitamin A per day. According to the National Institutes of Health (2013), individuals over the age of fourteen need between 700 and 900 micrograms of retinal activity equivalents per day. Consequently if those needs are not met, vitamin A deficiency may result from dietary deprivation, liver disorders, or fat malabsorption. In recent studies, Semba (2008) found that vitamin A deficiency is affecting approximately 125-130 million children and roughly 7 million pregnant women in developing countries. It was also discovered that, primarily, vitamin A deficiency is found in low income countries due to prolonged dietary deprivation. Furthermore, it is commonly found in eastern and southern Asia, because rice, a staple food, lacks nutritional β-carotene. Other causes of vitamin A deficiency have been linked to celiac disease, pancreatic insufficiency, cystic fibrosis, and bile duct obstruction (Johnson YEAR).
As discussed previously, vitamin A intake is essential for many parts of the body, but mostly vision because it greatly affects the anatomy of the cornea. The component of rhodopsin, a purplish-red light-sensitive pigment found in the retina of human eyes in rod cells helps convert light impulses to nerve impulses. Therefore, if you are consuming low quantities of vitamin A, you will have a low quantity of cis retinal. With prolonged vitamin A deficiency, rod cells will no longer be able to react to dim light, eventually leading to Xerophthalmia, or abnormal dryness of the eye. Xerophthalmia may result from keratinization of the eyes caused by the thickening and drying of the conjunctivae and corneas (Johnson, 2014). In extreme cases of this deficiency, the cornea develops erosions that lead to blindness (Johnson, 2014). Vitamin A deficiency is a serious cause of preventable childhood blindness and mortality among preschool-aged children in developing countries. According to the International Agency for the Prevention of Blindness (IAPB) (2015) just by improving a child’s vitamin A intake, you can decrease the risk of mortality by 24%. In order to prevent this deficiency an individual, young or old, should consume dark leafy greens, carrots, fish, and sweet potatoes. Through prevention by annual supplementation of vitamin A, the IAPB (2015) aims to stop childhood blindness caused by vitamin A deficiency.
Nevertheless, too much of anything, even vitamins, can cause issues. Excess amounts of vitamin A can accumulate in the body because it is fat soluble and this is known as hypervitaminosis A. According to the National Institutes of Health (2013), persisting symptoms of hypervitaminosis A include dizziness, nausea, skin irritation, joint pains, intracranial pressure, coma and possibly death. These symptoms are often caused from therapeutic retinoids or supplements often given out in low-income countries. After consuming too much vitamin A, tissue levels take a significant amount of time to decrease after discontinuing their intake, often causing irreversible liver damage. Even some total intakes of vitamin A seen in some medications like Accutane may cause congenital birth defects. The birth defects caused by the intakes of vitamin A in Accutane usually consisted of deformities of the eyes, skull, heat, and lungs.