Fundamentals of Human Nutrition/Vitamin A< Fundamentals of Human Nutrition
7.1 Vitamin AEdit
Vitamin A can come from plant or animal sources. Animal sources include liver and whole milk. Vitamin A is also added to foods like cereals. This vitamin is fat-soluble vitamin.Vitamin A has an recommended daily allowance that differs for both men and women. For females it would be 700 grams and for males if would be 900 grams.Vitamin A is an essential nutrient and, as with all nutrients, the good health of women throughout childbearing years, including pregnancy, depends on consuming ppropriate amounts and that is why you would want to focus on eating foods with the Beta-Carotene in them. The Beta-Carotene will have your body convert to vitamin A on its own. In order for an individual to obtain this amount it is advise to look to foods with significant amounts of vitamin A you will find it in fortified milk. Depending on the brand will depend on the exact amount plus you can get your calcium in at the same time. If your not a milk drinker then look to other dairy products such as fortified margarine, butter, cheese and cream. If someone doesn’t eat dairy or has an intolerance to dairy you would want to point them in the direction of eggs or liver. All of these you can find with retinol but if you want to look for those who has Beta-Carotene then you can look at spinach and other dark green leafy vegetables. For a vegetarian they can incorporate broccoli as well as deep orange fruits such as apricots and cantaloupe. Some other vegetables can include Squash, carrots, Sweet Potatoes, and raw pumpkin.Vitamin A can be found in other foods as well but to gain the most nutritional content you will want to look at fruits and vegetables that will give other nutritional content. Some people may think that it is hard to incorporate vitamin a into there diet so buy explaining where to find significant sources they will have a better chance at obtaining the recombined daily amount. But there are other options if someone does not have the means of obtaining these fresh fruits and vegetables. Other options could include supplements such as a multivitamin. Incorporating a multivitamin would assist in also adding in meeting the recommended daily amount. With anything you need to keep in mind that there is a recommended amount and this is recommended but not limited to. You will also want to make sure not to over intake of Vitamin A because it could lead to having toxic results.
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.
Vitamin A, along with vitamins D, E, and K, are fat-soluble vitamins. Carotenoids (popularly as beta-carotene) found from plant food sources act as the precursor to vitamin A with the ability to be converted to active forms of vitamin A as a retinoid, which are commonly found in animal food sources. Collectively known as retinoids, the three different forms of active vitamin A used in the body are retinol, retinal, and retinoic acid, each serving a specialized function. Retinol and retinal can convert interchangeably, however, the formation of retinoic acid cannot be reversed or reconverted back to retinal and cannot be stored in the body. Retinol, the alcohol form, supports reproduction and functions as the major transport and storage form as well. Retinal, the aldehyde form, promotes vision health and is also the intermediate form in the conversion of retinol to retinoic acid. Retinoic acid, the acid form, behaves similarly to hormones by regulating cell differentiation, growth and embryonic development.
protein synthesis/cell differentiation
maintaining health of epithelial tissues
Vitamin A in Vision
Vitamin A is involved with the conversion of light energy into nerve impulses which takes place at the retina. There are four different kinds of pigments located within the photosensitive cells of the retina. One of these pigment molecules are called rhodopsin; each rhodopsin consisting of the protein opsin that is bonded to a molecule of cis-retinal. As light passes through the cornea and strikes the rod cells, the rhodopsin absorbs the light, opsin is released, and retinal changes from cis to trans. This change in configuration generates an electrical impulse conveying a message to the brain. The cycle repeats as most of the trans retinal is converted back to the cis form then combined with opsin protein in order to recreate rhodopsin. Any retinal that was not converted from trans to cis may be oxidized to retinoic acid.
Vitamin A in Protein Synthesis and Cell Differentiation
The majority of the body’s vitamin A is not in the retina, aiding vision, but instead in the cells lining the body’s surfaces by promoting protein synthesis and cell differentiation. Vitamin A supports the health of epithelial tissues both outside the body as skin and inside the body as mucous membranes. Both vitamin A and beta-carotene can help protect the skin against skin damage from sunlight. Retinoids are most commonly used in the treatment of skin diseases such as skin cancer and acne. Vitamin A promotes differentiation of both epithelial cells and the goblet cells that secrete mucus thereby protecting the epithelial cells from harmful substances and organisms.
Vitamin A in Reproduction and Growth
Vitamin A participates in sperm development, embryogenesis, tissue differentiation and promotes the normal development of the fetus during pregnancy. Studies have shown that children lacking in vitamin A failed to grow.
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 is required for many systems, such as the visual, immune, and reproductive systems, in order for the human body to function properly. If humans do not meet the required the Vitamin A intake, the immune system becomes impaired and cannot stop disease from attacking the body or be converted while traveling down neural pathways to the brain to help humans see. The requirements for human Vitamin A intake, set forth by the National Academy of Sciences, have actually been decreasing over time. For example, adult men were asked to consume fifteen hundred micrograms of retinol per day in 1968. This number was decreased to one thousand micrograms per day just six years later. Currently, it is even lower than that. The recommendations for Vitamin A intake are actually well divided into sex and age groups, as well as specific recommendations for pregnant women, lactating women, and those with liver conditions. Recommended Daily Allowances, or RDA, “are set to meet the needs of almost all (97 to 98%) individuals in a group” (2001). For infants younger than six months, the recommended daily allowance is four hundred micrograms of retinol per day. For infants between seven and twelve months, the recommended daily allowance jumps to five hundred micrograms of retinol per day. For children between the ages of one and eight, the recommended daily allowance actually drops. Children between the ages of one and three are recommended three hundred micrograms of retinol per day. Children between the ages of four and eight are recommended four hundred micrograms of retinol per day. Adolescent males between the ages of nine and thirteen are recommended six hundred micrograms of retinol per day while adolescent males between the ages of fourteen and eighteen are recommended nine hundred micrograms or retinol per day. Adolescent females between the ages of nine and thirteen are recommended six hundred micrograms of retinol per day while adolescent females between the ages of fourteen and eighteen are recommended eight hundred micrograms or retinol per day. This difference appears again between men and women between the ages of nineteen and seventy. Men between nineteen and seventy are given a recommendation of nine hundred micrograms of retinol per day while non-pregnant and non-lactating women in the same age group are recommended seven hundred micrograms of retinol per day. Pregnant and Lactating women need more retinol, as they have to pass some of the Vitamin A to the fetus or infant. Lactating women need approximately twelve hundred to thirteen hundred micrograms of retinol while pregnant women need between seven hundred fifty and seven hundred seventy micrograms of retinol.
Recommendations are expressed as retinol activity equivalents (RAE). 1 RAE is equivalent to 1 ug of retinol or 12 ug of dietary beta-carotene. International Units (IU) can also be used to report vitamin A amounts. In this case, 1UI of retinol is equal to 0.3ug RAE and 1UI of dietary beta-carotene is 0.05ug RAE.
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).
Since this vitamin is fat soluble, the body stores it when intake is high and is able to use it during the following weeks, months or even years. This means that intake can be adequate as long as the average daily intakes meet the recommendations. Only chronic vitamin A deficiency will lead to the existence of symptoms. Part of the metabolic pathways used by retinol are also used by other drugs, leading to many drug interactions.  In fact, scientists using mice and baboons to model alcoholism found that regardless of vitamin A intake and absorption, alcohol was able to deplete liver stores of vitamin A.  Increased alcohol intake not only decreases the amount of dietary retinoids and carotenoids consumed, but also leads to the activation of derivative enzymes that break down retinol. Ethanol and retinoic acid also compete for the same precursor, leading to chronic vitamin A deficiency in those who abuse alcohol. Although supplementation is recommended for alcoholics, the toxic effects of both vitamin A and high intakes of alcohol wreck havoc on the liver and can promote carcinogenesis.
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.
In the developing world, measles kills as many as two million children every year. The severity of the illness tends to correlate with vitamin A deficiency and administering large doses reduces the risk of death from pneumonia and diarrhea. Along with protecting against measles, vitamin A supplementation also protects against malaria, lung diseases and HIV. A review of 43 studies shows that supplementation of at-risk children under the age of five decreased their mortality by 24%. 
Vitamin A toxicity can occur because of two reasons; a high ingestion of vitamin A or because of a chronic condition. When there is excess amounts of Vitamin A in the body that leads to toxic effects, it 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 commonly caused from the use of therapeutic retinoids or supplements often given out in low-income countries however rarely it can be caused by high dietary intake of vitamin A. 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.
The RDA for vitamin A is 700 micrograms for women and 900 micrograms for men. According to Mayo Clinic, vitamin A toxicity is a rare occurrence however those with liver disease or suffer from excessive use of alcohol are more at risk than the general population. Acute toxicity occurs at 7.500 ug RAE/kg of body weight and chronic toxicity occurs at 1,200 ug RAE/kg of body weight for 6-15 months. The Office of Dietary Supplements says that the majority of the US population reaches an adequate level of vitamin A intake however intake decreased with age which is where the use of a dietary supplement may be helpful. (Johnson, 2014)
Certain foods should be eaten in limited quantities due to their high retinoid content. For example, one ounce of beef liver contains more than three times the RDA for vitamin A. Since fat soluble vitamins are not excreted easily, over consumption of animal liver can cause vitamin A toxicity since there is a high level of vitamin A in liver, This is especially dangerous for pregnant mothers and their unborn babies. Beta-carotene from fruits and vegetables does not pose the same risk because it is not converted into retinol efficiently enough, although it may turn the skin yellow due to accumulation of the vitamin in subcutaneous fat. Overconsumption of beta-carotene from supplements, however can be harmful because it acts like a pro-oxidant: promoting cell division and destroying vitamin A. Vitamin A is also part of the bone remodeling complex and participates in the dismantling process, removing the parts of the bones that are not needed. This process can be overactive if a person is consuming too much vitamin A from supplements.
Signs and Symptoms
Symptoms of acute vitamin A toxicity include drowsiness, irritability, abdominal pain, nausea, vomiting, and increased pressure on the brain. Symptoms of chronic vitamin A toxicity include blurry vision or other visual changes, swelling of the bones, bone pain, poor appetite, confusion, dizziness, nausea and vomiting, dry, rough skin, cracked fingernails, mouth ulcers, jaundice, hair loss, osteoporosis, and respiratory infection. Infants and children are most vulnerable to toxicity because they need less vitamin A since their liver stores are smaller which makes them more sensitive to overdoses. Symptoms of toxicity for infants and children include the softening of the skull, bulging of the fontanel (in infants), double vision, bulging eyeballs, an inability to gain weight. Other possible complications of excess vitamin A include liver damage, excessively high calcium levels, kidney damage due to high calcium, and failure to thrive in infants (Pietrangelo, 2015).
In addition to preformed vitamin A called retinoids which are found in animal food sources and in supplements, there is also another type of vitamin A found in a variety of fruits and vegetables called Carotenoids. Carotenoids have not been shown to cause toxicity because it will not be converted to vitamin A when there is already an excess of vitamin A in the body. (Higdon, 2015) Although overconsumption of carotenoids such as beta-carotene from foods is not harmful, it may turn the skin yellow (Whitney & Rolfes, 2013).
Vitamin A and Pregnancy
Both vitamin A excess and deficiency are known to cause birth defects. During fetal development, retinoic acid is critical for the development of the heart, eyes, ears, lungs, as well as limbs and visceral organs (“Vitamin A”, 2015). Excessive vitamin A during pregnancy leads to abnormal cell death in the spinal cord, increasing the risk of birth defects such as spina bifida and cleft palate. In these cases, vitamin A is considered a teratogen (Whitney & Rolfes, 2013). During pregnancy, it is suggested that women do not receive their recommended vitamin A intake through a supplement but rather through proper nutrition. The only time it is recommended to take a vitamin A supplement is if night blindness and vitamin A is deficiency is prominent in your area. (WHO, 2015)
The simple, most effective way to treat hypervitaminosis A is to stop taking high-dose vitamin A supplements.
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-  Clugston, R. D., & Blaner, W. S. (2012). The Adverse Effects of Alcohol on Vitamin A Metabolism. Nutrients, 4(5), 356–371. http://doi.org/10.3390/nu4050356
-  Mayo-Wilson, E., Imdad, A., Herzer, K., Yakoob, M. Y., & Bhutta, Z. A. (2011). Vitamin A supplements for preventing mortality, illness, and blindness in children aged under 5: systematic review and meta-analysis. BMJ : British Medical Journal, 343, d5094. http://doi.org/10.1136/bmj.d5094