Functions, Food Sources, RDI & Bioavailability of Vitamin A - Research Paper Example

? VITAMIN A By Presented to [Indicate your Vitamin A Introduction Nutrition is one of the key elements in the health of a human being. In this case, it is important for human beings to be aware of the nutritional content of the foods they consumed in order to avoid over consuming or under consuming some nutrients. Therefore, this underlines the importance of the knowledge of nutritional value of foods that people consume and the right amount of the nutrients required in one serving or a day. Falling into two categories, in terms of solubility in either fat or water, vitamins are some of the nutrients important for the health of a human being. Although this expose focuses on vitamin A, it is important to point out that there are thirteen vitamins in number. By virtue of its name, vitamin A, which is a fat-soluble vitamin, has the merit of being the first vitamin recognized. As a fat-soluble vitamin, vitamin A requires bile for absorption since it is common in fats and oils of foods consumed by human beings (Whitney, et al., 2010b). It is crucial to point out that the absorption of vitamin A is in a similar way to the absorption of lipids. In effect, the liver and fatty tissues store the vitamin until when required by the body (Sizer, et al., 2011). In this case, the storage of the vitamin in the liver implies that the body can go for a number of days without consuming foods that contain this vitamin. In a similar way, the storage can result in build up of toxic levels once an individual consumed more than the recommended dietary intake which leads to hypervitaminosis. Whitney et al. (2010a) noted the presence of three active forms of vitamin A in the body. These three forms of vitamin A, whose collective name is retinoid, are retinol, retinal, and retinoic acid (Whitney, et al., 2010a). In effect, this shows the importance of bile in absorbing vitamin A in a similar way to other lipids. On the other hand, consuming foods derived from plants provides an individual with carotenoids, such as beta-carotene, which the body converts to vitamin A after consumption (Sizer, et al., 2011). In this regard, these two forms of vitamin A lose some of their nutritious content once exposed to heat and light since they are only stable in their unique environment. For example, milk is one of the food that is rich in retinoids, and it is common knowledge that exposure of milk to excess heat has an effect on the nutritional content. Conversely, exposing vegetables and fruits that are rich in beta-carotene to heat makes them lose their nutrients. Food Sources of Vitamin A As indicated earlier, there are two sources of vitamin A which define each form of vitamin A. In this regard, the first form of vitamin A is from animal derived foods, while the second form is from plants-derived foods. Whitney et al. (2010b, p. 360) noted, “The richest sources of retinoids are foods derived from animals-liver, fish liver oils, milk and milk products, butter, and eggs.” Therefore, animal-derived foods contain more vitamins than foods derived from plants. However, it is important to point out that foods derived from plants do not contain vitamin A. In this case, these foods contain precursors or provitamin A, which the body converts into vitamin A after consumption. Whitney et al. (2010a, p. 202) noted the importance of counting “both the amount of the actual vitamin and the potential amount available from its precursors.” In effect, this helps an individual consume the recommended daily intake of vitamin A, which is crucial to avoid toxicity. Therefore, the table below shows the amount of vitamins available in the key sources of vitamin A. FOOD Amount of vitamin A in µg Fortified Milk 1c = 150 µg Carrots (beta-carotene) ? c = 671 µg Beef Liver 90g = 6,582 µg Cooked Bok Choy (beta-carotene) ? c = 180 µg Baked Sweet potatoes (beta-carotene) ? c = 961 µg Cooked Spinach (beta-carotene) ? c = 472 µg 3 Apricots 100 µg Figure 1: The amount of vitamins available in key sources that qualify to be “rich” and “good” sources (Sizer, et al., 2011, p. 231). Based on the table shown above, it is evident that food from fast food chains lacks vitamin A. However, fast foods improve the quality of food by offering salads to the customers. On the other hand, it is important to point out that most fruits and vegetables contain a vitamin A precursor known as beta carotene, which means that plants do not have any active vitamin A. It is important to point out that there are other foods that offer vitamin A and the table above only provided a few of the principal sources. Functions of Vitamin A Vitamin A is one of the most important nutrients in the body since it influences body functions through regulating activities of genes, which direct the process of protein synthesis. It is important to point out that genes direct enzyme which in turn performs the metabolic work of body tissues (Whitney, et al., 2010a). For this reason, factors that control the expression of a gene also have an impact in the metabolism of body tissues and the health of a human being’s body. According to Whitney et al. (2010a, p. 203), “Hundreds of genes have been suggested as regulatory targets of the retinoic acid form of vitamin A.” In this case, retinoic acid, which is a form of vitamin A, can activate or deactivate some genes in the body that control the production of some proteins in the body necessary for growth and development. Conversely, vitamin A plays two vital functions in an eye, which are the most recognizable functions of vitamin A. The first role of vitamin A in the eye is during the process of the retina perceiving light. On the other hand, the second role is during the process of maintaining a healthy, crystal-clear outer window known as the cornea (Sizer, et al., 2011). The process of vision entails a series of steps with the first step being the entrance of light into the eye through the cornea and striking the retina which contains a pigment called rhodopsin. This pigment, called rhodopsin, is a molecule in the retina that contains a protein known as opsoid, usually bonded to a molecule of the retinal form of vitamin A. Once light strikes the retina, there is a change in shape of rhodopsin, which results in the process of photo-bleaching that initiates a nervous signal to the optic centre of the brain through the optic nerves (Whitney, et al., 2010a). In effect, this is the function of vitamin A in vision since the lack of this essential vitamin in the body will result to difficulties in the eye to adapt to different levels of light. In this regard, lacking vitamin A will hinder the process of changing shape in the rhodopsin and fail to initiate the process of photo-bleaching, which is the initial symptom of night blindness. Although vitamin A plays a crucial role in vision, there is only a few thousands of an individual’s vitamin A in the eye, which means that there are many others lined up in the surface of body cells and performing various functions. In effect, these cells perform the process of cell differentiation, which enables each body cell to perform a specific function in the body (Whitney, et al., 2010b). On the other hand, there are large layers of cells called the epithelial cells, which cover all body surfaces both on the inside and outside. In this case, it is obvious that the epithelial tissue outside the body of an individual is the skin, which is the largest body organ. Conversely, the epithelial tissues that cover the inside of an individual’s body are the linings such as stomach linings, and other linings, which should always be kept smooth. The role of making these linings smooth is of by secreting mucus, which protects the tissue from any foreign organisms and particles by applying a coat over the tissues. Whitney et al. (2010a, p. 204) noted, “Vitamin A, by way of its role in cell differentiation, helps to maintain the integrity of the epithelial cells.” On the other hand, vitamin A plays a pivotal role in reproduction, growth, and development. During the process of reproduction, vitamin A plays an essential role in the production of sperms, in men, while it promotes growth and development of the foetus in women. In this case, the mother transfers vitamin A to her growing foetus, which is crucial in the development of essential body organs (Whitney, et al, 2010a). Conversely, vitamin A plays an important function in the immunity of an individual. Sizer et al. (2011, p. 229) called vitamin A “an ‘anti-infective’ vitamin because so much of the body’s defence against infections depend on an adequate supply.” In effect, a body will have a reduced immunity in case there was inadequate supply of vitamin A, which will affect its role in fighting an infection and may result in amplifying infections to dangerous levels. Deficiency and Excess Effects of Vitamin A According to Checkley et al. (2010, p. 1785), “Vitamin A deficiency affects millions of preschool-aged children and 19 million pregnant women worldwide. It is the underlying cause of 650,000 early childhood deaths and has become recognized as an important problem among women of reproductive age in many developing countries.” In effect, this high number of deaths, which is due to vitamin A deficiency (VAD), emphasizes the essential role the vitamin plays in a living being. On the other hand, it is important to point out that VAD affects poor countries more than developed nations since there is a limitation in the choice of food. Indeed, the World Health Organisation (2012) recognises the role of VAD in night blindness and the increasing of maternal mortality. As explained earlier, an individual is unable to adapt to changes in the level of light, in case of VAD. Concerning maternal mortality, one of the functions of vitamin A in the body is to support the development of the growing foetus. Checkley et al. (2010, p. 1785) noted, maternal VAD affects lung formation of the growing foetus. On the other hand, excess vitamin A in the body may result in toxicity or hypervitaminosis. According to Caroll (2012, p. 32), “There is now compelling evidence that some high-dose vitamins, when taken continually on a longer-term basis, may produce adverse reactions.” It is common knowledge that nutrients found in fruits and vegetables, such as beta-carotene, reduce the risk of cancer and other cardiovascular diseases since they contain antioxidants. However, research suggests that a high than recommended intake of beta carotene increases the chances of lung cancer in smokers exposed to asbestos (Caroll, 2012). In addition, excessive vitamin A causes defects in bones and during birth. In this case, excessive intake of vitamin A may result to osteoporosis since it interferes with the ability of vitamin D to maintain the recommended amount of calcium in the blood. On the other hand, excessive vitamin A causes maternal mortality since it may contribute to the death of cells in the spinal cord (Whitney, et al., 2010b). RDI & Bioavailability The following table shows the upper RDI recommended levels by the Australian National Health and Medical Research Council (NHMRC). Age Upper Level of RDI Infants 0-12 months 600 µg/day Children and Adolescents 1-3 years 600 µg/day 4-8 years 900 µg/day 9-13 years 1,700 µg/day 14-18 years 2,800 µg/day Adults (+19 Years) Men 3,000 µg/day Women 3,000 µg/day Pregnancy 14-18 years 2,800 µg/day 19-50 years 3,000 µg/day Lactation 14-18 years 2,800 µg/day 19-50 years 3,000 µg/day Figure 2: RDI recommended upper levels (NHMRC). According to the NHMRC (2012), the upper level intake is the level that is unlikely to pose any adverse effect on an individual. While setting the upper level for pregnant women, the rationale used considered agents that interfered with the development of the embryo while the NHMRC focused on abnormal liver pathology when setting the upper level for other adults. However, scarcity of data on children and adolescents made the NHMRC set the upper level for these groups by inferring from adult data based on the relative body weight. Conclusion The crucial role of all the body nutrients cannot be overemphasized. Consequently, each vitamin, such as vitamin A, plays a crucial role in the body as this expose elucidates. From the foregoing, it is evident that there are two forms of vitamin A based on the source of food consumed, which is either plant-derived or animal-derived. While plants do not contain an active form of vitamin A, they contain beta-carotene that the body converts into vitamin A after consumption. On the other hand, animal-derived vitamins are active in the body and require bile for absorption into the body. Nonetheless, it is crucial for an individual to be aware of the amount of vitamin A in the foods in order for them to consume the recommended dietary intake. In this regard, over consumption of vitamin A has an adverse effect on an individual while vitamin A deficiency also affects the individual with the WHO recognising VAD as one of the causes of maternal mortality. Nonetheless, essential knowledge of the amount of vitamin A available in food and knowledge of the recommended amount of vitamin A is crucial in avoiding over consumption and under consumption. Reference List Australian National Health and Medical Research Council (NHMRC)., 2012. Vitamin A, [online]. Available at http://www.nrv.gov.au/nutrients/vitamin%20a.htm [Accessed 22 May 2012]. Caroll, P., 2012. Is high-dose vitamin use safe? Australian Journal of Pharmacy, 93, 32-33. Checkley, W., West, K. P., Wise, R. A., Baldwin, M. R., Wu, L., LeClerq, S. C., Christian, P., Katz, J., Tielsch, J., Khatry, S. and Sommer, A., 2010. Maternal Vitamin A Supplementation and Lung Function in Offspring. The New England Journal of Medicine, 362(9), 1784-1794. Sizer, F. S., Piche, L. A. and Whitney, E. N., 2011. Nutrition: Concepts and Controversies. 2nd ed. Stamford: Cengage Learning. Whitney, E., DeBruyne, L. K., Pinna, K. and Rolfes, S. H., 2010a. Nutrition for Health and Health Care. 4th ed. Stamford: Cengage Learning. Whitney, E., Whitney, E. A. and Rolfes, S. H., 2010b. Understanding Nutrition. 11th ed. Stamford: Cengage Learning. World Health Organization (WHO)., 2012. Vitamin A deficiency, [online]. Available at http://www.who.int/nutrition/topics/vad/en/ [Accessed 22 May 2012]. Read More