الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 237 |  |  |
| | | from | 237 | | |
Abstract
Vitamins are considered as co-enzymes. Co-enzymes can help enzymes to transform substances in the human body. Most co-enzymes are formed by the human body. The food also contains co-enzymes. The human body doesn’t have the capacity to produce the co-enzymes that human food abundantly contains and these co-enzymes are called vitamins. The loss of production capacity for vitamins doesn’t apply to all vitamins. For example, Vitamins A, K2 B2, B3, B5, B8, and D can be formed in the human’s body. Animals that consume little amounts of vitamin C can form vitamin C, but apes, humans and some rodents that consum fruits cannot form it. (Artist Cooperative Groove Union UA , 2000) .
Vitamin A, was the first fat-soluble vitamin to be discovered, identified in 1913. It is a light yellow crystalline compound. It is also known as retinol due to its participation in the functions of the eye retina. Vitamin A is also known as the ”anti-infective” vitamin because it has a role in supporting the activities of the immune system. (The George Mateljan Foundation , 2001-2010) .
2. Sources Of Vitamin A :
Vitamin A can be obtained from food either as pre-formed vitamin A, in the form of retinol or retinyl-esters which come from animal sources, or as provitamin A form from plants, i.e. provitamin A-carotenoids such as Beta-Carotene. The highest concentration of vitamin A is found in liver and fish liver oil. Yellow and green leafy vegetables provide provitamin A-carotenoids (Ross, 2006). Carotenoids are plant pigments, responsible for the red, orange, and yellow color of fruits and vegetables. The human body can alter certain members of the carotenoid family, such as alpha-carotene, beta-carotene and gamma-carotene, into vitamin A. Sometimes, these carotenoids are referred to as ”provitamin A,” and retinol as ”preformed vitamin A.” (The George Mateljan Foundation , 2001-2010) .About 50% of dietary vitamin A is derived from beta carotene and 50% is preformed vitamin A (Underwood, 1994). Vitamin A is fat soluble, and 90% of body stores are located in the liver. Excess vitamin A in the form of acute hypervitaminosis A can be caused by excess intake of liver, but it is much more commonly associated with overuse of supplements. Excess consumption of beta-carotene does not result in hypervitaminosis A because the efficiency of carotenoid absorption in the small intestine decreases as carotenoid intake increases (Bendich & Langseth , 1989) .
Beneficial Effects and Functions Vitamin A is essential in the treatment of many diseases. It is used in the treatment of some skin disorders such as acne, psoriasis and ichthyosis. It is also beneficial when used as a chemo-preventative or adjuvant agent in the treatment of some cancers and may improve lung function in chronic obstructive pulmonary disease. Vitamin A is used as a treatment for abnormal dark-adaptation and is included in eye drops used to treat blurred vision, cataracts, glaucoma, conjunctivitis and dry eyes. Vitamin A (along with zinc) has been suggested to be a useful adjunct in the treatment of anaemia with iron. It is also taken as an anti-oxidant dietary supplement. Vitamin A is essential to the processes of vision, reproduction, embryonic development, morphogenesis, growth, and cellular differentiation. With the exception of the visual process, most processes are related to the control of gene expression, with vitamin A metabolites, such as retinoic acid, acting as nuclear receptor-ligands (Expert group On Vitamin and Minerals, 2003). Vitamin A induces many immune system activities, probably by enhancing the growth, and prohibiting the stress-induced shrinkage of the thymus gland. Vitamin A is known to have an anti-viral activity, stimulate white blood cells function, and to increase the response of antibodies to antigens. In addition to this, retinoic acid is required to maintain the normal structure and function of epithelial and mucosal tissues which are found in many organs such as the lungs, trachea, skin, oral cavity, and gastrointestinal tract. These tissues, if healthy and intact, act as the first line of defense for the immune system, and provid a protective barrier the disease-causing microorganisms can’t penetrate (The George Mateljan Foundation, 2001-2010).4. Vitamin A Metabolism :
Most retinoic acid is manufactured in the target cells, although it circulates in the blood bound to albumin with concentrations close to 10 nmol/L (Eckhoff & Nau, 1990). Vitamin A exists in the human diet either preformed, in animal products as retinyl esters, or as provitamin carotenoids, mainly beta-carotene (β-carotene), which are cleaved partly to produce vitamin A inside the intestine enterocyte . Vitamin A is loaded into chylomicrons as retinyl esters after intestinal absorption. The chylomicron remains are then captured by hepatocytes, inside the hepatocytes the retinyl esters are hydrolyzed to form the retinol (Figure 1). When vitamin A is insufficient, i.e, when extrahepatic organs need vitamin A urgently, a specific retinol carrier known as retinol binding protein (RBP) binds to the newly formed retinol and then it was secreted into the blood. The time taken in the total process from ingestion to secretion about less than 5 hours. When vitamin A is satisfying, the newly formed retinol is transported in the form of retinyl esters to the hepatic stellate cells (HSCs) (a specific type of liver cells ). These cells are also called perisinusoidal liver cells or Ito cells .Also, these cells have the ability to store large quantities of vitamin A in peculiar lipid droplets. The lipid droplets are then packaged to kept the homeostatic concentration of retinol at 2 mmol/L to meet the needs of the organism (Blaner & Olson, 1994). The RBP-bound retinol is captured by cells via a mechanism thought to include a
membranous receptor specific for RBP. This receptor has been characterized.