Human vision requires the presence of the vitamin A in sufficient amounts - its deficiency can cause a number of visual problems, however, this vitamin is necessary for a lot of other things besides the mere adjustment of the human eyes to dark conditions. The vitamin A has a very important role in the protection and regulation of at least three primary senses in human beings. The presence of sufficient quantities of the vitamin A is vital to fight off infections and pathogens of all kinds. This vitamin also holds great promise in fighting off a variety of other problems and disorders; it has a potential use in the treatment of abnormally heavy menstrual bleeding and serious diseases such as cancer.
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The function of the vitamin A in eyesight is the inspiration behind vitamin A's chemical name of retinol. Some trace amount of this vitamin must always be present in a healthy retina as the vitamin is an active participant in all the chemical processes by which light entering the eye, stimulates the rod cells in the eye and the cone cells into the transmission of visual impulses to the human brain's visual region.
Vitamin A is not required by the eyes alone and many other processes and tissues need this vitamin for proper functioning. The healthy structure and proper functioning of the epithelial tissues, such as the layers of tissue that cover an organ or the entire organism - skin and mucous membranes depend on the presence of the vitamin A at the cellular level. The maintenance and proper development or growth of bones and teeth is also influenced by the vitamin A in the human body. The suspected role of vitamin A in the function and structure of all biological membrane systems is an area of focus in some of the current biological researches; such studies particularly focus on the "border" membranes that lie between different cells in the body.
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The formation of a proper structure and the functioning of the adrenal glands are also dependent on the vitamin A, this gland through its hormones controls and regulates the body's response to stress of all kinds. The adrenal glands secrete hormones whenever the body is under a lot of stress, these hormones then stimulate the various organ systems and enable the body to deal with the stress resulting in the fight or flight reflex. It is important to remember that the stress responses in human beings evolved when stress was primarily an answer to immediate physical danger. The body still prepares itself for a fight or a flight away from the danger even if only very little of stressors in the contemporary world can be dealt with by the use of physical force alone. When the body responds to stress, there is a speeding up of the heart rate, the blood pressure is raised and the rapid conversion of stored fats and proteins to produce extra energy occurs with the concomitant use of all the reserve sugars - all this energy is made immediately available to the muscles in seconds through hormonal stimulation.
There is a great variation in the human requirement for the vitamin A at any one time. A person can be prevented from receiving an adequate supply of the vitamin by a whole host of intrinsic and extrinsic factors. Oxidation for example destroys stores of the vitamin in the body, while heat is not destructive for the vitamin. However, the presence of heat increases the rate of oxidation of the vitamin. Therefore the content of vitamin A in foods can be drastically reduce after foods are cooked for long periods of time at very high temperatures, followed subsequently by exposure to the air during storage of the vitamin. At the same time, cooking is essential to get the vitamin A content in plants; this vitamin is not readily available to human digestion till some cooking has managed to soften the cell walls that make up the vegetable tissues. The cell walls of plants are made to release their store of vitamins by smashing, blending or liquefying vegetables before eating them, this make more of the vitamin A available to the person eating the food. Many factors also influence the vitamin A content of different vegetables and this varies greatly as well, some factors such as the richness and condition of the soil, the amount of sunlight received by the plant are important among other factors both intrinsic and extrinsic to the plant.
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The vitamin A is soluble in lipid rich mediums and is considered a fat soluble vitamin for this reason - this means its structure contains water repelling elements which cannot be mixed with water. Getting vitamin A from dietary sources does not invariably mean the consumption of a lot of fatty foods. Vitamin A is actually formed in the human intestine from the provitamin A - the pigment carotene, that is present in abundant amounts in the yellow and orange pigments that gives color to many vegetables and fruits used by man. The presence of a little fat in the intestine however, does help a little when the vitamin is ingested and this fat makes assimilation of the vitamin into the body a lot easier. The absorption of the vitamin A and all other fat soluble vitamins is also disrupted in the presence of any conditions in the body, which result in the interference with the absorption of fat - vitamin A absorption is also inhibited in such scenarios. The disorders that can lead to the inhibition of vitamin A absorption include chronic diarrhea, any biliary or pancreatic dysfunction, the presence of celiac disease or mineral oil consumption. The storage site for absorbed vitamin A lies in the liver and in all other fatty portions of the body with plenty of lipids.
The principal source of vitamin A is the liver, and preformed vitamin A is available only from animal sources of food. Other good sources of the vitamin A include the yolk of eggs and all kinds of dairy products. Carotene is found in vegetable sources such as all dark green and leafy vegetables, in vegetables colored a deep yellow as well as tomatoes and in carrots - after which it is named. Both, the natural and synthetic forms of the vitamin A supplements are available in the market in a wide range dosages. Individuals affected by digestive problems that interfere with the absorption of fat can use supplemental forms of the vitamin that are easier to absorb even in the presence of water. The price of the natural vitamin A from fish liver oils is usually competitive with respect to the synthetic forms of the vitamin.
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A vitamin A deficiency can develop in people who severely limit their consumption of foods like liver, dairy products and beta-carotene containing vegetables. Poor night vision is one of the earliest signs of vitamin A deficiency. The vitamin A deficiency produces physical symptoms such as dry skin, a great increase in the risk of infections as well as metaplasia -a tissue level pre-cancerous condition affecting some area of the body. In the modern Western world, the onset of severe vitamin A deficiencies leading to blindness are extremely rare these days.
Vitamin A has a RDA of about 5000 IU for men per day and about 4000 IU for women, the RDA for infants is 1400 IU, for children it falls between 2000 - 3300 IU. The RDA is 5000 IU for pregnant women and for lactating mothers is 6000 IU. The requirement for the different vitamins and minerals varies to a great extent for each person. The range in the requirements for the vitamin A for each animal was so varied during one animal test that the researchers could not come up with a definite figure - this experiment was supposed to measure vitamin A requirement for rats. The study include all kinds of vigorous and healthy animals in all age groups, the group include animals that did not receive any vitamin A for the entire length of the research.
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The New England Journal of Medicine published a report in the year 1995 that pointed out that women who are ready for pregnancy or those who could become pregnant soon are advised by doctors to take less than 10,000 IU or 3,000 mcg daily of vitamin A in remove the risk of giving birth to babies with developmental defects. The evidence from a recent study which measured several hundred women exposed to daily doses of 10,000 to 300,000 IU of vitamin A - where the median exposure was 50,000 IU daily suggest different things about the nature of vitamin supplements. The study found that three major malformations occurred in the course of the study, however, these malformations could have happened anyway even in the absence of vitamin A supplementation. At the same time, the absence of congenital malformations was in any of the 120 infants exposed to the high maternal intakes of vitamin A exceeding 50,000 IU daily was surprising. The reality is that when compared with infants not exposed to vitamin A, the infants in the high exposure group had a 50% decreased risk for developmental malformations. The studies that pointed out the development of birth defects in babies were found by the authors to have been based on very few cases of birth defects occurring in previous trials, some of these limited trials didn't even measure vitamin A intake in the mothers - some of these studies found no link between vitamin A and birth defects. Clinicians who too a closer look at the results of a recent study found that there was a 32 % higher than expected risk of developmental birth defects in infants who were exposed to 10,000 to 40,000 IU of vitamin A daily, surprisingly exposure to even higher levels of the vitamin actually resulted in a 37% decreased risk for developmental birth defects in the infants. Based on the evidence, the role of chance is a factor in both the "higher" and "lower" risks evident during vitamin A supplementation. Thus, unless further research reveals more, the level at which birth defects are induced by vitamin A supplementation is still not fully understood, however, it is likely to be higher than 10,000 IU daily for long periods of time. A nutritionally oriented doctor must be consulted by all women who are in a term of pregnancy or those who could get pregnant soon before they begin supplementing with more than 10,000 IU vitamin A daily.
Vitamin A intakes above 25,000 UI or 7,500 mcg daily in older adults can in the rarest of cases lead to the development of certain symptoms like persistent headaches, the drying out of skin, hair loss and constant physical fatigue, problems in the bones as well as long term liver damage. These problems can become very common if the doses of the vitamin A are at the higher dosage levels exceeding 100,000 UI per day.
An iron deficiency is corrected much more quickly when taken along with supplements of the vitamin A than when iron is supplemented alone.