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The fat-soluble vitamins
The fat-soluble vitamins are vitamin A, D, E, and K. They differ from the water-soluble vitamins in several ways. They appear in fat and oily parts of food. They are digested only in bile because they are insoluble in water. The following sections provide detailed descriptions of each of these vitamins.
It is difficult to determine the required amount of vitamin A. It is produced from two other substances that have to be converted in the body to vitamin A. In animal foods, it is present in the form of retinol; in plant food it is present as beta-carotene, which is less efficient than retinol at producing vitamin A. This is why the recommended amounts of vitamin A are given in terms of retinol equivalents, RE. The recommended amount of vitamin A is 1000 micrograms RE per day for men and 800 micrograms for women.
Vegetables and fruits are the richest vitamin A carriers. Most foods that contain vitamin A are brightly coloured (although not all brightly coloured food contains a lot of vitamin A). Vitamin A-rich vegetables are carrots, sweet potatoes, winter squash, spinach, and cantaloupe. Milk, cheese, butter and eggs also contain vitamin A.
Vitamin A is important for the maintenance of cornea and epithelial cells and thus, of vision. It is also aids growth and reproduction of bones and teeth. It plays a role in hormone synthesis and regulation and helps protect the body against cancer.
Vitamin A deficiency can have severe consequences. It is normally accompanied by protein and zinc deficiency. Up to a year's supply of vitamin A can be stored in the body. That means that deficiency symptoms do not appear immediately after stopping the intake of this vitamin. However, if they appear after a long period of no intake, they may be profound and severe. One of the first symptoms is night blindness. If the deficiency continues, it can also lead to degeneration of the cornea and cause blindness. It can also prevent bone growth, or cause changes in bone shape and development of cracks and decay in teeth and atrophy of dentin-forming cells. Anaemia is another consequence. In addition, it affects the nervous and muscular system, and can lead to paralysis.
Vitamin A toxicity occurs when the proteins that bind it are swamped so that free vitamin A can attack the cells. This normally does not occur when the vitamins come from the daily diet, but it can happen if a person takes supplements. The symptoms are nausea, vomiting, abdominal pain, diarrhoea and weight loss. The nervous and muscular system may also be affected, causing symptoms such as loss of appetite, irritability, fatigue, insomnia, restlessness, headache and muscle weakness.
Vitamin D has one characteristic that distinguishes it from all other vitamins: it can be produced by sunlight. That means that with regular exposure to sunlight, no additional intake of vitamin D is necessary. The RDA for vitamin D is 5 micrograms per day. Although the amount of synthesised vitamin D increases the longer the skin is exposed to sunlight, sunlight alone can never cause the vitamin D level to reach toxic levels.
Food sources of vitamin D are eggs, liver and fish, as well as fortified milk and fortified margarine.
Vitamin D works on bone mineralisation by increasing the absorption from the digestive tract of calcium and phosphorus, thus raising their levels in the blood, by withdrawing calcium from bones and by stimulating retention by kidneys.
Vitamin D deficiency causes the same symptoms as a calcium deficiency. Bones are not able to calcify in the normal way. They can become so weak that they bend under the body's weight. Lack of vitamin D can also cause deformities and pain of limbs, spine, thorax and pelvis. It can also damage the nervous or muscular system, causing muscle spasms.
Excess vitamin D causes high concentrations of calcium in the blood. The calcium can form kidney stones. High calcium levels in the blood can also cause blood vessels to calcify, which is especially dangerous in arteries of heart and lungs and can be fatal. Additional symptoms of vitamin D toxicity are loss of appetite, headache, weakness, fatigue, excessive thirst, irritability and apathy.
The RDA for vitamin E is 10 mg per day for men and 8 mg per day for women.
Vitamin E is abundant in vegetable and seed oils, which are found in margarine, salad dressings, and shortenings. Soybean oil and wheat germ oil has especially high concentrations of vitamin E. Corn and sunflower oil rank second. A tablespoon of any of these contains more than half of the RDA of vitamin E. In contrast, animal fats such as butter and milk contain almost no vitamin E. Because vitamin E is readily destroyed by heat, it is best to obtain it from fresh foods.
Like vitamin C, vitamin E is an antioxidant. It helps to stabilise cell membranes, regulate oxidation reactions and protect vitamin A. In its role as an antioxidant, vitamin E has its most important effect on the cells as well as on the red and white blood cells that pass through the lungs.
When the level of vitamin E in the blood becomes too low, red blood cells may break open. This process is called erythrodyte hemolysis And can be corrected by treatment with vitamin E Vitamin E deficiency can affect the nervous and muscular system and cause degeneration, weakness, difficulties in walking and pain in calf muscles.
Toxicity may occur if supplements are taken in large amounts, but it does not happen as easily as for vitamins A and D. Symptoms are headache, weakness, dizziness, fatigue and visual abnormalities.
Much of the body's supply of vitamin K is synthesised by bacteria in the digestive tract. Food sources of vitamin K are liver, green leafy vegetables, cabbage-type vegetables and milk.
The digestive tract of humans contains bacteria that synthesise vitamin K, which is partly absorbed and stored in the liver. The body needs to take in additional vitamin K in the diet.
Vitamin K is essential for the synthesis for some of the proteins involved in blood clotting. Vitamin K is also needed for bone formation.
If vitamin K is missing, blood cannot clot. This can lead to haemorrhagic disease. However, vitamin K deficiency is rare.; only babies are susceptible to it. This is because the digestive tracts of new-born babies are sterile and do not contain the bacteria that synthesise vitamin K Also, breast milk contains only small amounts of vitamin K. This is why babies are given a dose of vitamin K at birth.
Vitamin K toxicity occurs only in people who take water-soluble substitutes for vitamin K. Symptoms are red cell hemolysis, jaundice and brain damage.
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