Vitamin A is a fat-soluble nutrient that occurs in nature in two forms: preformed vitamin A and provitamin A, or carotene. Preformed vitamin A is concentrated only in certain tissues of animal products in which the animal has metabolized the carotene contained in its food into vitamin A. One of the richest natural sources of preformed vitamin A is fish-liver oil, which is classified as a food supplement. Some animal products, such as cream and butter, may contain both preformed vitamin A and carotene.
Carotene is a substance that must be converted into vitamin A before it can be utilized by the body. Carotene is abundant in carrots, from which its name is derived, but it is present in even higher concentrations in certain green leafy vegetables, such as beet greens, spinach, and broccoli. If, owing to any disorder, the body is unable to use carotene, a vitamin A deficiency may arise.
Vitamin A aids in the growth and repair of body tissues and helps maintain smooth, soft, disease-free skin. Internally it helps protect the mucous membranes of the mouth, nose, throat, and lungs, thereby reducing susceptibility to infection. This protection also aids the mucous membranes in combating the effects of various air pollutants. The soft tissue and all linings of the digestive tract, kidneys, and bladder are also protected. In addition, vitamin A prompts the secretion of gastric juices necessary for proper digestion of proteins. Other important functions of vitamin A include the building of strong bones and teeth, the formation of rich blood, and the maintenance of good eyesight. Heavy use of the eyes for watching television and working under glaring lights require more vitamin A. It is essential in the formation of visual purple, a substance in the eye which is necessary for proper night vision.
RNA production is greatly enhanced by vitamin A. RNA (ribonucleic acid) is a nucleic acid that transmits to each cell of the body instructions on how to perform so that life, health, and proper function can be maintained. The body must be able to synthesize new RNA or cell degeneration begins. Studies have revealed that new RNA can be produced in vitamin A deficient bodies; however, the rate of production of new RNA is much less than if sufficient A is available. One of the best sources of RNA is yeast.
The upper intestinal tract is the primary area of absorption of vitamin A; it is here that the fat-splitting enzymes and bile salts convert carotene into a usable nutrient. This conversion is stimulated by thyroxine, a hormone obtained from the thyroid gland. Once converted into vitamin A, carotene is absorbed in the same way as is the preformed vitamin. Vitamin A is carried through the bloodstream, readily accessible to tissues throughout the body. Preformed vitamin A as found in fish-liver oil or other animal products is absorbed by the body 3 to 5 hours after ingestion, whereas the conversion and absorption of carotene takes 6 to 7 hours.
The conversion of carotene into vitamin A is not 100 percent complete; approximately one-third of the carotene in food is converted into vitamin A. Less than one-fourth of the carotene in carrots and root vegetables undergoes conversion, and about one-half of the carotene in leafy green vegetables undergoes conversion. Some unchanged carotene is absorbed into the circulatory system and stored in the fat tissues rather than in the liver. Unabsorbed carotene is excreted in the feces.
The ability of the body to utilize carotene varies with the food and the form in which the food is in gested. Cooking, pureeing, or mashing of vegetable; ruptures the cell membranes and therefore makes the carotene more available for absorption.
Factors interfering with absorption of vitamin A and carotene include strenuous physical activity performed with 4 hours of consumption, intake of mineral oil, excessive consumption of alcohol, excessive consumption of iron, and the use of cortisone and other drugs. The intake of polyunsaturated fatty acids with carotene results in rapid destruction of carotene unless antioxidants also are present. Even cold weather can hinder the transport and metabolism of both vitamin A and carotene. Diabetics may not be able to convert carotene into vitamin A.
Approximately 90 percent of the body's vitamin A is stored in the liver, with small amounts deposited in the fat tissues, lungs, kidneys, and retinas of the eyes. Under stressful conditions the body will use this reserve supply if it is not receiving enough vitamin A from the diet. An adequate supply of zinc is needed so the liver can mobilize vitamin A out of its storage depots. Gastrointestinal and liver disorders, infections of any kind, or any condition in which the bile duct is obstructed may limit the body's capacity to retain and use vitamin A. Factors affecting absorption of vitamin A include the quantity given, influence of other substances present in the intestines, and amount of the vitamin stored in the body. A diet low in fat, resulting in little bile reaching the intestine, can cause carotene and vitamin A to be lost in the feces. For these reasons, the recommended dietary amounts vary for each individual.
Recommended Dietary Allowances of vitamin A, as established by the National Research Council, are 1500-4000 International Units (IU) for children and 4000-5000 IU for adults. These amounts increase during disease, trauma, pregnancy, and lactation. Requirements vary for people who smoke, those who live in highly polluted areas, people who easily absorb vitamin A, and those who have had their stored supply of vitamin A depleted by pneumonia or nephritis.
Recommended amounts of vitamin A may be supplied through food sources; e.g., 1/2 pound of calf's liver contains approximately 74,000 IU preformed vitamin A, whereas a carrot contains 11,000 IU of carotene.
Toxicity symptoms include nausea, vomiting, diarrhea, dry skin, hair loss, headaches, appetite loss, sore lips, and flaky, itchy skin. Bone fragility, thickening of long bones, deep bone pain, enlargement of the liver and spleen, blurred vision, and skin rashes are symptoms of prolonged excessive intake. Excessive daily use of vitamin A also may lead to reduced thyroid activity and abnormalities in the skin, eyes, and mucous membranes. Excessive serum calcium can be an indication of vitamin A overdose.
If toxicity is detected, the symptoms will disappear in a few days if the vitamin is withdrawn. Vitamin C can help prevent the harmful effects of vitamin A toxicity.
Deficiency Effects and Symptoms
The eyes are well-known indicators of vitamin A deficiency. One of the first symptoms is night blindness, an inability of the eyes to adjust to darkness. Another eye-related deficiency symptom is xerosis, a disease in which the eyeball loses luster, it becomes dry and inflamed, and visual acuity is reduced.
Other signs of deficiency include rough, dry, or prematurely aged skin; loss of sense of smell; loss of appetite; frequent fatigue; skin blemishes; sties in the eye; and diarrhea. Vitamin A may be lacking when the hair loses its sheen and luster, when dandruff accumulates and fingernails become brittle. A drastic drop of serum A has been found in severely injured patients. More severe symptoms are corneal ulcers and softening of bones and teeth. Deficiency of vitamin A leads to the rapid loss of vitamin C.
Vitamin A deficiency may occur when an inadequate dietary supply exists; when the body is unable to absorb or store the vitamin (as in ulcerative colitis, cirrhosis of the liver, and obstruction of the bile ducts); when an ailment interferes with the conversion of carotene to vitamin A (as in diabetes mellitus and hypothy-roidism); and when any rapid bodily loss of the vitamin occurs (as in pneumonia, hyperthyroidism, chronic nephritis, scarlet fever, and some respiratory infections).
Beneficial Effect on Ailments
Many people are unaware of the importance of vitamin A in fighting infections. By giving strength to cell walls, it helps protect the mucous membranes against invading bacteria. People who live in environments with high air-pollution counts are more susceptible to infections and colds than are people who live in environments with cleaner air. If infection has already occurred, therapeutic doses of vitamin A will help keep it from spreading.
Vitamin A can be used successfully in treating several eye disorders, such as Bitot's spots (white, elevated, sharply outlined patches on the white of the eye), blurred vision, night blindness, cataracts, crossed eyes, and nearsightedness. Therapeutic dosages of vitamin A are necessary for treatment of glaucoma and conjunctivitis, an inflammation of the mucous membrane that lines the eyelids.
Administration of vitamin A has helped shorten the duration of communicable diseases-measles, scarlet fever, the common cold, and infections of the eye, middle ear, intestines, ovaries, uterus, and vagina. It also has been effective in reducing high cholesterol levels and atheroma, fatty degeneration or thickening of the wall of the larger arteries.
Vitamin A has proved successful in treating cases of brochial asthma, chronic rhinitis, and dermatitis. Vitamin A has also been helpful in treating patients suffering from tuberculosis, cirrhosis of the liver, emphysema, gastritis, and hyperthyroidism. Patients with nephritis (inflammation of the kidney), migraine headaches, and tinnitus (ringing in the ear) have benefited from vitamin A therapy.
Vitamin A protects the epithelial tissues like the skin, the stomach, and the lungs from becoming cancerous. Little research has been done to discover just what happens to the cells of the body from the time of exposure to a cancer-causing agent to the actual development of a malignancy. This period of time can be as much as 20 years or more. It is known that many cells repair themselves during this period. Vitamin A has been found to be extremely important in this repair process. Studies of animals have shown that carcinogens remain much more active when there is a vitamin A deficiency. Researchers also believe that the vitamin counters the cancerous process by activating the body's immune system and preventing the thymus gland from shrinking. When animals injected with a tumor-virus are given large doses of vitamin A, their tumors diminish and the thymus returns to normal size.
There is increasing evidence that vitamin A is related to sexual development and reproduction. Studies conducted on men having varying levels of sperm deficiency showed that when vitamin A along with Vitamin E was given to them, their sperm levels returned to normal. Dr. Thomas Moore in Cambridge, England, reports that experiments done on animals deficient in vitamin A resulted in undersized, shrunken, and flabby testicles. He also states that a deficiency of vitamin A in females causes an inability to conceive and a higher susceptibility to miscarriage. In animal studies, females that were vitamin A deficient yet able to conceive still had problems such as difficult births, death of the fetus, cleft palate, or other congenital defects.
Vitamin A is essential in the chemical process whereby cholesterol is converted into female estrogens and male androgens. Insufficient supply of these sex hormones results in degeneration of the sex organs. Vitamin A given to animals in this condition resumed normal hormone activity. Diabetic men have a higher incidence of impotence than nondiabetics. As diabetics are unable to convert carotene to vitamin A, their impotence could possibly be related to a vitamin A deficiency.
Externally, vitamin A is used in treating acne; when applied locally, it can clear up impetigo, boils, carbuncles, and open ulcers. Vitamin A applied directly to open wounds hastens the healing process in cases where healing has been retarded because cortisone has been used. It also stimulates the production of mucus, which in turn prevents scarring. A treatment using injections of vitamin A has proved effective in the removal of plantar warts.
|
|
 |
Главная
Питание
