Hotmail
Messenger
My MSN
MSN Directory
|
|
Windows Live® Search Results
Windows Live® Search Results Article Outline
Vitamin, any of the organic (carbon-containing) compounds that the body requires in small amounts to maintain health and function properly. Children additionally need vitamins to grow. The body gets most of its vitamins from the foods we eat. A healthy diet with plenty of fruits and vegetables should provide nearly all of the vitamins a person needs. See also Human Nutrition. Scientists have classified 13 compounds as vitamins. They have given most of these vitamins letter or letter plus number names, such as A, B12, and D. Most vitamins are produced by plants. Some, such as vitamin D, are produced only by animals. A few vitamins are made by the body itself. For example, bacteria in the digestive tract help produce vitamin K, and the skin uses sunlight to produce vitamin D. Vitamins are also manufactured for sale as supplements for people who need additional vitamins to meet their body’s requirements. For example, doctors often prescribe vitamin supplements for pregnant and nursing women to provide the additional nutrients needed by a rapidly growing fetus or infant. Older adults may not meet their vitamin requirements through food because the body’s ability to absorb vitamins is impaired with age. In the United States, since 1940, the Food and Nutrition Board of the National Research Council has published recommended dietary allowances (RDA) for vitamins, minerals, and other nutrients. Expressed in milligrams (mg) or international units (IU) for adults and children of normal health, these recommendations provide useful guidelines for daily intake. Such guidelines are useful not only for professionals in nutrition but also for the growing number of families and individuals who eat irregular meals and rely on prepared foods, most of which are now required to carry nutritional labeling.
The word vitamine, later shortened to vitamin, was coined by Polish-American chemist Casimir Funk in the early 20th century. Funk was searching for the then-unknown substance in foods that prevents such diseases as beriberi, rickets, and scurvy. After experiments on pigeons, Funk guessed that an amine—a compound containing nitrogen and hydrogen—in foods was responsible. He called this substance a vital (necessary for life) amine, or vitamine. Scientists later discovered that not all vitamins were amines, however. Vitamins help the body carry out essential biochemical processes. Vitamins generally combine with proteins to create enzymes that promote chemical reactions. These enzymes play an important role in metabolism—the reactions that break down the fats, carbohydrates, and proteins in food so that the body can use them for energy and cell repair. The enzymes also promote reactions involved in the formation of bone, hormones, blood cells, nervous-system chemicals, and genetic material. Without vitamins, many of these reactions would slow down or cease. The different vitamins are not chemically related, and most differ in their actions in the body. Deficiency of particular vitamins can lead to various diseases. Too little vitamin C, for example, can cause rickets, a disease in which the bones fail to develop properly. Bowlegs or “knock knees” are signs of rickets. Rickets and the other vitamin-deficiency diseases that interested Casimir Funk have largely disappeared in the developed world as the result of fortified foods and improved nutrition. But these diseases still occur in the developing world, especially where malnutrition is common. Interest in vitamins grew during the 1990s, especially in those vitamins that act as antioxidants—vitamins A, C, and E. Antioxidants neutralize molecules known free radicals that cause cell damage. Free radicals have been linked to a number of disorders, including Alzheimer’s disease, arteriosclerosis, cancer, diabetes, and Parkinson disease. Researchers theorized that increased doses of antioxidants might also prevent and even cure such diseases. These claims have so far not panned out, however, as scientific studies have failed to demonstrate the preventive abilities of antioxidants. Research on antioxidants and other vitamins continues.
Scientists classify the 13 well-identified vitamins as fat-soluble or water-soluble. The fat-soluble vitamins—A, D, E, and K—dissolve in fat and are generally consumed in foods that contain fat. Because these vitamins also can be stored in the body’s fat, we do not have to consume them every day. The water-soluble vitamins—the eight B vitamins and vitamin C—do not dissolve in fat and cannot be stored. They pass from the body in urine and must be consumed frequently, preferably daily. Vitamins are unstable and can be destroyed during the cooking or processing of foods. Heat in combination with water can remove water-soluble vitamins from food. Because many vegetables are rich in these vitamins, nutritionists advise people to cook vegetables by steaming, roasting, or microwaving rather than by boiling in water. The longer vegetables cook, the more vitamins they lose. In addition, some vitamins can be destroyed by exposure to sunlight or air. Frozen vegetables, which generally are frozen while still fresh, are often preferable to vegetables that have been shipped long distances. Fresh vegetables should be cooked soon after purchase.
Vitamin A is a pale yellow, fat-soluble substance. It is formed from an orange pigment in plants called carotene, which animals convert into vitamin A. Vitamin A plays an important role in cell growth, vision, and the immune system. It helps skin develop and stay healthy and promotes the growth of bones and teeth. It is present in the retina (the light-sensitive membrane at the back of the eye) and aids vision in low light. It also helps maintain the mucous membranes that trap microbes and fight infection. Vitamin A deficiency is rare in the developed world but more common in the developing world. An early symptom is night blindness (difficulty in adapting to darkness). The skin, eyes, and mucous membranes may also become extremely dry. The long-term risks of excess vitamin A are not yet certain. Some studies indicate that too much vitamin A—more than 1,500 micrograms per day—over a prolonged period may reduce bone density and increase the likelihood of fractures. In the short term too much vitamin A can cause headaches, nausea, blurred vision, and dizziness. The body obtains vitamin A in two ways. One is by manufacturing it from beta-carotene, a vitamin precursor found in orange vegetables, such as carrots, squash, and sweet potatoes, as well as in broccoli, spinach, and other dark green, leafy vegetables. The other way of obtaining vitamin A is by absorbing it from plant-eating animals. In animal form, vitamin A is found in milk, butter, cheese, eggs, liver, and fish.
© 1993-2008 Microsoft Corporation. All Rights Reserved.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
© 2008 Microsoft
  |
