Vitamin K
April 20th, 2009
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Daily Requirements
April 20th, 2009
With a few exceptions, the body is unable to make vitamins; they must be supplied in the daily diet or through supplements. One exception is vitamin D, which can be produced in the skin when the skin is exposed to sunlight. Another vitamin, vitamin K, is not made by the human body but is formed by microorganisms that normally flourish in the intestinal tract only when green, leafy vegetables and vegetable oils are ingested.
The body’s vitamin requirements are expressed in terms of recommended dietary allowances, or RDA. These allowances are the amount of essential nutrients that, if acquired daily, are considered to be sufficient to meet the known nutritional needs of most healthy persons. In the United States, the RDA values are established by the Food and Nutrition Board of the National Academy of Sciences/National Research Council (NAS/NRC). In addition, two agencies of the United Nations–the Food and Agriculture Organization and the World Health Organization–develop RDA for different, worldwide population groups
In the past, the strength of a vitamin or the amount of the vitamin necessary to produce a certain effect in the body was often expressed in terms of international units, abbreviated IU. The unit corresponds to a weight of the purified vitamin, and its value differs from one vitamin to another. Today, the strength of a vitamin is generally expressed directly in metric weights–micrograms or milligrams.
How Vitamins Work
April 20th, 2009
In the body, proteins, carbohydrates, and fats combine with other substances to yield energy and build tissues. These chemical reactions are catalyzed, or accelerated, by enzymes produced from specific vitamins, and they take place in specific parts of the body.
The vitamins needed by humans are divided into two categories: water-soluble vitamins (the B vitamins and vitamin C) and fat-soluble vitamins (A, D, E, and K). The water-soluble vitamins are absorbed by the intestine and carried by the circulatory system to the specific tissues where they will be put into use. The B vitamins act as coenzymes, compounds that unite with a protein component called an apoenzyme to form an active enzyme. The enzyme then acts as a catalyst in the chemical reactions that transfer energy from the basic food elements to the body. It is not known whether vitamin C acts as a coenzyme.
When a person takes in more water-soluble vitamins than are needed, small amounts are stored in body tissue, but most of the excess is excreted in urine. Because water-soluble vitamins are not stored in the body in appreciable amounts, a daily supply is essential to prevent depletion.
Fat-soluble vitamins seem to have highly specialized functions. The intestine absorbs fat-soluble vitamins, and the lymph system carries these vitamins to the different parts of the body. Fat-soluble vitamins are involved in maintaining the structure of cell membranes. It is also believed that fat-soluble vitamins are responsible for the synthesis of certain enzymes.
The body can store larger amounts of fat-soluble vitamins than of water-soluble vitamins. The liver provides the chief storage tissue for vitamins A and D, while vitamin E is stored in body fat and to a lesser extent in reproductive organs. Relatively little vitamin K is stored. Excessive intake of fat-soluble vitamins, particularly vitamins A and D, can lead to toxic levels in the body.
Many vitamins work together to regulate several processes within the body. A lack of vitamins or a diet that does not provide adequate amounts of certain vitamins can upset the body’s internal balance or block one or more metabolic reactions.
Who need Vitamins
April 20th, 2009
All living things, plant or animal, need vitamins for health, growth, and reproduction. Yet vitamins are not a source of calories and do not contribute significantly to body mass. The plant or animal uses vitamins as tools in processes that regulate chemical activities in the organism and that use basic food elements–carbohydrates, fats, and proteins–to form tissues and to produce energy.
Vitamins can be used over and over, and only tiny amounts are needed to replace those that are lost. Nevertheless, most vitamins are essential in the diet because the body does not produce enough of them or, in many cases, does not produce them at all.
Thirteen different vitamins have been identified by nutritionists: A, eight B-complex vitamins, C, D, E, and K. Some substances, such as carnitine and choline, behave like vitamins but are made in adequate amounts in the human body.
Vitamins were originally placed in categories based on their function in the body and were given letter names. Later, as their chemical structures were revealed, they were also given chemical names. Today, both naming conventions are used.
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