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Vitamins
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The substances commonly known as vitamins are diverse in chemical structure and function. Although vitamins are essential for the normal growth and functioning of the human body, they are effective in extremely small amounts, and act mainly as regulators of the body’s metabolic processes.
Vitamins are classified as either water-soluble or fat-soluble. Fat-soluble vitamins are vitamin A (retinol), vitamin D (calciferol), vitamin E (tocopherol), and vitamin K (phytomenadione). Water-soluble vitamins are vitamin C (ascorbic acid), and the vitamin B group: vitamin B1 (thiamine), vitamin B2 (riboflavine), vitamin B6 (pyridoxine), vitamin B12 (cyanocobalamin), biotin, folic acid, and pantothenic acid.
A daily intake of vitamins is not essential, since the body can store fat-soluble vitamins for many months, and water-soluble vitamins for several weeks. A balanced diet should contain sufficient vitamins to maintain these stores. Growing children need proportionately more vitamins than adults. The body’s vitamin requirements are also increased during illness, pregnancy, and breast-feeding.
The vitamins regulate reactions that occur in metabolism, in contrast to other dietary components known as macronutrients (e.g., fats, carbohydrates, proteins), which are the compounds utilized in the reactions regulated by the vitamins. Absence of a vitamin blocks one or more specific metabolic reactions in a cell, and eventually may disrupt the metabolic balance within a cell and in the entire plant or animal as well.
With the exception of vitamin С (ascorbic acid), all of the water-soluble vitamins have a catalytic function; i.e., they act as coenzymes of enzymes that function in energy transfer or in the metabolism of fats, carbohydrates, and proteins. The metabolic importance of the water-soluble vitamins is reflected by their presence in most plant and animal tissues involved in metabolism.
Some of the fat-soluble vitamins form part of the structure of biological membranes or assist in maintaining the integrity (and therefore, indirectly, the function) of membranes. Some fat-soluble vitamins also may function at the genetic level by controlling the synthesis of certain enzymes. The roles of the fat-soluble vitamins have not yet been established with certainty. Unlike the water-soluble ones, fat-soluble vitamins are necessary for specific functions in highly differentiated and specialized tissues; therefore, their distribution in nature tends to be more selective than that of water-soluble vitamins.
Vitamins are organic substances required in the diet either because the human body cannot manufacture them, or because the body makes them in inadequate quantities. Altogether, the required amounts of the 13 vitamins needed in the daily diet compose only about one-eighth of a teaspoon. Vitamins in excess of the required amount are either excreted from the body, stored in the body fat and other organs, or used by the body as drugs to perform other, non-vitamin functions. It is the body buildup or storage of the excess that often causes serious side effects.
A committee of experts appointed by the National Academy of Sciences has determined certain levels, called Recommended Dietary Allowances, or RDAs, that meet the vitamin and mineral needs of the overwhelming majority of people. A megadose has been arbitrarily defined as more than ten times the RDA (except for vitamins A and D: more than five times the RDA of these vitamins can be considered a megadose).
In March 1979 an FDA advisory panel evaluated the safety and efficacy of vitamins and minerals sold over-the-counter for therapeutic purposes. The panel recommended that even relatively small doses of vitamin K, copper, fluoride, iodine, magnesium, manganese, phosphorus and potassium not be sold as medication, and that vitamin A be sold only in limited doses because of potential hazards associated with misuse of these substances.
Megadosing vitamins can also be dangerous. Vitamins A, D, and K, for example, are especially risky in large amounts, because, as fat-soluble vitamins, they are stored in the body. Too much vitamin D can cause kidney stones, irreversible kidney damage, abnormal heart rhythms, lethargy, coma, and even death. Megadoses of vitamin E can interfere with blood-clotting.
As for the most popular member of the megadosing set, i.e. vitamin C, excessive quantities can create a dependence, and early symptoms of scurvy when intake is suddenly discontinued. Other side effects include diarrhea, and urinary tract irritation.
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