A vitamin is an organic molecule required by a living organism in minute amounts for proper health. An organism deprived of all sources of a particular vitamin will eventually suffer from disease symptoms specific to that vitamin.
Vitamins can be classified as either water soluble, which means they dissolve easily in water, or fat soluble, which means they are absorbed through the intestinal tract with the help of lipids.
In general, an organism must obtain vitamins or their metabolic precursors from outside the body, most often from the organism's diet. Examples of vitamins that the human body can derive from precursors include vitamin A, which can be produced from beta carotene; niacin from the amino acid tryptophan; and vitamin D through exposure of skin to ultraviolet light.
The term vitamin does not encompass other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids, nor is it used for the large number of other nutrients that merely promote health, but are not strictly essential.
The word vitamine was coined by the Polish biochemist Casimir Funk in 1912. Vita in Latin is life and the -amine suffix is for amine; at the time it was thought that all vitamins were amines. This is now known to be incorrect.
In 1881, Russian surgeon Nikolai Lunin fed mice upon an artificial mixture of all the separate constituents of milk known at that time, namely the proteins, fats, carbohydrates, and salts. They died, while the mice fed by milk itself developed normally. He made a conclusion that "a natural food such as milk must therefore contain besides these known principal ingredients small quantities of unknown substances essential to life"  However, his conclusion was rejected by other researchers who were unable to reproduce his results. One difference was that he used table sugar (sucrose), while other researchers used milk sugar (lactose) which still contained small amounts of vitamin B.
In 1905, William Fletcher discovered that eating unpolished rice instead of polished helped prevent the disease beriberi. The following year, Frederick Hopkins postulated that foods contained "accessory factors"—in addition to proteins, carbohydrates, fats, etc.—that are necessary to the human body. When Casimir Funk isolated the water-soluble complex of micronutrients whose bioactivity Fletcher had identified, he proposed that it be named "Vitamine". The name soon became synonymous with Hopkins' "accessory factors", and by the time it was shown that not all vitamins were amines, the word was already ubiquitous. In 1920, Jack Cecil Drummond proposed that the final "e" be dropped, to deemphasize the "amine" reference, after the discovery that vitamin C had no amine component, and the name has been "vitamin" ever since.
The reason the alphabet soup of vitamins seems to skip from E to the rarely-mentioned K is that most of the "letters" were reclassified, as with fatty acids, discarded as false leads, or renamed because of their relationship to "vitamin B", which became a "complex" of vitamins. Vitamin G, Riboflavin, for example, is now known as B2.
Throughout the early 1900s, scientists were able to isolate and identify a number of vitamins by depriving animals of them. Initially, lipid from fish oil was used to cure rickets in rats, and the fat-soluble nutrient was called "antirachitic A". The irony here is that the first "vitamin" bioactivity ever isolated, which cured rickets, was initially called vitamine A, this bioactivity is now called vitamin D, which is subject to the semantic debate that vitamin D is not truly a vitamin because it is a steroid derivative. What we now call "vitamin A" was identified in fish oil because it was inactivated by ultraviolet light. Most of what we now recognize as the water-soluble organic micronutrients were initially referred to as just one entity, "vitamin B".
Other nutrients that are not classified as vitamins include carnitine (meat, fish, dairy), DMAE (fish, eggs, soy, brains), lipoic acid (liver), folinic acid (liver), bioptrin (fish, liver), PPQ (below) and coenzyme Q (meat, yogurt, soy).
Vitamin deficiency and excess
An organism can survive for some time without vitamins, although prolonged vitamin deficit results in a disease state, often painful and potentially deadly. Body stores for different vitamins can vary widely; an adult may be deficient in vitamins A or B12 for a year or more before developing a deficiency condition, while vitamin B1 stores may only last a couple of weeks.
Fat-soluble vitamins may be stored in the body and can cause toxicity when taken in excess. Water-soluble vitamins are not stored in the body, with the exception of vitamin B12, which is stored in the liver.
Vitamin F was the designation originally given to essential fatty acids that the body cannot manufacture. They were "de-vitaminized" because they are fatty acids. Fatty acids are a major component of fats which, like water, are needed by the body in large quantities and thus do not fit the definition of vitamins which are needed only in trace amounts.
The toxins laetrile and amygdaline are sometimes referred to as vitamin B17. Both pangamic acid and laetrile were first proposed as vitamins by Ernst T. Krebs; neither are recognized by the medical community as vitamins and their claimed anticancer activities have been disproven by many experiments.
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