Vitamin

2007 Schools Wikipedia Selection. Related subjects: Food and agriculture; General Chemistry

Retinol (Vitamin A)

Vitamins are nutrients required in very small amounts for essential metabolic reactions in the body . The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids. Nor does the term refer to the large number of other nutrients that promote health, but are not strictly essential.

Vitamins act both as catalysts and substrates in chemical reactions. When acting as a catalyst, vitamins are bound to enzymes and are called cofactors, for example vitamin K forms part of the proteases involved in blood clotting. Vitamins also act as coenzymes to carry chemical groups between enzymes, for example folic acid carries various forms of carbon groups (methyl, formyl or methylene) in the cell.

Until the 1900s, vitamins were obtained solely through food intake. Many food sources contain different ratios of vitamins. Therefore, if the only source of vitamins is food, a seasonal, yearly or even daily change in diet also alters the ratio of ingested vitamins. Many vitamins can be stored by the body over a range of dosages and short term deficiencies (e.g. during a particular food growing season), do not always result in disease.

Vitamins have been produced as commodity chemicals and made widely available as inexpensive pills for several decades allowing for consistent supplementation to dietary intake.

Fruits and vegetables are often a good source of vitamins.

History

The value of eating certain foods to maintain health was recognized long before vitamins were identified. The ancient Egyptians knew that feeding a patient liver would help cure night blindness, now known to be caused by a vitamin A deficiency. In 1747, the Scottish surgeon James Lind discovered that citrus foods helped prevent scurvy, a particularly deadly disease in which collagen is not properly formed, and is characterized by poor wound healing, bleeding of the gums, and severe pain. In 1753, Lind published his Treatise on the Scurvy, which recommended using lemons and limes to avoid scurvy, which was adopted by the British Royal Navy. This led to the nickname Limey for sailors of that organization. Lind's discovery, however, was not widely accepted by individuals in the Royal Navy's Arctic expeditions in the 19th century, where it was widely believed that scurvy could be prevented by practicing good hygiene, regular exercise, and by maintaining the morale of the crew while on board, rather than by a diet of fresh food. As a result, Arctic expeditions continued to be plagued by scurvy and other deficiency diseases. In the early 20th century, when Robert Falcon Scott made his two expeditions to the Antarctic the prevailing medical theory was that scurvy was caused by "tainted" canned food.

In 1881, Russian surgeon Nikolai Lunin studied the effects of scurvy while at the University of Tartu (in present day Estonia). He fed mice an artificial mixture of all the separate constituents of milk known at that time, namely the proteins, fats, carbohydrates, and salts. The mice that received only the individual constituents 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 conclusions were rejected by other researchers when they were unable to reproduce his results. One difference was that he had used table sugar ( sucrose), while other researchers had used milk sugar ( lactose) which still contained small amounts of vitamin B.

In 1897, Christiaan Eijkman discovered that eating unpolished rice instead of the polished variety helped to prevent the disease beriberi. The following year, Frederick Hopkins postulated that some foods contained "accessory factors"—in addition to proteins, carbohydrates, fats, etc.—that were necessary for the functions of the human body. Hopkins was awarded the 1929 Nobel Prize for Physiology or Medicine, with Christiaan Eijkman, for their discovery of several vitamins.

Kazimierz Funk was the first to isolate the water-soluble complex of micronutrients, whose bioactivity Fletcher had identified, and Funk proposed the complex 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.

Riboflavin (Vitamin B₂)

Throughout the early 1900s, the use of deprivation studies allowed scientists to isolate and identify a number of vitamins. 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 "vitamin A", the bioactivity of which is now called vitamin D, What we now call "vitamin A" was identified in fish oil because it was inactivated by ultraviolet light.

In 1931, Albert Szent-Györgyi and his research fellow Joseph Svirbely, determined that "hexuronic acid" was actually vitamin C and noted its anti-scorbutic activity, and 1937 Szent-Györgyi was awarded the Nobel Prize for his discovery. In 1943 Edward Adelbert Doisy and Henrik Dam were awarded the Nobel Prize for their discovery of vitamin K and its chemical structure.

Human vitamins

Vitamins are classified as either water soluble, meaning that they dissolve easily in water, or fat soluble, and are absorbed through the intestinal tract with the help of lipids. Each vitamin is typically used in multiple reactions and therefore, most have multiple functions.

In humans there are thirteen vitamins, divided into two groups; four fat-soluble vitamins (A, D, E and K), and nine water-soluble vitamins (eight B vitamins and vitamin C).

Vitamins in nutrition and disease

Vitamins are essential for normal growth and development. Using the genetic blueprint inherited from its parents, a fetus begins to develop, at the moment of conception, from the nutrients it absorbs. The developing fetus requires certain vitamins and minerals to be present at certain times. These nutrients facilitate the chemical reactions that produce, among other things, skin, bone, and muscle. If there is serious deficiency in one or more of these nutrients, a child may develop a deficiency disease. Even minor deficiencies have the potential to cause permanent damage.

For the most part, vitamins are obtained through food sources. However, a few vitamins are obtained by other means: for example, microorganisms in the intestine - commonly known as " gut flora" - produce vitamin K and biotin, while one form of vitamin D is synthesized in the skin with the help of natural ultraviolet in sunlight. Some vitamins can be obtained from precursors that are obtained in the diet. Examples include vitamin A, which can be produced from beta carotene and niacin, from the amino acid tryptophan.

Once growth and development are completed, vitamins remain essential components of the healthy maintenance of the cells, tissues, and organs that make up the human body, and enable the body to efficiently use the calories provided by the food that we eat, and to help process proteins, carbohydrates, and fats.

Vitamin deficiencies

Deficiencies of vitamins are classified as either primary or secondary. A primary deficiency occurs when you do not get enough of the vitamin in the food you eat. A secondary deficiency may be due to an underlying disorder that prevents or limits the absorption or use of the vitamin, due to a “lifestyle factor”, such as smoking, excessive alcohol consumption, or the use of medications that interfere with the absorption or the body's use of the vitamin. Individuals who eat a varied diet are unlikely to develop a primary vitamin deficiency. Whereas, restrictive diets have the potential to cause prolonged vitamin deficits, which may result in often painful and potentially deadly diseases.

Because most vitamins are not stored in the body, a person must consume them regularly to avoid deficiency. Body stores for different vitamins vary widely; vitamins A, D, and B₁₂ are stored in significant amounts in the body, mainly in the liver, and an adult may be deficient in vitamin A and B₁₂ for long periods of time before developing a deficiency condition. Vitamin B₃, is not stored in the body in significant amounts, and stores may only last a couple of weeks.

Well-known vitamin deficiencies involve thiamine (beriberi), niacin (pellagra), vitamin C (scurvy) and vitamin D ( rickets). In much of the developed world, such deficiencies are rare due to; an adequate supply of food and the addition of vitamins and minerals, often called fortification, to common foods.

Vitamin side effects and overdose

In large doses some vitamins have documented side effects. Vitamin side effects tend to increase in severity with increasing dosage. The likelihood of consuming too much of any vitamin from food is remote, but overdosing from vitamin supplementation does occur. At high enough dosages some vitamins cause side effects, such as nausea, diarrhea, and vomiting. Unlike some of the side effects caused by drugs, vitamin side effects rarely cause any permanent harm. When vitamin side effects emerge, recovery is often accomplished by reducing the dosage. Furthermore, the concentrations of vitamins an individual can tolerate vary widely, and appear to be related to age and state of health.

It is for these reasons that physicians and scientists carefully review the clinical data on supplement use in order to determine upper dosage thresholds for each vitamin that can be tolerated as a daily dose by the entire population without side effects. This dosage is known as the tolerable upper intake level (UL).

Supplements

Dietary supplements are often used to ensure that adequate amounts of nutrients are obtained on a daily basis, if the nutrients cannot be obtained through a varied diet. Scientific evidence supporting the benefits of some dietary supplements is well established for certain health conditions, but others need further study.

Supplements are, as required by law, not intended to treat, diagnose, mitigate, prevent, or cure disease. In some cases, dietary supplements may have unwanted effects, especially if taken before surgery, with other dietary supplements or medicines, or if the person taking them has certain health conditions. Vitamin supplements may also contain levels of vitamins many times higher, and in different forms, than one may ingest through food. Before taking a supplement, it is important to check with a knowledgeable health care provider, especially when combining or substituting supplements with other foods or medicine.

Governmental regulation of vitamin supplements

Most countries place dietary supplements in a special category under the general umbrella of "foods," not drugs. This necessitates that the manufacturer, and not the government, be responsible for ensuring that its dietary supplement products are safe before they are marketed. Unlike drug products, that must implicitly be proven safe and effective for their intended use before marketing, there are often no provisions to "approve" dietary supplements for safety or effectiveness before they reach the consumer. Also unlike drug products, manufacturers and distributors of dietary supplements are not generally required to report any claims of injuries or illnesses that may be related to the use of their products however, side effects have been reported for several types of vitamin supplements.

Names in current and previous nomenclatures

The reason the set of vitamins seems to skip directly from E to the rarely-mentioned K is that the vitamins corresponding to "letters" F-J were either reclassified over time, were discarded as false leads, or were renamed because of their relationship to "vitamin B", which became a "complex" of vitamins. The following table lists chemicals that had previously been classified as vitamins, as well as the earlier names of vitamins that later became part of the B-complex.