A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. A compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and the particular organism. For example, ascorbic acid functions as vitamin C for some animals but not others, and vitamins D and K are required in the human diet only in certain circumstances.The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids, nor does it encompass the large number of other nutrients that promote health but are otherwise required less often.
Vitamins are classified by their biological and chemical activity, not their structure. Thus, each "vitamin" may refer to several vitamer compounds that all show the biological activity associated with a particular vitamin. Such a set of chemicals are grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A," which includes the compounds retinal, retinol, and many carotenoids.Vitamers are often inter-converted in the body.
Vitamins have diverse biochemical functions, including function as hormones (e.g. vitamin D), antioxidants (e.g. vitamin E), and mediators of cell signaling and regulators of cell and tissue growth and differentiation (e.g. vitamin A). The largest number of vitamins (e.g. B complex vitamins) function as precursors for enzyme cofactor bio-molecules (coenzymes), that help act as catalysts and substrates in metabolism. When acting as part of a catalyst, vitamins are bound to enzymes and are called prosthetic groups. For example, biotin is part of enzymes involved in making fatty acids. Vitamins also act as coenzymes to carry chemical groups between enzymes. For example, folic acid carries various forms of carbon group – methyl, formyl and methylene - in the cell. Although these roles in assisting enzyme reactions are vitamins' best-known function, the other vitamin functions are equally important.
Until the 1900s, vitamins were obtained solely through food intake, and changes in diet (which, for example, could occur during a particular growing season) can alter the types and amounts of vitamins ingested. Vitamins have been produced as commodity chemicals and made widely available as inexpensive pills for several decades, allowing supplementation of the dietary intake.
In humans
Vitamins are classified as either water-soluble or fat soluble. In humans there are 13 vitamins: 4 fat-soluble (A, D, E and K) and 9 water-soluble (8 B vitamins and vitamin C).
Water-soluble
Water-soluble vitamins dissolve easily in water, and in general, are readily excreted from the body, to the degree that urinary output is a strong predictor of vitamin consumption.Because they are not readily stored, consistent daily intake is important.Many types of water-soluble vitamins are synthesized by bacteria.
Fat-soluble
Fat-soluble vitamins are absorbed through the intestinal tract with the help of lipids (fats). Because they are more likely to accumulate in the body, they are more likely to lead to hypervitaminosis than are water-soluble vitamins. Fat-soluble vitamin regulation is of particular significance in cystic fibrosis.
List of vitamins
Each vitamin is typically used in multiple reactions and, therefore, most have multiple functions.
Vitamin generic
descriptor name Vitamer chemical name(s) (list not complete) Solubility Recommended dietary allowances
(male, age 19–70)Deficiency disease Upper Intake Level
(UL/day) Overdose disease
Vitamin A Retinoids
(retinol, retinoids
and carotenoids) Fat 900 µg Night-blindness and
Keratomalacia 3,000 µg Hypervitaminosis A
Vitamin B1 Thiamine Water 1.2 mg Beriberi, Wernicke-Korsakoff syndrome N/D Rare hypersensitive reactions resembling anaphylactic shock—injection only;
Drowsiness
Vitamin B2 Riboflavin Water 1.3 mg Ariboflavinosis N/D ?
Vitamin B3 Niacin, niacinamide Water 16.0 mg Pellagra 35.0 mg Liver damage (doses > 2g/day) and other problems
Vitamin B5 Pantothenic acid Water 5.0 mg Paresthesia N/D ?
Vitamin B6 Pyridoxine, pyridoxamine, pyridoxal Water 1.3–1.7 mg Anemia peripheral neuropathy. 100 mg Impairment of proprioception, nerve damage (doses > 100 mg/day)
Vitamin B7 Biotin Water 30.0 µg Dermatitis, enteritis N/D ?
Vitamin B9 Folic acid, folinic acid Water 400 µg Deficiency during pregnancy is associated with birth defects, such as neural tube defects 1,000 µg Possible decrease in seizure threshold
Vitamin B12 Cyanocobalamin, hydroxycobalamin, methylcobalamin Water 2.4 µg Megaloblastic anemia N/D No known toxicity
Vitamin C Ascorbic acid Water 90.0 mg Scurvy 2,000 mg Vitamin C megadosage
Vitamin D Ergocalciferol, cholecalciferol Fat 5.0 µg–10 µg Rickets and Osteomalacia 50 µg Hypervitaminosis D
Vitamin E Tocopherols, tocotrienols Fat 15.0 mg Deficiency is very rare; mild hemolytic anemia in newborn infants.1,000 mg Increased congestive heart failure seen in one large randomized study.
Vitamin K phylloquinone, menaquinones Fat 120 µg Bleeding diathesis N/D Increases coagulation in patients taking warfarin.
In nutrition and diseases
Riboflavin (Vitamin B2)
Vitamins are essential for the normal growth and development of a multicellular organism. Using the genetic blueprint inherited from its parents, a fetus begins to develop, at the moment of conception, from the nutrients it absorbs. It 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 may cause permanent damage.
For the most part, vitamins are obtained with food, but a few 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 the natural ultraviolet wavelength of sunlight. Humans can produce some vitamins from precursors they consume. Examples include vitamin A, produced from beta carotene, and niacin, from the amino acid tryptophan.
Once growth and development are completed, vitamins remain essential nutrients for the healthy maintenance of the cells, tissues, and organs that make up a multicellular organism; they also enable a multicellular life form to efficiently use chemical energy provided by food it eats, and to help process the proteins, carbohydrates, and fats required for respiration.
Deficiencies
Deficiencies of vitamins are classified as either primary or secondary. A primary deficiency occurs when an organism does not get enough of the vitamin in its food. 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 use of the vitamin. People who eat a varied diet are unlikely to develop a severe primary vitamin deficiency. In contrast, restrictive diets have the potential to cause prolonged vitamin deficits, which may result in often painful and potentially deadly diseases.
Because human bodies do not store most vitamins, humans must consume them regularly to avoid deficiency. Human bodily stores for different vitamins vary widely; vitamins A, D, and B12 are stored in significant amounts in the human body, mainly in the liver,and an adult human's diet may be deficient in vitamins A and B12 for many months before developing a deficiency condition. Vitamin B3 is not stored in the human body in significant amounts, so stores may only last a couple of weeks.
Well-known human vitamin deficiencies involve thiamine (beriberi), niacin (pellagra), vitamin C (scurvy) and vitamin D (rickets). In much of the developed world, such deficiencies are rare; this is due to (1) an adequate supply of food; and (2) the addition of vitamins and minerals to common foods, often called fortification.
Some evidence also suggests that there is a link between vitamin deficiency and mental disorders.
Side effects and overdose
In large doses, some vitamins have documented side effects that tend to be more severe with a larger 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.
When side effects emerge, recovery is often accomplished by reducing the dosage. The concentrations of vitamins an individual can tolerate vary widely, and appear to be related to age and state of health. In the United States, overdose exposure to all formulations of vitamins was reported by 62,562 individuals in 2004 (nearly 80% of these exposures were in children under the age of 6), leading to 53 "major" life-threatening outcomes and 3 deaths;a small number in comparison to the 19,250 people who died of unintentional poisoning of all kinds in the U.S. in the same year (2004).
Supplements
Dietary supplements, often containing vitamins, are used to ensure that adequate amounts of nutrients are obtained on a daily basis, if optimal amounts of 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.A meta-analysis in 2006 suggested that Vitamin A and E supplements not only provide no tangible health benefits for generally healthy individuals, but may actually increase mortality, although two large studies included in the analysis involved smokers, for which it was already known that beta-carotene supplements can be harmful. Another study released in May 2009 found that antioxidants such as vitamins C and E may actually curb some benefits of exercise.
In the United States, advertising for dietary supplements is required to include a disclaimer that the product is not intended to treat, diagnose, mitigate, prevent, or cure disease, and that any health claims have not been evaluated by the Food and Drug Administration.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.
Intake of excessive quantities can cause vitamin poisoning, often due to overdose of Vitamin A and Vitamin D. (The most common poisoning with multinutrient supplement pills does not involve a vitamin, but is rather due to the mineral, iron). Due to toxicity, most common vitamins have recommended upper daily intake amounts.
Since 2005, suppliers have distinguished their products as either Medical Grade or Pharmaceutical Grade products. Both of these classifications indicate products that are manufactured to be easily absorbed by the body. Normal vitamin manufacturing is not regulated in the United States to the same standards as are medicinal pharmaceuticals, although U.S. vitamins which are manufactured for food consumption by humans or animals must be manufactured to Food Chemicals Codex (FCC), grade, commonly called "food grade".
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, which must explicitly 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.
