Description
A vitamin is an organic compound required by an organism as a vital
nutrient in limited amounts. An organic chemical compound (or
related set of compounds) 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 on the particular organism. For example, ascorbic
acid (vitamin C) is a vitamin for humans, but not for most other
animals, and biotin and vitamin D are required in the human diet
only in certain circumstances.
By convention, the term vitamin includes neither other essential
nutrients, such as dietary minerals, essential fatty acids, or
essential amino acids (which are needed in larger amounts than
vitamins) nor the large number of other nutrients that promote
health but are otherwise required less often.Thirteen vitamins are
universally recognized at present.
Vitamins are classified by their biological and chemical activity,
not their structure. Thus, each "vitamin" refers to a number of
vitamer compounds that all show the biological activity associated
with a particular vitamin. Such a set of chemicals is grouped under
an alphabetized vitamin "generic descriptor" title, such as
"vitamin A", which includes the compounds retinal, retinol, and
four known carotenoids. Vitamers by definition are convertible to
the active form of the vitamin in the body, and are sometimes
inter-convertible to one another, as well.
Vitamins have diverse biochemical functions. Some have hormone-like
functions as regulators of mineral metabolism (such as vitamin D),
or regulators of cell and tissue growth and differentiation (such
as some forms of vitamin A). Others function as antioxidants (e.g.,
vitamin E and sometimes vitamin C). The largest number of vitamins
(such as B complex vitamins) function as precursors for enzyme
cofactors, that help enzymes in their work as catalysts in
metabolism. In this role, vitamins may be tightly bound to enzymes
as part of prosthetic groups: For example, biotin is part of
enzymes involved in making fatty acids.
Vitamins may also be less tightly bound to enzyme catalysts as
coenzymes, detachable molecules that function to carry chemical
groups or electrons between molecules. For example, folic acid may
carry methyl, formyl, and methylene groups in the cell. Although
these roles in assisting enzyme-substrate reactions are vitamins'
best-known function, the other vitamin functions are equally
important.
Until the mid****0s, when the first commercial yeast-extract
vitamin B complex and semi-synthetic vitamin C supplement tablets
were sold, vitamins were obtained solely through food intake, and
changes in diet (which, for example, could occur during a
particular growing season) usually greatly altered the types and
amounts of vitamins ingested. However, vitamins have been produced
as commodity chemicals and made widely available as inexpensive
semisynthetic and synthetic-source multivitamin dietary and food
supplements and additives, since the middle of the *0th century.
