Definition and classifi cation of vitamins

Vitamins are a group of organic compounds which
are essential in very small amounts for the normal
functioning of the body. Thirteen vitamins are recognized
in human nutrition and these have been classifi
ed, according to their solubility, into two groups. The
fat-soluble vitamins are represented by vitamins A, D,
E and K; also included are the 50 or so carotenoids
that possess varying degrees of vitamin A activity. The
water-soluble vitamins comprise vitamin C and the
members of the vitamin B group, namely thiamin
(vitamin B1), ribofl avin (vitamin B2), niacin, vitamin
B6, pantothenic acid, biotin, folate and vitamin
B12. Vitamins have widely varying biochemical and
physiological functions and are broadly distributed in
natural food sources.
For several of the vitamins, biological activity is
attributed to a number of structurally related compounds
known as vitamers. The vitamers pertaining
to a particular vitamin display, in most cases, similar
qualitative biological properties to one another, but,
because of subtle differences in their chemical structures,
exhibit varying degrees of potency.
It is often stated that vitamins cannot be produced
in the body and must, therefore, be supplied in the
diet. This statement is valid for many of the vitamins,
but is not strictly true for others. For example, vitamin
D can be formed in the skin upon adequate
exposure to ultraviolet radiation; vitamin K is normally
produced in suffi cient amounts by intestinal
bacteria; and niacin can be synthesized in vivo from
an amino acid precursor, L-tryptophan. With the
possible exception of vitamins D and K, vitamins
must be supplied by the diet because they cannot
be produced in adequate amounts by the human
body. Plants have the ability to synthesize most of
the vitamins and serve as primary sources of these
dietary essentials.