is also known as provitamin A, because it is one of the most important
precursors of vitamin A in the human diet. If you compare the two
molecules, it is clear that vitamin A (retinol) is very closely related
to half of the beta-carotene molecule.
- click on image to view Chime version
A - click on image to view Chime version
There are two ways in which beta-carotene can be converted to vitamin
A: either by cleavage at the centre or by breaking the molecule down from
one end (see
here for a diagram illustrating these processes). The second
of these is thought to be most important biologically. The breakdown
of beta-carotene occurs in the walls of the small intestine (intestinal
mucosa) and is catalysed by the enzyme beta-carotene dioxygenase. The
retinol formed is stored in the liver as retinyl esters. This is why
cod liver oil used to be taken as a vitamin A supplement. It is also
why you should never eat polar bear liver if you run out of food in the Arctic;
vitamin A is toxic in excess and a modest portion of polar bear liver contains
more than two years supply!
Beta-carotene, on the other hand, is a safe source of vitamin A.
The efficiency of conversion of beta-carotene to retinol depends on
the level in the diet. If you eat more beta-carotene, less is converted,
and the rest is stored in fat reserves in the body. So too much beta-carotene
can make you turn yellow, but will not kill you with hypervitaminosis.
Vitamin A has several functions in the body. The most well known
is its role in vision - hence carrots "make you able to see in the dark".
The retinol is oxidised to its aldehyde, retinal, which complexes with a
molecule in the eye called opsin. When a photon of light hits the complex,
the retinal changes from the 11-cis form to the all-trans form, initiating
a chain of events which results in the transmission of an impulse up the
optic nerve. A more detailed explanation is given
here. Other roles of vitamin A are much less well understood.
It is known to be involved in the synthesis of certain glycoproteins, and
that deficiency leads to abnormal bone development, disorders of the reproductive
system, xerophthalmia (a drying condition of the cornea of the eye) and