Vitamin A
Vitamin A (retinol) is required by humans for the normal
functioning of the visual system. Retinol is transported to ocular tissue and
to the retina of the eye by intracellular binding and transport protein in
which it plays an important part in the formation of rhodopsin, an important
visual pigment, particularly for dim-light vision. All-trans retinol is
converted to retinaldehyde, isomerised to the 11-cis form and bound to opsin to
form rhodopsin. When there is insufficient amount of retinol available,
rhodopsin synthesis is affected and night blindness may result. The condition
can, however, also be due to a lack of other nutrients which are critical to
the regeneration of rhodopsin such as protein and zinc.
The second main function of vitamin A is in the maintenance
of growth and epithelial cellular integrity and immune function in the body.
Thus, in vitamin A deficiency, the number of goblet cells are reduced in
epithelial tissues, resulting in a reduction in mucous secretions with their
antimicrobial components. Cells lining protective tissue surfaces flatten and
accumulate keratin because they fail to regenerate and differentiate. All these
changes result in diminished resistance to invasion by potentially pathogenic
organisms. The immune system is also adversely affected by direct interference
with production of some types of protective secretion and cells. As these
changes in internal epithelial tissues occur, the external reflections of such
changes are seen in the classical eye changes in xerophthalmia and xerosis.
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