Human Nutrition 3(3+0)

Lesson 20 : Vitamin – A

Physiological and Biochemical Functions

1. Vitamin A and Vision: Vitamin A plays a critical role in vision in dim light. The vitamin A visual cycle is shown in figure.
   
   
   According to Wald (1935) the characteristic pigments of the rods and cones in the retina are rhodopsin and iodopsin. They differ only in respect of the protein moieties. The specific pigment common to both is a cis-isomer of retinene. Vitamin A alcohol is oxidized to vitamin A aldehyde in the epithelium of the rods by alcohol dehydrogenase in the presence of NAD. The protein opsin reacts with retinene to form rhodopsin. Light initiates a series of photochemical changes in rhodopsin, beginning with bleaching of the purple pigment and ending with formation of all–Trans-retinene and its isomerisation. The resynthesis of rhodopsin is isomer specific, requiring the 11-cis-isomer of retinene.


2. Vitamin A and epithelial tissues: Vitamin A is essential for the integrity of the mucous secreting cells of epithelial tissues. In vitamin A deficiency, the epithelial tissues are keratinized. The tissues affected are salivary glands, respiratory tract, eyes, skin and sex organs.


3. Vitamin A and nerves: Vitamin A deficiency causes degeneration of the myelin sheath.


4. Vitamin A and bone: Vitamin A is essential for normal bone formation. Excess of vitamin A, however, is toxic and causes brittleness of bone and hence bone fractures.


5. Vitamin A and protein deficiency: The absorption and mobilization of vitamin A is impaired in protein malnutrition. When protein intake is adequate, vitamin A is mobilized from liver.


6. Mucopolysaccharides and sulphate metabolism: Vitamin A takes part in the incorporation of inorganic sulphate in mucopolysaccharide synthesis.


7. Vitamin A and mucoprotein synthesis: Vitamin A is essential for the synthesis of mucoproteins and glycoproteins.


8. Vitamin A and reproduction: In vitamin A deficiency, reproduction does not take place due to (I) infertility in the male and (II) failure of the female to conceive, if conceived resorption or abortion of the fetus occurs.