Vitamin A
Vitamin A is available to the body in two forms: 1.Pure vitamin A (Retinol), or 2.Pro-vitamin A (Beta carotene).
Retinol is a yellow fat soluble alcohol, and is found in foods which come from animal sources (in this case, fish and dairy products). It is insoluble in water but is soluble in organic solvents such as alcohol. It is heat stable, but can be destroyed by prolonged exposure to high temperatures. It can also be destroyed by oxygen when it is exposed to light and air.
It is necessary for the production of rhodopsin, a pigment in the retina of the eye which allows the eye to adapt to dim light. It is also needed to maintain healthy lining membranes for example, in the eye, and in the digestive and respiratory tracts. This helps to prevent the entry of microorganisms. Retinol also keeps skin and hair healthy, and helps to regulate growth.
Good sources of retinol include 1.Fish liver oils such as cod or halibut; 2.Oily fish; 3.Butter; 4. Margarine; 5.Cheese; 6.Egg yolk. The Recommended Daily Allowance (RDA) of retinol for children aged 1 to 7 is 500 micrograms (i.e. 0.5 mg), and for adults is 700 micrograms (i.e. 0.7mg). During lactation (breast-feeding) about 950 micrograms is recommended (i.e. 0.95mg).
Retinol deficiency may result in: 1.Night blindness, in which the eyes are unable to adjust to the dim light; 2.Xerophthalmia, an eye infection which can lead to blindness; 3.Reduced resistance to infection; 4. Follicular keratosis (rough dry skin); 5.Retarded growth.
If the diet contains too much vitamin A the excess is accumulated in the liver, and causes hypervitaminosis. Symptoms of hypervitaminosis A include pain in bones, an enlarged liver and hair loss. It can cause death. Hypervitaminosis occurs very rarely, and is much more likely to be due to overconsumption of dietary supplements (for example, taking more than the recommended dose of cod liver oil), especially in babies and young children, than overconsumption of foods rich in...