Vitamin A is not synthesized in the human body and must be supplied by food or supplements. Dietary adequacy of proteins and vitamin ‘E’ are necessary for proper and maximum utilization of the vitamin A.
One of the most important roles of vitamin A is its requirement for normal vision. Vitamin A is necessary for the health of the epithelial cells. It has also been mentioned as important in the body’s resistance to infection. This reaction may be explained by its relationship to the health of epithelial cells.
Effects of Deficiency of Vitamin A
(a) Changes in the Eye: The early symptoms start itching, burning and inflammation of eyelids followed by various eye diseases. Prolonged vitamin A deficiency results in a condition known as night blindness in which one is unable to see well in dim light.
Night blindness is more common in poor people. Males suffer more than females. Pregnant women suffer frequently from night blindness due to increased demand for vitamin A during pregnancy.
Xerophthalmia due to vitamin A deficiency is the most important single cause of blindness in many developing countries.
Various stages of Xerophthalmia are classified below:
(i) Only night blindness.
(ii) Xerosis of the conjunctiva with or without blindness and with or without Bitot’s spots.
(iii) Xerosis of cornea, superficial but reversible changes of the corneal epithelium.
(iv) Irreversible corneal changes, involving the corneal stroma, leading to the Keratomalacia (softening of cornea).
(v) scars giving rise to corneal opacity(nebula), a total or partial milk-white opacity, a protrusion of the cornea or shrinkage and wasting of the eye.
(b) Epithelial changes:
Vitamin A deficiency leads to degeneration and hardening (Keratinization) of epithelium. There is increased infection of eyes, nasal passages, sinuses, middle ear, pharynx, mouth, respiratory tract, lungs and urinogenital tract. The epithelium of eye gets so severely damaged that the condition is almost incurable.
(c) Skin changes: Deficiency of Vitamin A leads to xeroderma in which the skin becomes dry, rough and scaly. Follicle-like goose-pimples appear on the upper arms and thighs, shoulders, back, abdomen and buttocks. This condition is known as follicular hyperkeratosis.
(d) Changes in the Elementary Tract: Elementary tract dries up resulting in diminished secretion of digestive juices, lack of their absorption and increased susceptibility to intestinal infection and diarrhoea.
(e) Lowered Resistance to Infection: Vitamin A is needed for the maintenance and functioning of the mucous membranes of the body. Due to deficiency of Vitamin A, the membranes become thin, dry, porous and fleshy. They are unable to perform their normal protective functions.
(f) It has been found that Vitamin A deficiency results into retarded growth. Apparently it contributes in the growth of skeletal and soft tissues through an effect on protein synthesis, mitosis or stability of cell membranes.
(g) The rationoids are necessary for reproductive functions in both males and females. The deficiency of these rationoids causes glandular degeneration and sterility, testicular degeneration in off spring in female.
Requirement of Vitamin A ranges from 10,000 µg of β carotene to 4600 µg of β-carotene.
Vitamin A can be accumulated and stored in the liver and it has been found that administration of a single oral dose of about 2, 00,000 I.U. of vitamin A is safe and it can build upon sufficient stores to last for about six months.
But it is also important to note that vitamin A in high doses becomes toxic, therefore, dose of 2, 00, 000 I.U. should not be given more than once in six months.
Sources of Vitamin A:
The animal sources of vitamin A are liver, kidney, whole milk, egg and butter. The vegetable sources of Vitamin A (Precursor of Vitamin A) are yellow and green leafy vegetables. The amount of Vitamin A available in fruits and vegetables is related to intensity of yellow and orange color in them. Vitamin A is found in the following fruits and vegetables:
Green Leafy Vegetables:
Spinach, turnip tops, beet greens.
Green Stem Vegetables:
Apricot, peaches, papaya, mango, orange.
Vitamin A is stable to moderate heat and but it is destroyed by oxidation.