Nutritional Pathology Online
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Importance of Vitamin A in our daily diet
Pathology of Vitamin A deficiency
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As a toddler I enjoyed eating carrots and green leafy vegetables like spinach which surprised everyone.
Years later when I became a doctor I realized that my mother was not only an expert nutritionist, she also understood child psychology.
She prepared mouth watering vegetarian delicacies and awesome sweet dishes with sliced carrots in a way that I could never turn up my nose and throw tantrums.
Carrots and other green vegetables are rich in Vitamin A and there is no toxic effect on children, unlike cod-liver oil which can cause toxicity if taken in large quantities.
In this post I will highlight few important facts about Vitamin A.
Vitamin A is one of the fat-soluble vitamins which is important to the health of skin, teeth and bones.
In herbivorous animals the carotenoid pigments, such as the carotenes of plants and vegetables, are converted into vitamin A in the intestine, absorbed, and subsequently stored in the liver.
Human beings acquire their supply either directly from the carotenoids in vegetable matter (Example: Carrots, beetroot, and green vegetables in general) or indirectly from animal or dairy products (Example:Preformed Vitamin A are highest in liver and fish oils. Milk and eggs also contain preformed vitamin A).
The concurrent absorption of fat and the presence of bile salts in the intestine favour vitamin A absorption.
Vitamin A deficiency is usually due to a deficient diet which is a common problem in the developing world.
Vitamin A deficiency may also occur as a complication of the malabsorption syndrome.
The blood level of carotene or vitamin A is used as a measure of the severity of the malabsorption.
Effects of Vitamin A deficiency:
In vitamin A deficiency the most important effect is on the eye.
Vitamin A forms an essential component of rhodopsin, a pigment in the rods of the retina.
By absorbing light, rhodopsin initiates an electrical impulse that is transmitted to the brain and is interpreted as light.
The first sign of vitamin A deficiency, therefore, is night blindness.
It is known that vitamin A regulates the structure of certain epithelial cells.
The conjunctival epithelium loses its mucus-secreting goblet cells and becomes keratinized, thereby taking on the characteristics of epidermis.
An early sign is the formation of desquamating hyperkeratotic plaques on the conjunctiva at the lateral margin of the cornea.
These are triangular in shape, and appear as areas of dry foam. They are known as Bitot’s spots.
The lacrimal ducts show hyperkeratosis, so that the eye becomes dry and subject to cracking and infection.
The condition is called Xerophthalmia.
In due course the cornea becomes cloudy, with infection it can soften (keratomalacia), so that the globe may become perforated.
The lens is then extruded, and blindness results.
Indeed vitamin-A deficiency is a leading cause of blindness in the world.
The skin shows dryness and follicular plugging. Stratified squamous epithelium keratinizes and the keratin debris blocks the sweat glands and lacrimal glands.
Follicular hyperkeratosis is a skin disorder that results from occluded sebaceous glands.
This feature is often noted in patients with vitamin A deficiency.
Whether these changes are specific for vitamin A deficiency is not clear.
Other effects: The replacement of respiratory-type epithelium (with its goblet cells and cilia) by keratinized squamous epithelium is a factor in the production of respiratory infections.
The nasal passages, trachea and bronchi shows a transformation of the lining cells into a stratified epithelium which undergo extensive keratinization.
Vitamin A deficiency has been noted in patients with tuberculosis. Low serum retinol levels return to normal after antituberculosis treatment.
The deficiency of vitamin A in patients with tuberculosis may have contributed to the development of tuberculosis.
In the presence of a normal amount of vitamin A there is no evidence that an excessive intake can prevent respiratory infections.
In vitamin A deficiency the epithelial lining of the genitourinary tract (renal pelvis) tend to undergo squamous metaplasia.
Epithelial changes in the renal pelvis are occasionally associated with kidney stones.
Kidney Stones Uterus Pancreas with duct
The lining epithelium of the pancreatic ducts, uterus, and salivary glands are also commonly affected.
In spite of the changes seen in the epithelium in vitamin-A deficiency, the biochemical nature of the defect is unknown.
If a small piece of skin is grown in organ culture and subjected to vitamin-A deficiency, it soon shows hyperkeratosis.
If an excess quantity of vitamin A is now added to the medium, the epithelium changes to a goblet-cell, mucous-secreting type.
There is no evidence, however, that excess vitamin-A in the human can convert normal keratinized epidermis into mucus secreting epithelium.
Toxicity of vitamin A.
Very large doses of vitamin A are toxic and can cause an increase in intracranial pressure with headache, blurring of vision,vomiting, and drowsiness.
This effect has been described by Arctic explorers when they ate polar bear liver, which is a very rich source of vitamin A.
Chronic poisoning can cause loss of hair, bone pains, calcification of ligaments and tendons, hyperpigmentation of the skin, liver damage, and psychiatric symptoms.
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