Vitamin A is found in nature in the form of its precursors the carotenoid provitamins (the yellow and orange parts of most fruits and vegetables). It is formed in the body of man from these precursors after hydrolysis which gives one or two molecules of Vitamin A per carotene molecule. The conversion of cartene to Vitamin A takes place chiefly in the cells of the intestines.
Vitamin A plays an important role in normal vision. It is found in the retina in the form of visual pigments in the photoreceptor system of rods and cones. Rods are sensitive to light of low intensity and cones perceive light of high intensity and colours. It is required for proper growth of developing bony structures and teeth. It is needed for good, healthy, moist and glossy skin, bright clear eyes, normal secretion of tear glands, and is regarded as an anti-infective vitamin.
The liver stores approximately 95% of the body’s Vitamin A reserves. This amount is smallest at birth and in childhood, but increases as age advances. In a healthy person Vitamin A, which is reserved in the liver, is sufficient for one or more years. Normal adult liver contains 100 to 300 mg per gram of tissue. Vitamin A need is increased during pregnancy and lactation.
Causes of Vitamin A Deficiency
(i) A low dietary intake of Vitamin A or its precursors, the carotenoids,
A low dietary intake of Vitamin A or its precursors, the carotenoids, can result in a deficiency. This is usually seen only in people whose diet has been lacking for a long time in dairy products, green leafy vegetable and suitable fruits. Infants develop Vitamin A deficiency more easily as they are born with relatively small reserves.
(ii) Impaired absorption of Vitamin A
Impaired absorption of Vitamin A can be a major cause of deficiency. Failure to convert dietary carotene into biologically active Vitamin A can take place as a result of severe liver disease, diabetes, hypertension and body reserve may be depleted, as during pregnancy and lactation.
Lack of fat in the diet leads to improper absorption of the fat soluble vitamins. Absorption is also impaired in a condition where a major portion of the intestines has been surgically removed or in a condition called steatorrhea (small gut insufficiency). Excessive intake of mineral oil is another cause of malabsorption of Vitamin A, as much of it gets excreted in the faeces.
Bile helps in the absorption of Vitamin A and carotene. When there is an obstruction in the bile duct or low fact intake, absorption of Vitamin A is seriously impaired. The conversion of carotene into Vitamin A in the intestinal mucosa is helped by the presence of insulin and thyroxine. An absence of these hormones can be detrimental to Vitamin A production in the body.
When the absorption of Vitamin A is poor, the reserve in the liver is used up, which in turn can lead to a deficiency.
(iii) Nutritional status of mothers.
The breast milk of a healthy woman contains about 137 I.U. of Vitamin A per 100 g. When a mother’s diet does not provide adequate Vitamin A its concentration in breast milk is decreased. The infant feeding on it is likely to have a diminished amount of Vitamin A. Studies done at the National Institute of Nutrition (NIN) have shown that the liver reserves of the foetus are also diminished by maternal stress.
(iv) Dietary deficiency like PEM.
Regarding inter-dependence of protein of Vitamin A there are two observations. In patients suffering from kwashiorkor, the total serum protein as well as globulin ratio is considerably reduced. This finally reduces the flow of liver Vitamin A into the blood stream. The second is that if protein intake is unbalanced it reduces Vitamin A requirements in its mobilisation. Protein energy malnutrition (PEM) is a major cause of preventable blindness and occurs because of poverty, ignorance and poor dietary practices.
(v) Role of infection in the absorption of Vitamin A.
According to studies carried out at NIN the widespread occurrence of Vitamin A deficiency amongst children is due to infections which lower the absorption of Vitamin A. Infections such as meningococcal meningitis (inflammation of the meanings or specific brain cells), diarrhea, convulsions, tuberculosis with fever, measles, whooping cough, severe chicken pox and other acute infections result in the decrease of Vitamin A stores and the blood. Vitamin A deficiency results in keratomalacia or permanent blindness. Among children, during pneumonia, reheumatoid arthritis, acute tonsillitis and rheumatic fevers, blood Vitamin A levels is reduced.