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Vitamin A

Vitamin A covers both a pre-formed vitamin, retinol, and a provitamin, beta carotene, some of which is converted to retinol in the intestinal mucosa.

One International Unit (IU) of vitamin A is equivalent to 0.3 micrograms of retinol (or 0.55 micrograms of retinol palmitate).

Sources of Vitamin A

Vitamin A is widely distributed in animal and plant foods. It is found in animal foods as preformed vitamin A (retinol) and in plant foods as provitamins (carotenes).

Animal foods:

Liver, eggs, butter, cheese, whole milk, fish and meat. Fish liver oils are the richest natural sources of retinol, but they are generally used as nutritional supplements rather than as food sources.

Plant foods:

The cheapest source of vitamin A is green leafy vegetables such as spinach and amaranth, which are found in great abundance in nature throughout the year. The darker the green leaves, the higher its carotene content. Vitamin A also occurs in most green and yellow fruits and vegetables (e.g. papaya, mango, pumpkin) and in some roots (e.g. carrots). The most important carotenoid is beta carotene which has the highest vitamin A activity. Carotenes are converted to vitamin A in small intestine. This action is poorly accomplished in malnourished children and those suffering from diarrhoea.

Fortified foods:

Foods fortified with vitamin A (e.g. vegetable oils, margarine, milk, bread) can be an important source.

Retinol content of selected foods (micrograms per 100 grams)
Foods of Animal origin
Foods of plant origin
Halibut liver oil 900,000 Carrot 1167
Cod liver oil 18,000 Spinach 607
Liver (ox) 16,500 Amaranth 515
Margarine 900 Mango, ripe 313
Butter 825 Green leaves 300
Cheese 350 Papaya 118
Egg 140 Tomato 84
Fish 40 Orange 25
Milk (cow) 38    

The liver has an enormous capacity for storing vitamin A, mostly in the form of retinol palmitate. Under normal conditions, a well-fed person has sufficient vitamin A reserves to meet his needs for 6 to 9 months or more. Free retinol is highly active, but toxic, and is therefore transported in the blood stream in combination with retinol-binding protein, which is produced by the live. In severe protein deficiency, decreased production of retinol-binding protein prevents mobilisation of liver retinol reserves.

Functions of vitamin A

Indispensable for normal vision: It contributes to the production of retinal pigments which are needed for vision in dim light.

It is necessary for maintaining the integrity and the normal functioning of glandular and epithelial tissue which lines the intestinal, respiratory and urinary tracts, as well as the skin and the eyes.

It supports growth, especially skeletal growth.

It is anti-infective. There is increased susceptibility to infection and lowered immune response in vitamin A deficiency.

It may protect against some epithelial cancers such as bronchial cancers, but the data are not fully consistent.

Deficiency of Vitamin A

Despite the large storehouse of vitamin A in human liver, deficiency can occur in periods of prolonged vitamin a deficiency. Vitamin A deficiency is especially common in growing children and adolescents.

The signs of vitamin A deficiency are predominantly ocular (related to eyes). They include the following:

Nightblindness: Lack of vitamin A first causes nightblindness, or the inability to see in dim light. The affected person becomes progressively unable to see clearly in late evenings. Vision in darkened atmospheres (such as a dimly lit room) is poor. Night blindness is due to impairment in dark adaptation. Unless vitamin A intake is increased, the condition may get worse, especially when children also suffer from diarrhoea and other infections.

Conjunctival xerosis: This is the first clinical sign of vitamin A deficiency. The conjunctiva becomes dry and non-wettable. Instead of looking smooth and shiny, it appears muddy and wrinkled.

Bitot's spots: these are triangular, pearly white, or yellowish-foamy spots on the bulbar conjunctiva, on either side of the cornea. They are frequently bialteral. Bitot's spots in young children usually indicate vitamin A deficiency. In older individuals, these spots are often inactive sequelae of earlier disease.

Corneal xerosis: This stage is particularly serious. The cornea appears dull, dry and non-wettable, and eventually opaque. It does not have a moist appearance. In more severe deficiency, there may be corneal ulceration. The ulcer may heal, leaving a corneal scar which can affect vision.

Keratomalacia: This means liquefaction of the cornea. It is a grave medical emergency. The cornea (a part, or the whole) may become soft, and may burst open. The process is a rapid one. If the eye collapses, vision is lost. Keratomalacia is one of the major causes of blindness in India and is frequently associated with protein energy malnutrition.

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This page last updated on:
April 12, 2005