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).
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.
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.
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
|Cod liver oil
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
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
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.