Discovered in the year 1913, it was initially termed as the "anti-infective vitamin", mainly
because it was found to confer resistance against various infectious diseases. Even before
the discovery of this fatsoluble vitamin, cod liver oil was being used for the treatment of
xerophthalmia and rickets. It was in 1922 that McCollum and his team identified the presence
of two fat-soluble vitamins (A&D) in cod liver oil which were effective for treatment of these
conditions. Further study of this vitamin aided chemists to classify it as a fat-soluble organic
compound which exists in two main forms, viz. preformed vitamin- retinol and provitamin
carotenoids. Provitamin carotenoids include acarotene, B-carotene and y-carotene. Retinol is
mainly obtained from animal and dairy products, while B-carotene is obtained from plant
sources.
A recent report has stated that worldwide approximately 4.4 million
preschool kids develop visible vision damage
due to Vitamin A deficiency, and close to 20 million pregnant females in developing
countries suffer from this deficiency.
Out of these affected population, nearly one-half of the
cases occur in India.
Mechanism of absorption in the body
Since vitamin A is fat-soluble in nature, it cannot be directly absorbed from the
diet, and hence it is first solubilised to micelles in the intestinal lumen and further absorbed
by the duodenal mucosal cells.
The provitamin A obtained from plant source, is absorbed
through the linings of the small intestine which converts it to retinol and can be stored in
the liver as retinyl esters.
Retinol is transported in the bloodstream by the Retinol Binding
Protein (RBP), which is synthesised in the liver.
Thus, carotenoids undergo a series of enzymatic conversion from retinol, further oxidised to
retinal and finally retinoic acid.
Prevalence of deficiency
Vitamin A deficiency is the leading cause of preventable blindness in children. It also
increases risk of various diseases and in some cases death from severe infections. In
many pregnant women, deficiency has also been associated with increased risk of
maternal mortality. Globally, preschool aged children and women of reproductive age
are the worst affected groups and are at the highest risk of developing nutritional
deficiency related disorders. A recent report has stated that worldwide approximately
4.4 million preschool kids develop visible vision damage due to vitamin A deficiency,
and close to 20 million pregnant females in developing countries suffer from this
deficiency. Out of these affected population, nearly one-half of the cases occur in
India.
Sources
The major sources for this powerful vitamin include vegetables like sweet potato, spinach,
carrots, tomato, etc. Animal sources include hard boiled egg, chicken breast, salmon, tuna,
etc. Apart from these, yogurt, pistachios, mangoes and dried apricots are also packed with
vitamin A. Fortified ready-to-eat cereals are also available these days, and in case of detected
deficiency, supplements of vitamin A are also available to be prescribed by the physician. It
usually contains vitamin A in the form of retinyl palmitate or retinyl acetate."
Functions
- Promotes growth of strong teeth and bones.
- Plays an important role in normal brain develop
ment and functioning of the nervous system.
- Maintenance of a healthy skin texture and prevents dry skin.
It is essential for maintaining a good eye-sight and promoting vision especially in low light
as it is involved in synthesis of certain pigments in the retina.
- Carotene acts as an antioxidant and protects cell damage by free radicals, and plays a
crucial role in preventing cancer development.
Deficiency disorders
Vitamin A deficiency, clinically can cause several complications, majorly Xeropthalmia.
Xerophthalmia
This condition encompasses all the defects which affect the eyes. It can occur secondary to
many clinical autoimmune disorders like Sjogren's syndrome, rheumatoid arthritis, systemic
lupus erythematosus, as side affects of certain medicines and also due to deficiency of vitamin
A. It results due to inadequate functioning of the lacrimal glands, which is responsible for forming
a tear film over the cornea. Thus, this condition is also termed as "dry eyes". If xerophthalmia is
due to vitamin A deficiency, the condition begins with night blindness and conjunctival xerosis
(dryness of the eye membranes), progresses to bitot's spot and corneal xerosis (dryness of the
cornea), and in its later stages develops into keratomalacia (softening of the cornea).
1. Night blindness (XN)
As the name implies, XN indicates complete lack of vision at night, or inability to see at night
or in low light and is the first symptom of vitamin A deficiency Human retina has specialised
photo receptor cells called rods (for light vision) and cones (for colour vision), of which rods
are imperative for good vision especially in low light or night. XN arises due to reduced synthesis
of the pigment rhodopsin present in the rods due to vitamin A deficiency. Though XN can be both
hereditary and acquired, most of the cases in developing countries are acquired due to malnutrition.
2. Conjunctival xerosis (X1A)
It is a condition accompanied by extreme dryness of the eye surfaces caused by deficiency
of tears or conjunctival secretions. It is mainly caused due to loss of goblet cells of the
conjunctival epithelium. This condition precedes the more severe stage of corneal xerosis.
3. Bitot's spot (X1B)
It involves build-up of keratinised material on the surface of conjunctiva, and can be oval,
triangular or irregular in shape and is usually presented on the temporal (outer) side of the
conjunctiva which is not covered by the eyelids. Extreme case of conjunctival xerosis leads
to Bitot's spot. Histopathologically it is characterised by presence of a thickened superficial
layer of flattened cells having a frayed keratinised surface with prominent gram-positive
bacteria or in some cases even fungi.
4. Corneal xerosis (X2)
This is a more advanced stage of lesions which represent severe vitamin A deficiency.
However, it is highly reversible and can be treated using external
vitamin A supplements
5. Keratomalacia (X3A, X3B)
This is the last stage of xerophthalmia where there is complete destruction of cornea.
It progresses rapidly and arises due to sudden and severe vitamin A deficiency. It usually
affects both eyes and the severity can vary from mild to severe. In extreme cases, it can
also cause blindness and this stage is seen to affect younger children at a higher rate
6. Corneal scars (XS)
Corneal scars occur due to injury, infection or xerophthalmia. Survivors of keratomalacia
develop scars of various sizes on one or both sides of cornea. These can be of two forms
viz. active and inactive. Inactive form of corneal scars are very commonly seen in young
children as a sequelae to keratomalacia
Apart from Xeropthalmia, other conditions like cancer, measles and age-related macular
degeneration (AMD) also occur due to vitamin A deficiency. AMD has been highlighted
to be one of the major causes of vision loss in the older age group. Lower levels of
vitamin A has also been shown to increase the severity and complications associated
with measles.
Diagnosis of "Vitamin A" deficiency disorders
Assessing physical signs and clinical symptoms is essential to determine any nutritional
deficiency disorder. Once the signs and symptoms have been assessed by the physician,
following tests may be recommended for accurate diagnosis of deficiency. These are:
1. Lissamine green staining
It involves staining of the conjunctiva, and is a new and widely heralded aid for diagnosing
early xerophthalmia. Extent of damage or staging of the condition is done by observing the
stained cornea.
2. Estimating vitamin A levels - Serum estimation of vitamin A levels is also
considered to be an efficient diagnostic tool to detect deficiency. Technologies like LC-MS/MS
(Liquid Chromatography-Mass Spectrometry) have been widely recognised for their sensitivity
and specificity for diagnosis of micro-vitamin deficiency disorders as well as toxicity.
Treatment of disorders
Treatment of any nutritional deficiency involves diet pattern modification to include foods rich
in the deficient nutrient. In case of vitamin A deficiency, inclusion of coloured vegetables rich
in carotene along with supplementation is recommended. Limiting uptake of solid fats,
cholesterol and salt while maintaining a healthy calorie intake is also a must.
Vitamin A toxicity
Hypervitaminosis can be majorly caused either due to excessive dietary intake of vitamin
A or over consumption of supplements. Since, these are fat-soluble, they tend to accumulate
in the liver and are not easily excreted by the body. Though, excess retention of B-carotene
and other provitamin A is not deleterious, high levels of vitamin A can cause symptoms like
nausea, headache, dizziness, skin irritation, joint pain, coma and even death.
