'Vitamin A' includes both retinol(a performed vitamin) and beta carotene(a provitamin).The international unit(IU) of vitamin A is equivalent to 0.3 microgram of retinol or 0.55 mcg of retinol palmitate.There are seperate values for retinol and beta-carotene in different foods.The term retinol equivalent (RE) has been conventionally adopted to convert these into a single value.The equivalents of RE are as follows:
1 mcg of retinol=1 mcg of RE.
1 mcg of B-carotene=0.167 mcg of RE.
1 mcg of other carotenoids=0.084 mcg of RE.
Functions of vitamin A:
The role of vitamin A at molecular level is unknown.This vitamin is essential for vision.It helps in the production of retinol pigments needed for vision in dim light.Vitamin A maintains functioning and integrity of gladular and epithelial tissues.It helps in skeletal growth and has an anti-infective action.It provides protection against some epithelial cancers like bronchial cancer.
Sources of vitamin A:
The animal sources are the important sources of retinol and include fish,liver,butter,ghee,egg,yolk,and whole milk.The plant sources are rich in carotenes and include carrot,green leafy vegetables,mango and orange etc.
The recommended daily intake of vitamin A is 750 micrograms as retinol(or 3000 microgram as B-carotene)in adults and adolescents.In children the requirement varies from 250 to 600 micrograms of retinol or 1000 to 2400 microgram of B-carotene.
Deficiency of vitamin A:
Vitamin A deficiency commonly affects the eyes but extaocular effects can also be noted.
The short description of common eye manifestations due to vitamin A deficiency is given below.
a)Night blindness is the inability to see in dim light due to impairment in dark adaptation.The condition worsens when child suffers from recurrent infections or diarrhoea.
b)Conjunctival xerosis is the first sign of vitamin A deficiency.In this condition the secretions of tears is decreased and the conjunctiva of the eye becomes dry,wrinkled and muddy.
c)Bitot's spots are seen on the bulbar conjuctiva on either side of the cornea.They are mostly bilateral and appear as triangular pearly white or yellowish foamy spots.
d)Corneal xerosis is a condition of eye in which the cornea becomes dry,dull non-wettable and opaque.In severe deficiency cases even corneal ulcers are seen.
e)Keratomalacia also called liquification of the cornea.In this condition the cornea at places becomes soft and even burst open.The eye may collapse and the vision is lost.Keratomalacia is one of the major causes of blindness in our country.