Vitamins are nutritionally essential organic compounds required in small quantities for normal biochemical functions. They cannot be synthesized in the body and hence, have to be supplied through the diet. Some of them may be synthesized in the body by the intestinal microorganisms. Vitamin A is cannot be synthesized by the body, and has to be taken from the external source.
Absence of relative deficiency of vitamins in the diet leads to characteristic deficiency states and diseases. Excess intake of some vitamins leads to toxicity, also known as Hypervitaminosis. Vitamins are broadly classified into two groups, i.e. water-soluble vitamins and fat soluble vitamins. Water-soluble vitamins are vitamin B complex, vitamin C and fat soluble vitamins are vitamin A, vitamin D, vitamin E, vitamin K.
3MEDS-online chemist shop, briefs on the sources, functions and deficiencies of vitamin A.
Vitamin A has a cyclohexenyl group and polyisoprenoid chain with a functional group. Depending on the functional groups present there are 3 different biologically active forms of vitamin A i.e. Retinol, Retinal, and Retinoic acid.
Vitamin A is absorbed in the intestinal lumen where the intestinal mucosal cell converts beta carotene into chylomicrons which is converted into retinol and stored in the liver. This stored retinol is then transported in the blood to target tissues.
Sources of vitamin A
Animal source: fish liver oils (cod liver oil & shark liver oil), milk, butter, cream, cheese, egg-yolk, liver, are very rich sources of vitamin A.
Vegetable source: vegetable containing yellow pigment- carotene like carrot, pumpkin, papaya, apple, mango and green leafy vegetables like spinach.
Recommended dietary allowance (RDA):
Men : 750 to 1000 micrograms/day
Women : 750 micrograms/day
Pregnancy : 1000 micrograms/day
Lactation : 1200 micrograms/day
Functions of vitamin A
· Vision: retinal form of vitamin A is responsible for the vision. Rods and cones present in the retina are responsible for vision, rods for dim light vision/ black and white vision and cones for bright light vision/ colour vision. Rods contain visual pigment rhodopsin and cones contain the visual pigment conopsin. When light falls on retina, it induces a conformational change in the protein leading to generation of nerve impulse responsible for vision.
Deficiency of vitamin A increases the dark adaptation time. It is ability of the eyes to adjust to the dim light area on sudden shifting from bright light.
· Cell differentiation: retinol and retinoic acid forms of vitamin-A is responsible for cell differentiation. Retinol and retinoic acid act as a hormone and affect the transcription of specific proteins.
Ø Maintenance of epithelial tissue: required for normal differentiation and mucous secretion of epithelial tissue, prevents keratinisation of epithelial tissues.
Ø Normal reproduction: supports spermatogenesis, oogenesis, and placental development.
Ø The preservation of proper immune system to combat against various infections.
· Antioxidant: beta-carotene is known to prevent the incidence of epithelial cancers, heart attacks.
Deficiencies of vitamin A:
Vitamin A deficiency is caused by malnutrition, steatorrhea as in obstructive jaundice and pancreatitis, cirrhosis of liver, chronic nephrosis.
Deficiency of vitamin A is manifested in eyes, skin and epithelium, and reproductive failure.
Eye manifestations:
Ø Night blindness or Nyctalopia: it is the earliest manifestation. Visual acuity is diminished in dim light. The dark time adaptation is increased.
Ø Conjunctival xerosis: conjunctiva becomes dry, thick, wrinkled & loses its normal transparency due to keratinisation of the conjunctiva. When aridness spreads to cornea, it turns into glazy & lustreless due to keratinization of corneal epithelium.
Ø Bitot’s spots: they appear as greyish-white triangular plaques firmly adherent to conjunctiva. This is due to increased thickness of conjunctiva in few areas.
Ø Keratomalacia: softening of cornea. Degeneration of corneal epithelium leading to corneal opacities, corneal ulceration, perforation of cornea and total blindness.
Skin, and epithelium manifestations: the skin becomes rough and dry. Keratinization of the epithelial cells of gastrointestinal tract, urinary tract and respiratory tract.
Toxicity of vitamin A:
Also known as Hypervitaminosis A is caused by excessive intake can lead to toxicity since vitamin A is stored. Symptoms include anorexia, headache, irritability, dry itching, peeling of skin, drowsiness, and vomiting. Sometimes, swelling of bones occurs.
Outlook:
Early reporting of the symptoms to the doctor will prevent the deficiencies caused by vitamin shortage. Consumption of balanced diet will meet up the daily requirements of the individual. In the case of deficiencies, vitamin A supplements can be ordered from 3MEDS-online pharmacy store in India. Do not consume supplements without prescription as it leads to toxicity.