Vitamin A

INTRODUCTION:
  • Vitamin A is a fat soluble vitamin
  • Required for vision, repair, reproduction, growth and tissue differentiation
  • Vitamin A consist of Retinol, retinal and retinoic acid
  • Two groups of compounds have vitamin A activity
  1. Retinoids and
  2. Carotenoids
SOURCES:
  • Liver, milk,fish, and eggs
  • Dark green and deeply colored fruits and vegetables.
  • The highest concentrations of Vitamin A is seen in Green leafy vegetables then Cod liver oil
DIGESTION & ABSORPTION:
  • Dietary Vitamin A, from animal sources is available in the form of Retinyl esters, which is hydrolyzed to retinol and fatty acid by pancreatic hydrolases
  • The absorption of retinol requires the presence of bile salts, fat
  • In the intestinal cells, retinol is esterified back and secreted with chylomicrons
TRANSPORTATION & STORAGE:
  • The liver contains approximately 90% of the vitamin A reserves and secretes vitamin A in the form of retinol, which is bound to retinol-binding protein.
  • The retinol-binding protein complex interacts with a second protein, Transthyretin.
  • Vitamin A is stored mainly as retinol esters in Liver
  • This trimolecular complex functions to prevent vitamin A from being filtered by the kidney glomerulus, to protect the body against the toxicity of retinol and to allow retinol to be taken up by specific cell-surface receptors that recognize retinol-binding protein.
FUNCTIONS:
  • Vitamin A is essential for vision (especially dark adaptation),
  • Immune response,
  • Bone growth,
  • Reproduction,
  • Maintenance of the surface linings of the eyes, epithelial cell growth and repair, and the epithelial integrity of the respiratory, urinary, and intestinal tracts.
  • Vitamin A is also important for embryonic development and the regulation of adult genes.
  • Role in vision:
  1. In the retina, retinaldehyde functions as the prosthetic group of the light-sensitive opsin proteins, forming Rhodopsin (in rods) and iodopsin (in cones).
  2. Any one cone cell contains only one type of opsin, and is sensitive to only one color.
  3. The absorption of light by Rhodopsin causes isomerization of the retinaldehyde from 11-cis to all-trans, and a conformational change in opsin.
  4. This results in the release of retinaldehyde from the protein, and the initiation of a nerve impulse.
  • Wald’s visual cycle:
  1. The formation of the initial excited form of Rhodopsin, bathorhodopsin, occurs within picoseconds of illumination.
  2. There are then a series of conformational changes leading to the formation of metarhodopsin II, which initiates a guanine nucleotide amplification cascade and then a nerve impulse. The final step is hydrolysis to release all-trans-retinaldehyde and opsin.
  3. The key to initiation of the visual cycle is the availability of 11-cis-retinaldehyde, and hence 
  • vitamin A.
  1. In deficiency, both the time taken to adapt to darkness and the ability to see in poor light are impaired.
  • Role in the Regulation of Gene Expression and Tissue Differentiation:
  1. Retinoic acid is a ligand for certain nuclear receptors that act as transcription factors.
  2. The retinoid receptors play an important role in controlling cell proliferation and differentiation.
  3. Binding of all-trans-RA and 9-cis-RA to RAR and RXR respectively allows the complex to regulate the rate of gene transcription.
  • Role of vitamin A in Immunity:
  1. Development and differentiation of WBC
  2. The skin and mucosal cells (function as a barrier and form the body's first line of defense against infections. Retinol and its metabolites are required to maintain the integrity and functioning of these cells.
  • Retinol binding protein (RBP) is a negative ‘Acute phase protein’, that results in decreased circulatory concentration of the vitamin with further deterioration of the immune system. Role of vitamin A in growth:
  1. Brain development
  2. General body growth
  3. Bone 
DAILY DOSE:
  • First dose of vitamin A should be given at 9 months
  • Vitamin A prophylaxis is given to children every 6 months
Age Retinol
(mcg)
Infants 0 - 12 months 350
Children 1 - 6 years 400
Children 7 - 9 years
Adolescents 10 -17 years
Adult Man and Woman
600
(200000 iu)
Pregnancy 800
Lactation 950
Exam Question

  • Vitamin A is present in milk, egg yolk, fish , liver, fruit and vegetables
  • Vitamin A promotes maintenance of epithelial tissue
  • Vitamin A is stored mainly as retinol esters in Liver
  • Vitamin A consist of Retinol, retinal and retinoic acid
  • During the dark phase of visual cycle 11-cis-Retinaldehyde combines with opsin to make Rhodopsin
  • Anti infective vitamin is vitamin A
  • Daily dose of vitamin A in a 6-12 months old child is 300 microgram
  • The highest concentrations of Vitamin A is seen in Cod liver oil
  • Vitamin 'A' requirment for a Pregnant woman is 800 mg/d.
  • Dose of vitamin A prophylaxis given in age group 1­6 years 200000 iu
  • Absorption of vitamin A can be enhanced by giving the child a diet rich in Fat
  • Highest vitamin A content is seen in Green leafy vegetables
  • First dose of vitamin A should be given at 9 months
  • Vitamin A prophylaxis is given to children every 6 months
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