Vitamin a

1. OPTOM FASLU MUHAMMEDOPTOM FASLU MUHAMMED

2. CHEMISTRY OF VITAMIN A
• VITAMIN A is fat soluble vitamin
• Pro-Vitamin Beta-Carotene present in plant tissues give
rise two molecules of Vitamin A.
• All compounds with Vitamin A activity is called retinods.
• Retinol (alcohol) Retinal (aldehyde) & Retinoic acid
(Vitamin A acid)
• Retinal on reduction gives Retinol & on oxidation
Retoinic acid.
• Vitamin A1 – all trans variety of retinal is most common
• Vitamin A2 – found in fish oils.
• Biologically important compounds is 11 Cis – Retinal.

3.  Intestine is the major site
 Cleavage of Beta Carotene by di-
oxygenase results in the formation of
retinal.
 Intestine retinal is reduced to retinol.
 Absorption requires fat & Bile salts.
 It is transported to the liver and stored as
retinal Palmitate.

4. Conversion of Carotenoids to
Retinoids
• Enzymatic conversion of
carotenoids occurs in liver
or intestinal cells, forming
retinal and retinoic acid
• Provitamin A carotenoids
– Beta-carotene
– Alpha carotene
– Beta-cryptoxanthin
• Other carotenoids
– Lutein
– Lycopene
– Zeaxanthin

5. Absorption of Vitamin A
• Retinoids
– Retinyl esters broken down to free retinol in
small intestine - requires bile, digestive
enzymes, integration into micelles
– Once absorbed, retinyl esters reformed in
intestinal cells
– 90% of retinoids can be absorbed
• Carotenoids
– Absorbed intact, absorption rate much lower
– Intestinal cells can convert carotenoids to
retinoids

6. Transport and Storage of Vitamin A
• Liver stores 90% of vitamin A in the body
• Reserve is adequate for several months
• Transported via chylomicrons from intestinal
cells to the liver
• Transported from the liver to target tissue as
retinol via retinol-binding protein.
• Retinol- RBP Complex binds to specific
receptors on retina, skin,gonads & other
tissues.

7. Vitamin A in Foods
• Preformed
– Liver, fish oils, fortified milk,
eggs, cheese, yolk other fortified foods
– Fish liver oils (shark and code liver oil)
– Contributes ~70% of vitamin A intake for
• Provitamin A carotenoids
– green leafyvegetables , yellow-orange /fruits
– Carrot, papaya, mango, Pumpkins.

8. NORMAL CONCENTRATION IN BLOOD
&
RECOMMENDED DAILY ALLOWANCE
• 0.15 to 0.6 mg/dl
• Children (400-600) microgram/day
• Men-(750-1000) microgram/day
• Women -750 microgram/day
• Pregnancy- 1000 microgram/day
• Lactation - 1200µg/day

9. Functions of Vitamin A: Vision
• Retinal turns visual light into nerve signals
in retina of eye
• Retinoic acid required for structural
components of eye
– Cones in the retina
• Responsible for vision under bright lights
• Translate objects to color vision
– Rods in the retina
• Responsible for vision in dim lights
• Translate objects to black and white vision

10. Functions of Vitamin A:
Growth and Differentiation of Cells
• Retinoic acid is necessary for cellular
differentiation
• Retinol is Important for embryo
development, gene expression.
• Retinoic acid influences production,
structure, and function of epithelial cells
that line the outside (skin) and external
passages (mucus forming cells) within the
body

11. The Visual Cycle

12.  During Bright light Entire Rhodopsin is
found to be in bleached condition and
when the person moves in to dark area it
takes a few seconds to resynthesize the
rhodospin to activate the Rod cells to
restore vision in the Dim light.
 This time interval is called Dark
adaptation time and it is found to be high
in Vitamin A deficiency.

13. Functions of Vitamin A:
Immunity
• Deficiency leads to decreased resistance
to infections
• Supplementation may decrease severity of
infections in deficient person

14.  Vitamin A can act as antioxidant
 Due to the antioxidant property it can
exhibit anticancer activity.
 Effect of vitamin A is complementary to
VitaminE
 Beta Carotene Prevents cancer
 Retinol can act as steroid hormone
controlling the expression of genes
 Retinoic acid is found to have important
role in Glycoprotein synthesis.

15. Terminal Ends of Retinoids

16. Deficiency of Vitamin A
• Most susceptible
populations:
– Preschool children
with low F&V intake
– Urban poor
– Older adults
– Alcoholism
– Liver disease (limits
storage)
– Fat malabsorption
• Consequences:
– Night blindness
– Decreased mucus
production
– Decreased immunity
– Bacterial invasion of
the eye
– Conjunctival xerosis
– Bitot’s spots
– Xerophthalmia
– Irreversible blindness
– Follicular
hyperkeratosis
– Poor growth

17.  Nyctalopia;-Night Blindness
 Xeropthalmia;- conjuctiva become thick
and wrinkled. Conjuctiva become
keratinised.
 Bitot’s Spot:- Greyish Traingular spots
adherent to Conjuctiva.reversed if proper
supply of Vitamin A maintained.
 Keratomalacia;-Softening of the cornea.
 Corneal epithelium gets degenerated
and vascularised.

18.  Hyperkeratinisation of respiratory,
gastrointestinal tract and urinary tract.
 Epithelium atrophied, Urinary Calculi.
 Generalised infections observed in
skin.

19. Health Effects of Vitamin A