Vitamin A is a fat-soluble vitamin that exists in multiple forms including retinol, retinal, retinoic acid, and beta-carotene. It is absorbed in the small intestine and transported to tissues via retinol binding protein. Vitamin A acts by controlling gene expression and protein production. Its key functions are maintaining vision, supporting growth, enabling reproduction, and promoting epithelial cell differentiation. A daily requirement of vitamin A is 900 RAE for adult males and 700 RAE for adult females.

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Vitamin A
Dr Kavindya Fernando
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Contents
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 Vitamin classification
 Structure of vitamin A
 Sources of vitamin A
 Absorption and transport
 Mechanism of action
 Functions & body requirement

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Vitamin classification
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Vitamins
Water soluble Fat soluble
Non-B complex B complex
Energy - releasing Hamatopoietic Other
• Vitamin A • Vitamin D
• Vitamin E • Vitamin K
• Vitamin C
• Thiamin (B1)
• Riboflavin (B2)
• Niacin (B3)
• Folic acid
• Vitamin B12
• Pyridoxine (Vitamin B6)
• Biotin
• Pantothenic
acid

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Vitamin A
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Structure of Vitamin A
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 Vitamin A – fat soluble vitamin
 Retinoids includes
 natural & synthetic forms of vitamin A
 that may or may not show vitamin A activity
Structure
Retinol Retinal Retinoic acid β-Carotene

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Structure of Vitamin A
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 Retinol
 Found in animal tissue, as retinyl ester
 Primary alcohol,
 containing a β-ionone ring,
 with an unsaturated side chain
 β-carotene
 Found in plant food
 Oxidatively cleaved in the intestine to yield two molecules of retinal
 In human this conversion is inefficient,
 Vit A activity of β-carotene is only about 1/12 that of retinol

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Structure of Vitamin A
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 Retinal
 Aldehyde derived from the oxidation of retinol
 Retinal and retinol can readily be interconverted
 Retinoic acid
 Acid derived from the oxidation of retinal
 Cannot be reduced in the body
Retinol Retinal Retinoic acid
Oxidation
Aldehyde Acid
Oxidation

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Sources of Vitamin A
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 Preformed vitamin A
 Liver, kidney, cream, butter and egg yolk
 Beta carotene – precursor of vitamin A
 Yellow & dark green vegetables & fruits

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Absorption and transport of vitamin A
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Retinyl esters
β-Carotene
Fatty
acids
Retinol
β carotene
Retinal
Retinol
Retinyl
esters
Hydrolysed
Enterocyte
Fatty acyl
CoA
Re-
esterification
to long chain
fatty acid
Secreted as a
component of
chylomicrons lymph
Chylomicron
remnants

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Absorption and transport of vitamin A …cnt
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RBP
Retinyl
pulmitate
All-trans-Retinol
Retinol- RBP
complex
Retinol Retinol
Chylomicron
remnants
RBP

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Mechanism of action of vitamin A
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 Eg: retinoids control
the expression of
the gene of keratin
in most epithelial
tissues of the body
Retinoic acid
Retinol
oxidation
Specific
receptor proteins
Epithelial cell
Activated
retinoic acid-
receptor complex
Interact with
nuclear chromatin
To regulate retinoid-
specific RNA
synthesis
Control the
production of
specific proteins

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Mechanism of action of vitamin A
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Retinoic acid
Retinol
oxidation
Epithelial cell
Plasma retinol-RBP
RBP
Retinoic acid
Inactivate
receptor
Activated
receptor complex
gene
mRNA
mRNA Specific proteins
Cellular
differentiation
cytasol
nucleus

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Functions of Vitamin A
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Retinyl
esters
Retinol
Carotenes
Retinal All-trans
retinoic acid
(tretinoin)
13-cis retinoic
acid
(isotretinoin)
Psoriasis Tx
Promyelocytic
leukemia tx
Severe acne
Tx
Vision Growth
Reproduction
Epithelial tissue
differentiation

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Functions of Vitamin A
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Retinol- RBP
complex
All- trans-Retinol
RBP
Retinal rod/cone
All- trans-retinyl
esters
11-cis Retinol
11-cis Retinal
Rhodopsin
All- trans-Retinal
Opsin
Opsin
When exposed
to light
Bleaching
reduced
esterified
isomerized
oxidized

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Requirement of vitamin A
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 Measured with – retinol activity equivalents (RAE)
 900 RAE daily  adult male
 700 RAE daily  adult female
 1 RAE equals to
 1 mg of retinol
 12 mg of β carotene
 24mg of other carotinoids

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In Summary
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 Vitamin A is a fat soluble vitamin
 There are 4 forms of vitamin A
 Retinol, Retinal, Retinoic acid & β- carotene
 They act by controlling the production of specific proteins
 It is important in maintaining vision, growth, reproduction &
epithelial differentiation
 It has a therapeutic importance in the treatment of psoriasis,
promyelocytic leukemia and severe acne

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Thank You!
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