Transcript Vitamins

VITAMIN A & VISUAL
CYCLE
EYE
 The
most sophisticated organ in all
creatures
 Human eye is unique
 Retina contains special types of
photo receptor cells
 3 million Rods and
 1 Billion Cones
Vitamin A deficiency is a major public health
problem globally
Vitamin A deficiency is the most important
preventable cause of blindness.
 The earliest sign of deficiency is a loss of
sensitivity to green light.
 Impairment to adapt to dim light.
 Night blindness.
 More prolonged deficiency leads to
xerophthalmia ( keratinization of the cornea and

blindness.)
Other health Problems
Vitamin A, also has an important role in
differentiation of immune system cells,
and even mild deficiency leads to
increased susceptibility to infectious
diseases.
 Also, the synthesis of Retinol Binding
Proteins (RBP) is reduced in response to
infection, decreasing the circulating
concentration of the vitamin, and further
impairing immune responses.

What are Vitamins ?
Vitamins may be regarded as natural organic
compounds required in the diet in small
amounts to perform specific biological
functions for normal maintenance of optimum
growth and health of the organism.
WHAT IS VITAMIN A?
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The term “vitamin A” makes it sound like
there is one particular nutrient called
“vitamin A”, but this is not true. It is a
broad group of related nutrients,
RETINOIDS
Vitamin A is a broad term for group of
unsaturated nutritional organic
compounds, that
includes retinol, retinal, retinoic acid, and
several provitamin A carotenoids, among
which beta-carotene is the most
important.
HISTORY
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It is recorded in history that HIPPOCRATES cured
night blindness(about 500 B.C.)
He prescribed to the patients Ox liver(in
honey)which is now known to contain high
quantity of vitamin A.
By 1917, Elmer McCollum et al at the University
of Wisconsin–Madison, studied the role of fats in
the diet and discovered few accessory factors.
These "accessory factors" were termed "fat
soluble" in 1918 and later "vitamin A" in 1920.
History
In 1919, Harry Steenbock (University of
Wisconsin) proposed a relationship
between yellow plant pigments (betacarotene) and vitamin A.
 In 1931, Swiss chemist Paul
Karrer described the chemical structure of
vitamin A.
 Vitamin A was first synthesized in 1947
by two Dutch chemists, David Adriaan van
Dorp and Jozef Ferdinand Arens.
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NOMENCLATURE
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PROVITAMIN A : β-Carotene
VITAMIN A1 : Retinol ( Vitamin A alcohol)
VITAMIN A2 : 3 –Dehydro-retinol
VITAMIN A ALDEHYDE : Retinal
VITAMIN A ACID : Retinoic acid
VITAMIN A ESTER : Retinyl ester
NEO VITAMIN A : Stereoisomer of Vitamin
A1, has 70 –80% of biological activity of
Vitamin A1.
Structure of Vitamin A ( Chemistry)
Vitamin A is composed of ‘β-IONONE RING’
(CYCLOHEXENYL) to which ‘POLY ISOPRENOID
SIDE CHAIN’ is attached
 Polyisoprenoid chain –all trans configuration,
contains 4 double bonds, has 2 methyl
groups with terminal carbon having ‘R’ group
 ‘R’ Group –alcohol/aldehyde/acid
 β-Ionone ring –contains 1 double bond with
3 methyl groups
Retinol: -(CH2OH)
-found in animal tissues
as ‘Retinyl esters’ with
long chain fatty acids
•Retinal:
-(CHO)
-Aldehyde derived from
oxidation of retinol by
‘retinal reductase’
requiring NAD/NADP
Retinol & Retinal are
inter-convertible
•Retinoic
acid : -(COOH)
-Acid derived from
oxidation of retinal
-Retinoic acid cannot be
reduced in body therefore
cannot form retinal or
retinol.
•β-Carotene
:
-Hydrolysed by βcarotene dioxygenase in
presence of oxygen & bile
salts to two molecules of
retinal.
Dietary Sources of Vitamin
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Animal sources
– Fish
– Milk
– Butter
– Cheese
– Liver
– Eggs
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Vegetable sources
– Olive
– Deep green & yellow
plants
– Carrots
– Turnip
– Mango
Sources
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Major sources of vitamin A are the carotenes ,
synthesized by plants
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Animals including humans can convert carotenes in
plants to Vit A.
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Alpha, beta and gamma carotene are of nutritional
importance.
Sources
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Alpha and gamma carotenes yield 1 molecule of Vit A
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Beta carotene yields 2 molecules of vit A upon hydrolysis
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Vitamin A is present in 2 forms in diet
1. Preformed Vit A (retinal)
2. Provitamin A carotenoid (Vitamin A precursors).
STRUCTURE OF CAROTENES
To be active,
– Pro vitamins A, carotenoids must be converted into
retinol during absorption.
– Hydrolysis of carotenes occurs in the mucosa of small
intestine.
Absorption
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Absorption occurs in the intestine
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Absorption of Vit A is facilitated by dietary fat, bile,
thyroid hormone and Vit E.
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Vit A is also absorbed directly into portal circulation
and transported in chylomicrons
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Enters the blood circulation and reach the liver.
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Mineral oil prevents the absorption of Vit A
ABSORPTION,TRANSPORT AND STORAGE OF
VITAMIN A
Vitamin A
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Vitamin A is an important fat soluble vitamin
required
for
normal
reproduction and growth
eye
function,
IMPORTANT FUNCTIONS
Role in gene expression
 Glycoprotein synthesis
 Mucopoly-saccharides synthesis
 Mitochondrial membrane function
 Anti cancer role
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IMPORTANT FUNCTIONS
Improper
development of
testes
Steroid hormone synthesis
Bone and teeth development
Catract in old age
General metabolism
INTRODUCTION
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The visual cycle is the biological conversion of a
photon into an electrical signal in the retina.
The processing of visual information begins in the
retina with the detection of light by photoreceptor
cells.
The photoreceptor cells involved in vision are :
1. Rods.
2. Cones.
VISUAL CYCLE
The term “visual cycle” was coined by
George Wald in the mid 1900’s to describe
the ability of the eye to recycle vitamin A
for the synthesis of visual pigments.
(wald,1968)
 According to Wald ,the rod visual cycle
requires the involvement of both retina
and the retinal pigment epithelium(RPE) in
order to properly process the retinal
chromophore released from bleached rod
pigment(or rhodopsin)
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Both the rods and cones contain
chemicals that decompose on
exposure to light and in the process,
excite the nerve fibers present in the
eye.
 light sensitive chemical in the rods is
called rhodopsin and that in the
cones is called Iodopsin.
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Functions
In the retina of the eye there are two types of light receptors
1. Cones for vision in bright light & color vision
2. Rods for vision in dim light
The photo sensitive receptors (rhodopsin visual purple in
rods and iodopsin or visual violet in cones) contains Vit A
plus different proteins.
When struck by light the pigments are split apart into Vit A
and the constituent protein.
Recommended Dietary Allowances
Infants
Children
00 – 0.5
420 g
0.5 – 1.0
400 g
1–3
400 g
4–6
500 g
7 – 10
700 g
Recommended dietary allowances
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The allowances for children and adolescents are based on average
body weight and growth needs.
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Adults Males RDA is 1000 retinol equivalents(5000iu)
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Adults Female RDA is 800 retinal equivalents (4000 iu)
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In pregnancy and Lactation it is increased to 1000 and 1200 retinal
equivalents respectively.
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Vit A is stored in liver and normal individuals have a three month
supply
FUNCTIONS OF VITAMIN A
VISION
 GENE TRANSCRIPTION
 IMMUNE FUNCTION
 EMBRYONIC DEVELOPMENT AND
REPRODUCTION
 BONE METABOLISM
 HAEMATOPOESIS
 SKIN AND CELLULAR HEALTH
 ANTIOXIDANT ACTIVITY
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VITAMIN A DEFICIENCY
Most susceptible populations:
– Preschool children with low F&V intake
– Urban poor
– Older adults
– Alcoholism
– Liver disease (limits storage)
– Fat mal-absorption
Vitamin A deficiency may result from
 Dietary insufficiency of Vitamin A / Precursors
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Interference with absorption from intestines
eg: diarrhoea, malabsorption syndrome, bile salt
deficiency
Defect in the transport due to protein
malnutrition –‘Kwashiorkar’
Defect in the storage due to diseases of liver
Tissues chiefly affected
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–‘Epithelial’ principally which are not
normally keratinised
Includes epithelium of respiratory tract,
gastrointestinal tract, genitourinary tract,
eye & para occular glands, salivary glands,
accessory glands of tongue & buccal cavity
and pancreas
Fundamental change: Metaplasia of
normal non-keratinised living cells
into keratinising type of epithelium
Effects
1. Night blindness (nyctalopia)
Inability to see well in dim light easily when
entering a dark space form bright light
2. Night blindness occurs when there is insufficient Vit A
in the blood to quickly regenerate visual purple .
3. Alcoholic liver disease (cirrhosis) causes
night blindness which is due to hepatic damage affecting
Vit A release.
Effects
a.
In the eye, the 1st symptom of Vit A deficiency include
photophobia (sensitivity to bright light)
b.
Inflammation of eyes and eyelids due to impaired
functioning of lacrimal glands
c.
Xerophthalmia (dry, inflamed and edematous cornea)
d.
Keratomalcia – permanent blindness results when
infection leads to ulceration and softening of cornea
Skin and mucous membrane changes
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Keritinization of the epithelial tissues
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Increase susceptibility to infections of all membranes,
protected by mucous
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Follicular hyperperatosis – The sebceous glands
becomes clogged and skin takes on a gooseflesh like
appearance.
Vitamin A is Toxic in Excess
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There is only a limited capacity to
metabolize vitamin A, and excessive
intakes lead to accumulation beyond the
capacity of binding proteins, so that
unbound vitamin A causes tissue damage.
 Symptoms
of toxicity affect the
central nervous system (headache,
nausea and anorexia, all associated
with increased cerebrospinal fluid
pressure);
 Liver (hepatomegaly with histologic
changes and
 Hyperlipidemia’s
Toxic effects
1.
Drying and desquamation of skin
2.
Anorexia
3.
Loss of hair
4.
Bone pain and fragility
5.
Enlargement of liver
Remember us in you Diet
and enjoy the beautiful
nature
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