2. VITAMIN A & VISUAL CYCLE

3. 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

4. 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.)

5. 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.

6. 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.

8. WHAT IS VITAMIN A?
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.

9. HISTORY
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.

10. History
In 1919, Harry Steenbock (University of Wisconsin) proposed a relationship between yellow plant pigments (beta-carotene) 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.

11. NOMENCLATURE 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.

12. 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

13. 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

14. 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 β-carotene dioxygenase in presence of oxygen & bile salts to two molecules of retinal.

15. Dietary Sources of Vitamin
Animal sources
Fish
Milk
Butter
Cheese
Liver
Eggs
Vegetable sources
Olive
Deep green & yellow plants
Carrots
Turnip
Mango

16. Sources
Major sources of vitamin A are the carotenes , synthesized by plants
Animals including humans can convert carotenes in plants to Vit A.
Alpha, beta and gamma carotene are of nutritional importance.

17. Sources Alpha and gamma carotenes yield 1 molecule of Vit A
Beta carotene yields 2 molecules of vit A upon hydrolysis
Vitamin A is present in 2 forms in diet
Preformed Vit A (retinal)
Provitamin A carotenoid (Vitamin A precursors).

18. STRUCTURE OF CAROTENES

20. To be active,
Pro vitamins A, carotenoids must be converted into retinol during absorption.
Hydrolysis of carotenes occurs in the mucosa of small intestine.

21. Absorption Absorption occurs in the intestine
Absorption of Vit A is facilitated by dietary fat, bile, thyroid hormone and Vit E.
Vit A is also absorbed directly into portal circulation and transported in chylomicrons
Enters the blood circulation and reach the liver.
Mineral oil prevents the absorption of Vit A

22. ABSORPTION,TRANSPORT AND STORAGE OF VITAMIN A

23. Vitamin A
Vitamin A is an important fat soluble vitamin required for normal eye function, reproduction and growth

24. IMPORTANT FUNCTIONS Role in gene expression Glycoprotein synthesis
Mucopoly-saccharides synthesis
Mitochondrial membrane function
Anti cancer role

25. IMPORTANT FUNCTIONS Improper development of testes
Steroid hormone synthesis
Bone and teeth development
Catract in old age
General metabolism

26. INTRODUCTION
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.

27. 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)

28. 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.

29. Functions
In the retina of the eye there are two types of light receptors
Cones for vision in bright light & color vision
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.

31. Recommended Dietary Allowances
Infants 00 – g
0.5 – g
Children 1 – g
4 – g
7 – g

32. Recommended dietary allowances
The allowances for children and adolescents are based on average body weight and growth needs.
Adults Males RDA is retinol equivalents(5000iu)
Adults Female RDA is 800 retinal equivalents (4000 iu)
In pregnancy and Lactation it is increased to 1000 and 1200 retinal equivalents respectively.
Vit A is stored in liver and normal individuals have a three month supply

33. FUNCTIONS OF VITAMIN A VISION GENE TRANSCRIPTION IMMUNE FUNCTION
EMBRYONIC DEVELOPMENT AND REPRODUCTION
BONE METABOLISM
HAEMATOPOESIS
SKIN AND CELLULAR HEALTH
ANTIOXIDANT ACTIVITY

34. 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

35. Vitamin A deficiency may result from
Dietary insufficiency of Vitamin A / Precursors
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

36. Tissues chiefly affected
–‘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

37. 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.

38. Effects
In the eye, the 1st symptom of Vit A deficiency include photophobia (sensitivity to bright light)
Inflammation of eyes and eyelids due to impaired functioning of lacrimal glands
Xerophthalmia (dry, inflamed and edematous cornea)
Keratomalcia – permanent blindness results when infection leads to ulceration and softening of cornea

39. Skin and mucous membrane changes
Keritinization of the epithelial tissues
Increase susceptibility to infections of all membranes, protected by mucous
Follicular hyperperatosis – The sebceous glands becomes clogged and skin takes on a gooseflesh like appearance.

40. Vitamin A is Toxic in Excess
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.

41. 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

42. Toxic effects Drying and desquamation of skin Anorexia Loss of hair
Bone pain and fragility
Enlargement of liver

43. Remember us in you Diet and enjoy the beautiful nature

44. THANKS