BIOCHEMISTRY OF VISION AND

ROLE OF VITAMIN A

Hira Nath Dahal

VITAMINS• Potent organic compounds needed in minute

amount for growth and good health.

• Not used for energy but for utilization of other nutrients like carbohydrate, proteins and fats.

• Most vitamins act as co-enzymes.

• Most vitamins are not made in the body , they must be taken via food or vitamin supplements.

CLASSIFICATION OF VITAMINS

1. Fat soluble vitamins:

Vit A

Vit D : are made in skin

Vit E

Vit K : are synthesized by intestinal bacteria

2. Water soluble vitamins:

Vit B Inositol , choline ,para-amino benzoic acid

Vit C are other water soluble vitamins.

VITAMIN A

• Vitamin A is necessary for a variety of functions such as vision, proper growth and differentiation, reproduction and maintenance of epithelial cells.

• Active form is present only in animal tissue.

• Retinoids are the compounds

with vitamin A activity.

• Anti infection vitamin

SOURCES OF VITAMIN A

•Animal sources (Retinal esters and

retinol) Fish liver oil (Cod and Helibnt)

Egg yolk,milk,liver, butter,cream,cheese)

• Liver is the best source of Vit A.

•Plant sources ( -carotene β pro →vitamin)

Carrot, green leafy vegetables

(Spinach), papaya, mango, pumpkin, brinjal .

• Plant foods contain Vit A in the form of precursors “ the carotenoid pigment ( Carotenes).

• Carotenes are converted to Vit A(Retinol) by

metabolic activityin the wall of small intestine.

Types of carotenes

i. α-carotenes: yields 1-molecule of Vit A.

ii.β-carotenes: yields 2-molecule of Vit A.

iii. -carotenes: yields 1-molecule of Vit A.γ

VIT A ABSORPTION AND STORAGE

In intestine Vit A is esterified

Reaches blood stream through intestinal

lymphatics

Most retinol reaches liver (stored as well)

Retinol binds with RBP(Retinol Binding Protein)

TRANSPORT FROM LIVER TO EYE

Retinol-protein complex enters circulation

Becomes attached to specific receptors

present in basal surface of RPE cells

RBP is left outside and only retinol enters RPE

SYNTHESIS OF VISUAL PIGMENTS Retinol from RPE pass to outer segments of

photoreceptors 11-Cis

Retinol oxidized Retinene Retinal

opsin + NAD Oxidative system

Rhodopsin (Rods) Photopsin

(for peripheral vision & vision

(for highly discriminating or

of low illumination)

central vision)

LIGHT INDUCED CHANGES IN VISUAL PIGMENTS

• Light when falls on eye are absorbed by rods and cones in retina.

• Initiates photochemical changes which in turn initiates electrical changes.

LIGHT INDUCED CHANGES AS STUDIED IN RODS

CAN BE DESCRIBED UNDER 3-HEADINGS

i. Rhodopsin bleaching

ii.Rhodopsin regeneration

iii.Visual cycle

I) RHODOPSIN BLEACHING & II)REGENERATION

RhodopsinBathorodhopsin

Lumirhodopsin

Metarhodopsin I

Metarhodopsin II

opsin

isomerase

isomerase

11-Cis-Retinal

11-Cis-Retinol

all-trans-Retinal

all-trans-RetinolNAD NAD

NADHNADH

Light energy

III) VISUAL CYCLE

Rhodopsin Light energy

all-trans-Retinal11-Cis-Retinal

Retinal isomerase

opsinExcitation of nerve

Rate of photochemical bleaching Rate of rhodopsin regenerationUnder constant

Light condition

US FOOD & DRUG ADMINISTRATIONRECOMMENDED DAILY

ALLOWANCE(RDA) OF VIT A

Groups RDA (IU) Infants 1500

Children (< 4yrs) 2500

Children (> 4 yrs) 5000

Lactating or Pregnant women

8000

IU= International Unit1 IU= 0.3 μg of retinol

ROLE OF VITAMIN A IN EYE

•Vision in dim light

• It is the main cause for preventable blindness.

BIOCHEMICAL FUNCTION

1. Vision in dim light

2. Necessary for the maintenance of normal epithelium

3. Necessary for reproduction

4. Acts as an anti-oxidant

5. - carotene prevents heart attackβ .

CAUSES OF VITA DEFICIENCY

a) Inadequate dietary intake.

a) Poor absorbtion due to:

- abnormal fat metabolism (biliary

obstruction or pancreatic disease)

- Chronic diarrhoea

- habitual intake of liquid paraffin

c) Inadequate conversion of -carotene toβ

retinol as in liver or intestinal disease.

d) Exhaustion of Vit A store in liver due to

increased demand/intake ratio eg. In pregnancy or in low birth weight infants or after measles,diarrhoea.

e) Deficiency of zinc and proteins( in PEM ).

OCULAR MANIFESTATION(XEROPHTHALMIA)

a) Nigt blindness is first symptom due to

impairment in dark adaption.

b) Conjunctival changes:

- Conjunctival xerosis is first clinical sign.

- Bitot’s spot on bulbar conjunctiva.

c) Corneal changes

- Corneal xerosis

- ulceration

- keratomalacia may lead to blindness.

NON- OCULAR MANIFESTATIONS

a) Hyperkeratinization of epithelium and

follicular hyperkeratosis.

b) Increase in infection of gastrointestinal and

respiratory tract.

c) Cessation of bone growth and changes in

teeth.

d) Anorexia

e) Injury to nerves and brain.

f) Calcium deposition in urogenital tract & bladder disorders.

g) Atrophy of germinal epithelium of testes.

VIT A PROPHYLAXIS SCHEDULE IN TREATMENT OF XEROPTHALMIA

Individuals oral dose Timing

Children < 12 months age

1 lakh IU Once every 4-6 months

Children >12 months of age

2 lakh IU “

New born 0.5 lakh IU At birth

Women of child bearing age

3 lakh IU Within 1 months of giving birth

Pregnant and lactating women

5 thousand IUOr 20 thousand IU

Every day once every week

MANAGEMENT OF VITAMIN A DEFICIENCY IN COMMUNITY/PREVENTION OF XEROPHTHALMIA IN COMMUNITY

a) Identification of severity by history,ocular and systemic examination.

b) Short term interventions(treatment)

- Breast feeding & proper supplementary

food.

- Vit A supplementation

c) Long term sustainable solutions:(prevention)

- advocacy for change in dietary behavour, breast-feeding & weaning through nutrition education.

- support home gardening to increase production of vitamin rich foods and vegetables.

- improved methods for production, preparation & preservation of Vit A rich foods.

- Fortification of skimmed milk and margarine with Vit A.

- Control of malnutrition, ARI , measles & diarrhoea.

- Vit A supplementation.

HYPERVITAMINOS A

• If exceeds daily intake > 10,000 IU

• Results:

abdominal pain , blurred vision , drowsiness,

headache , irritability , nausea and vomiting.