Vitamin A (Retinol)

 

Vitamin A (Retinol)

(Antixerophthalmic factor)

Introduction

Vitamin A is a fat soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal also known as retinaldehyde, retinoic acid, and several provitamin A carotenoids most notably  beta-carotene . Vitamin A occurs as two principal forms in foods-

·        Retinol- found in animal-sourced foods, either as retinol or bound to a fatty acid to become a retinyl ester

·        Carotenoids- alpha carotene, Beta-carotene, gamma-carotene, and the  xanthophyll beta-cryptoxanthin all of which contain β-ionone rings 

Chemical structure

The term vitamin A Includes A₁ (retinol₁), and A₂ (retinol₂) both are functionally almost similar, their chemical structure being slightly different. Vitamin A₂ has one more double bond and chemically is 3- dehydro retinal.

Chemistry

Vitamin A exists naturally in several isomeric forms. This is a cis- trans isomerism which arises from configurational differences about the double bonds in the side chain. The most naturally occurring form of vitamin A is the all-trans isomer.

Retinene (Retinal) is the aldehyde form of vitamin A.

Absorption

In the small intestine, the vitamin A esters of food are hydrolyzed to fatty acids and free vitamin. Vitamin A₁ and carotene are absorbed through the lymphatics and appear in the blood plasma as the ester, indicating re-esterification in the intestinal wall. Presence of fat and bile salts helps the process. Vitamin A₁ is more quickly and completely absorbed than carotene and the later which is not converted to vitamin A here is carried to the blood. The simultaneous presence of tocopherols and other antioxidants protects them against destruction in the intestinal lumen.

Storage

Chiefly in the liver (95%) as esters, in lesser amount in lungs, lactating breasts, kidney, skin, etc.

Excretion

Excreted mainly in the feces

Synthesis

It is synthesized in the liver from carotene. One molecule of β-carotene gives two molecules of vitamin A. Hence, carotene is called provitamin A. It has also been artificially synthesized.

Properties

·        Soluble in fat-solvents

·        Insoluble in water

·        Ordinarily it is viscid colorless oil but by careful fractionation it has also been isolated as pale yellowish needles

·        Heat-stable, provided contact with air is prevented

·        Easily destroyed on exposure to air or ultra-violet rays

·        Gives blue color with antimony tri chloride in chloroform. The reaction is known as Carr-Price reaction

Distribution

Vitamin A is present in animal fats and vegetable substances which are rich sources of carotene.

Animal sources

·        Cod-liver oil (400- 4,000 i.u. per gm)

·        Halibut-liver oil (20,000 i.u. per gm)

·        Milk (2,000 Lu per 600 ml)

·        Butter (20-50 i.u. per gm)

·        One egg (200 i.u.)

·        Fish are also rich sources.

Plant sources

·        Carrots (20 i.u. per gm)

·        Spinach (100 i.u. per gm)

·        Green leaves

·        Yellow fruits such as mangoes, tomatoes

·        Vegetable oils contain very little

Functions

(1) Essential for growth.

(2) It is a component of Rhodopsin, hence essential for eye sight.

(3) Maintains the integrity and activity of epithelial tissues and glands.

(4) Prevents infection (secondary to normal epithelial tissues).

(5) It plays some part in protein synthesis.

(6) Controls the action of the bone cells, so that normal growth and shape of bone are maintained.

(7) Vitamin A is directly concerned both in the formation of mucopolysaccharides and also in the sulphation of it.

(8) Helps in keeping normal fertility.

(9) Vitamin A participates in reactions which affect the stability of cell membranes, and membranes of sub cellular particles.

(10) It also plays a stabilizing role on the normal permeability of Lysozyme and mitochondria.

(11) It is probable, that vitamin A has some specific functions on carbohydrate metabolism.

(12) Prevents the condition known as urolithiasis where urinary calculi in the form of calcium phosphate are present.

Deficiency signs

1.     General growth- failure of growth.

2. Eye

(1) Night-blindness (Nyctalopia).

(2) Degeneration of lachrymal glands.

(3) Redness, abnormally dry and lusterless condition of the eye-ball (Xerophthalmia) with consequent keratinization and degeneration of cornea (Keratomalacia).

3. Epithelial tissues

(a) Skin-thickening and keratinization, follicular and papular eruptions (Phrynoderma or Toad skin), sebaceous and sweat glands degenerate.

(b) Alimentary tract-the glands in the alimentary tract and the epithelial linings degenerate.

(c) Kidney and Urinary tract- epithelium degenerates and becomes cornified, hence favours formation of renal stone.

(d) Respiratory passages- Epithelium becomes stratified and degenerates.

(e) Nervous system- degeneration

(d) Bones- abnormal growth

International unit

It is equivalent to the activity of 0.6 micro-gram of carotene (same as 0.344 micro-gram of pure preparation of vitamin A acetate).

Daily requirement

·        adults- 5,000 i.u.

·        growing children, puberty, lactation and pregnancy-6,000-8,000 i.u

 

Hypervitaminosis A

·        drowsiness

·        sluggishness

·        headache

·        vomiting  

·        peeling of skin of mouth, hands and other areas

Vitamin interrelationship

The hemorrhagic manifestations associated with Hypervitaminosis A may be prevented by simultaneous administration of vitamin K. The effect may be due to interference with bacterial synthesis of vitamin K in the intestine. The anti oxidant action of tocopherols is probably responsible for its sparing action on vitamin A and carotene. This protective effect of vitamin E is enhances by other antioxidants, e.g., ascorbic acid, etc.