Vitamin A
Vitamin A
VITAMIN A (Beta Carotene)

-Vitamin A is fat soluble. It requires fats as well as minerals to be properly absorbed by you digestive
tract.
-It can be stored by your body and need not to be replenished every day.
-It occurs in two forms
-Performed vitamin A, called retinol (found only in the foods of animal origin)
-Provitamin A known as carotene (found only in the foods of both plant and animal origin).
Vitamin A must be supplied from the environment, most of it’s actions are exerted through hormone-like
receptors, a property that is shares with the vitamin D.
Vitamin A have diverse action in the cellular regulation & differentiation that’s why vitamin A have found
important therapeutic application in the treatment of a variety of dermatological conditions.

Terminology, Occurrence, and chemistry:

Vitamin A has been used to donate specific chemical compound, such as retinol or it’s ester. Retinoid
refers to the chemical entity retionol or to the other closely related naturally occurring derivatives.
The most active B-carotene found in the plant

Retinol (vitamin A1)
A primary alcohol, is present in the esterified form in the tissues of animals and salt water fish. Mainly
in the liver.

Retinoic acid (vitamin A acid)
In which the alcohol group has been oxidized share some but not all the action of retinol. Although
retinoic acid is ineffective in the restoring visual or reproductive function in the certain species where
retinol is effective, it is very potent in the promotion growth and controlling differentiation and
maintenance of epithelial tissue in the vitamin A- deficient animals.
In deed all trans-retinoic acid appears to be the active form of vitamin A in all tissues except the retina,
and is 10-to 100 fold more potent than retinol in various system invetro.

Physiological Function & Pharmacological action:

*Vitamin A has number of important functions in the body.
-It plays an essential role in the function of the retina.
-It is necessary for growth and differentiation of epithelial tissues
-Required for the growth of bones, reproduction, emperionic development.
-Together with certain vitamin carotenoids, vitamin A appear to inhance the immune system, to reduce
the consequence of some infectious disease.
-Protects against the development of certain malignancies
-Retinoid is used for the treatment of various premalignancy conditions. Because of the effect of vitamin
A on the epithelial tissue.
-Ritenoid and their analogs are used in the treatment of number of skin diseases, including some of
the consequences of aging and prolonged exposure to the sun.
-The function of vitamin A is mediated by different forms of the molecule.
        Retinal is the functional vitamin in vision,.
Retinoic acid appear to be the active form in the function associated with the growth, differentiation, &
transformation.

Rhodopsin (visual purple), the retinal pigment that is necessary for vision in dim light (scotopic vision)
consists of a protein (opsin) combined with vitamin A. In bright light rhodopsin is destroyed. It is partly
regenerated in the dark, but because the regeneration id not quantitative complete, vitamin A is needed
to maintain retinal levels.

Retinal & visual cycle:
It has long been known that vitamin A deficiency interferes with the vision in the dim light, a condition
known as night blindness (nyctalopia).
Photoreception is accomplished by two type of specialized retinal cell, termed rods and cones.
Rods are especially sensitive to light of low intensity; Cones act as receptors of high-intensity light and
are responsible for color vision.

The visual cycle initiated by the absorption of a photon of light, followed by photocomposition, or
bleaching, of rhodopsin through the cascade of unstable conformation state, leading ultimately to
isomarization of 11-cis-retinal to the all-trans form and dissociation of the opsin moiety.


The initial step is the absorption of the light by chromophore attached to the receptor protein. The
chromophore of both rods and cones is 11–cis- retinal. The holoreceptor is termed rhodopsin- a
combination of the protein opsin and 11-cis retinal attached as prosthetic group.

The three different types of cone cells (red, green, and blue) contain individual related photoreceptor
proteins and respond optimally to light of different wave lengths.

When human beings are fed diet deficiency in vitamin A. Their ability for dark adaptation is gradually
diminished. Rod vision is affected more than cone vision.
Upon depletion of retinol from liver and blood, usually at plasma concentration of retinol of less than 20
μg/dl (0.7 μm), the concentration of retinol and of rhodopsin in the retina falls.         Unless the deficiency
is over come, opsin, lacking the stabilizing effect of retinal, decays and anatomical deterioration of the
rods outer segments takes place.

Vitamin A epithelial structure:

The function and structural integrity of the epithelial cells throughout the body is dependent upon an
adequate supply of vitamin A. The vitamin plays a major role in the induction and control of epithelial
differentiation in the mucous secretion or keratinizing tissue.

In the presence of retinol or retinoic acid, basal epithelial cells are stimulated to produce mucus.
Excessive excretion of retinoids lead to the production of mucin the inhibition of keratinization and
display of goblet cells
Goblet cells (specialized, flask–shaped glandular cell scattered among the columnar cell of this tissue.
Cell called goblet cells, secrete protective fluid (mucus) onto the free surface of the tissue.)
Mucin: (is secreted by mucous cells which is a thick fluid riches with glycoprotein and is secreted
abundantly by glands)

In the absence of vitamin A goblet mucous cells disappear and are replaced by the basal cells that
have been stimulated to proliferate. The Suppression of normal secretion lead to the irritation and
infection.

Hypervitaminosis:
An intake of retinoids greatly in excess of requirement result in toxic syndrome known as
hypervitaminosis

Human requirements:

Vitamin A measured in USP (united state pharmacopoeia), IU (Inter National Unit) and most recently –
RE (Retinol Equivalent).
•Normal daily recommended intakes in the United States for vitamin A are generally defined according
to age or condition and to the form of vitamin A. In the past, the RDA and RNI for vitamin A have been
expressed in Units. This term Units has been replaced by retinol equivalents (RE) or micrograms
(mcg) of retinol, with 1 RE equal to 1 mcg of retinol. This was done to better describe the two forms of
vitamin A, retinol and beta-carotene.
One RE of vitamin A is equal to 3.33 Units of retinol and 10 Units of beta-carotene. Some products
available have not changed their labels and continue to be labeled in Units.
However, taking too much vitamin A (more than 1800 RE [6000 Units]) during pregnancy can also
cause harmful effects such as birth defects or slow or reduced growth in the child.
Dosing—The combination of retinol and beta-carotene in the diet is based on 1980 U.S.
Recommended Dietary Allowances (RDAs).
•Adult and teenage males—1000 retinol equivalents (RE) (3330 Units of retinol or 5000 Units as a
combination of retinol and beta-carotene) per day.
•Adult and teenage females—800 RE (2665 Units of retinol or 4000 Units as a combination of retinol
and beta-carotene) per day.
•Pregnant females—800 RE (2665 Units of retinol or 4000 Units as a combination of retinol and beta-
carotene) per day.
•Breast-feeding females—1200 to 1300 RE (4000 to 4330 Units of retinol or 6000 to 6500 Units as a
combination of retinol and beta-carotene) per day.

Food Source:
Major dietary source of vitamin A are liver, butter cheese, whole milk, egg yolk, and fish. B – carotin
present in a various yellow or green fruits and vegetables. These foods also contain numerous
carotinoids that cannot be converted to retinol. Nevertheless, many of these can function as antioxidant
and may have useful health promoting effects.

Deficiency Symptoms:
Lack of vitamin A may lead to a rare condition called night blindness (problems seeing in the dark), as
well as dry eyes, eye infections, skin problems, and slowed growth.
Increased susceptibility to infections; rough, dry, scaly skins; loss of smell & appetite; frequents fatigue;
lack of tearing; defective teeth & gums' retarded growth.
Xerophthalmia “dryness and thickening of the conjunctiva”.

Some conditions may increase your need for Vitamin A. these include:
Diarrhea
Eye diseases
Intestine diseases
Infections (continuing or chronic)
Measles
Pancreas disease
Stomach removal
Stress (continuing)
Vitamin A absorption will be decreased in any condition in which fat is poorly absorbed.