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Vitamin A was identified as a necessary
growth factor in 1913 and was the first vitamin to be
discovered. It is obtained from food in a combination of
two different forms: as pre-formed vitamin A and as pro-vitamin A which the body can convert to vitamin A as
necessary. Pre-formed vitamin A, often in the form of
retinol or retinal, is found in foods of animal origin
and pro-vitamin A, of which beta carotene is the best
known form, is found in orange, yellow and dark green
vegetables and fruits. Both forms are fat soluble.
What! does
vitamin A do for
your body?
EYES - Vitamin A is essential for eyes to
function effectively. It is involved in the growth and
repair of the eye and in the production of a chemical
called visual purple, which helps in night vision.
EPITHELIAL CELLS - Vitamin A is involved in the growth
and repair of epithelial cells. These cells cover the
internal and external surfaces of the body and are found
in the skin, lungs, developing teeth, inner ear, cornea
of the eye, sex organs, glands and their ducts, gums,
nose, cervix and other areas. This growth and maintenance
role is vital for many bodily functions. For example, the
good health of the digestive tract lining is important in
protecting against ulcers and maintenance of the lining
of the vagina and uterus is important in fertility.
PREGNANCY - Vitamin A is necessary in pregnancy for the
development of the embryo.
NERVES - Vitamin A is involved in the production of
membranes and of myelin, which coats the nerves.
GLANDS AND HORMONES - Vitamin A plays a role in the
maintenance of the adrenal gland and synthesis of certain
hormones such as thyroid hormone.
THE IMMUNE SYSTEM - Vitamin A is known as "the
anti-infective vitamin" as it is vital for the development of the body’s barriers to infection. It
stimulates and enhances many immune functions including
antibody response and the activity of various white blood
cells. This immune enhancing function promotes healing of
infected tissues and increases resistance to infection.
Adequate vitamin A intake, either from diet or
supplements, is very important in preventing sickness and death in children. Many studies have found that vitamin A
supplementation reduces the risk of infectious diseases
in areas where vitamin A deficiency is widespread. A
recent research review analyzing the results of several
studies found that adequate vitamin A intake in children
resulted in a 30% decrease in deaths from all causes.
Children in developing countries are often at high risk
of vitamin A deficiency. In developed countries, ensuring
adequate vitamin A intake is particularly important in
those with life threatening infections such as measles
and in those at risk of relative deficiency, such as
premature infants.
GROWTH AND BONE FORMATION - Vitamin A is necessary for
growth and the formation of bones and teeth, collagen
synthesis, cartilage synthesis and wound healing.
CANCER - Many studies suggest that high blood levels of
vitamin A can help prevent certain forms of cancer,
particularly cancers of epithelial tissue. A study of 89
000 nurses found that those in the top 20% for vitamin A
intake were significantly less likely to develop breast
cancer than those in the bottom 20%.
This may be due to
the importance of vitamin A in maintaining healthy
epithelial cells, strengthening the
immune system and
stimulating the response to abnormal cells.
ANTIVIRAL ACTIVITY - Laboratory experiments have shown
vitamin A to have antiviral activity.
Absorption - The presence of fat
and bile in the intestines is necessary for vitamin A
absorption. Around 80 to 90% of vitamin A in the diet is
absorbed although this is reduced in older people and
those who have trouble absorbing fat, such as pancreatitis, celiac disease and
cystic fibrosis
sufferers, who may run the risk of vitamin A deficiency.
Vitamin A is joined to fatty acids in the intestinal
lining, combined with other substances and transported to
the liver, which stores 90% of the body’s vitamin A.
Deficiency - Vitamin A deficiency
is relatively rare in developed countries and is usually
limited to those who have absorption difficulties, liver
disease or who drink a lot of alcohol. Vitamin A
deficiency is common in alcoholics and contributes to
some of the disorders of alcoholism such as night
blindness, skin problems, cirrhosis of the liver and
susceptibility to infections.
Vitamin A deficiency symptoms:
EYES - One of the first symptoms of deficiency is night
blindness due to lack of visual purple. Prolonged
deficiency leads to xerophthalmia, a condition in which
eyes become dry, ulcers appear on the cornea, the eyelids
become swollen and sticky and which eventually leads to
blindness. Vitamin A deficiency is the leading
preventable cause of blindness in developing countries.
SKIN - Prolonged deficiency leads to thickened dry skin
which is prone to infections. Small hardened bumps of a
protein known as keratin may develop around the hair
follicles.
GROWTH - Deficiency causes growth retardation, weight
loss, diarrhea, thickening of bone shafts, congenital
malformations, impaired hearing, taste and smell, wasting
of testicles and reduced sperm count. Inadequate vitamin
A intake can may lead to improper tooth formation in
children and to gum disease.
IMMUNE SYSTEM - Epithelial surfaces are adversely
affected by vitamin A deficiency causing increased
susceptibility to skin and respiratory infections. Immune
cells and antibody functions are also affected which may
lead to an increase in pre-cancerous cells in the
epithelial tissues of the mouth, throat and lungs.
THYROID GLAND - A deficiency of vitamin A can contribute
to lower levels of active thyroid hormone with symptoms
of low body temperature, depression, difficulty in losing
weight, headaches and lethargy.
Therapeutic uses: Vitamin A
supplements are used to prevent or treat deficiency and
in developing countries to protect against or treat
measles and other viral infections.
CANCER - Many studies suggest that high blood levels of
vitamin A can help prevent certain forms of cancer,
particularly cancers of epithelial tissue. A recent study
in Western Australia found that miners who had been
exposed to carcinogenic substances had less risk of
cancer when given 7500 mg RE per day.
SKIN DISORDERS - The vitamin A derivatives etretinate and
isotretinoin are used to treat psoriasis.
These compounds
inhibit the formation of some of the toxic compounds
which may be responsible for the high rate of cell
division causing the scaly build up on the skin.
OTHER USES:
Vitamin A eye drops have been used to treat dry eyes.
Creams containing vitamin A have been used to heal wounds
in patients taking corticosteroid drugs.
Interactions:
Vitamin E and zinc are necessary for vitamin A
metabolism, including absorption, transport and release
from the liver. Vitamin E may protect against some of the
effects of excess vitamin A.
Vitamin A is necessary for calcium metabolism in the
formation of healthy bones and teeth.
Smokers may have increased needs for vitamin A and beta
carotene in order to protect against free radical damage
(see antioxidants) .
Vitamin A is destroyed in the body by mineral oil
laxatives, which bind it. Antacids, the anti-gout drug colchicine, and the cholesterol reducing drug,
cholestyramine inhibit vitamin A absorption.
Alcohol irritates the digestive tract and inhibits the
absorption of vitamin A while also depleting the body’s tissue stores.
Cautions:
Vitamin A supplements in doses of more than 3000 mg RE probably should not be taken by women who may become pregnant.
Vitamin A supplements should not be taken with
isotretinoin or etretinate for skin disease or in cases
of impaired liver or kidney function. If vitamin A
supplements are taken with large amounts of alcohol liver damage may occur.
Broad spectrum antibiotics should not be taken with high
doses of vitamin A.
Vitamin A is also known as:
Retinol
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