Vitamin - A deficiency
Definition
Vitamin A deficiency exists when the chronic failure to eat
sufficient amounts of vitamin A or beta-carotene results in levels of
blood-serum vitamin A that are below a defined range. Beta-carotene is a form of
pre-vitamin A, which is readily converted to vitamin A in the body. Night
blindness is the first symptom of vitamin A deficiency. Prolonged and severe
vitamin A deficiency can produce total and irreversible blindness.
Description
Vitamin A (called retinol in mammals) is a fat-soluble vitamin. The
recommended dietary allowance (RDA) for vitamin A is 1.0 mg/day for the adult
man and 0.8 mg/day for the adult woman. Since beta-carotene is converted to
vitamin A in the body, the body's requirement for vitamin A can be supplied
entirely by beta-carotene. Six mg of beta-carotene are considered to be the
equivalent of 1 mg of vitamin A. The best sources of vitamin A are eggs, milk,
butter, liver, and fish, such as herring, sardines, and tuna. Beef is a poor
source of vitamin A. Plants do not contain vitamin A, but they do contain
beta-carotene and other carotenoids. The best sources of beta-carotene are
dark-green, orange, and yellow vegetables; spinach, carrots, oranges, and sweet
potatoes are excellent examples. Cereals are poor sources of beta-carotene.
Vitamin A is used for two functions in the body. Used in the eye, it
is a component of the eye's light-sensitive parts, containing rods and cones,
that allow for night-vision or for seeing in dim-light circumstances. Vitamin A
(retinol) occurs in the rods. Another form of Vitamin A, retinoic acid, is used
in the body for regulating the development of various tissues, such as the cells
of the skin, and the lining of the lungs and intestines. Vitamin A is important
during embryological development, since, without vitamin A, the fertilized egg
cannot develop into a fetus.
Causes and
symptoms
Vitamin A deficiency occurs with the chronic consumption of diets
that are deficient in both vitamin A and beta-carotene. When vitamin A
deficiency exists in the developed world, it tends to happen in alcoholics or in
people with diseases that affect the intestine's ability to absorb fat. Examples
of such diseases are celiac disease (chronic nutritional disorder), cystic
fibrosis, and cholestasis (bile-flow failure or interference). Vitamin A
deficiency occurred in infants during the early 1900s in
Denmark.
The deficiency resulted when milk fat was made into butter for export, leaving
the by-product (skimmed milk) for infant feeding. Vitamin A deficiency has taken
place in infants in impoverished populations in India, where the only foods fed
to the infants were low in beta-carotene. Vitamin A deficiency is also common in
areas like Southeast Asia, where polished rice, which lacks the vitamin, is
a major part of the diet.
The earliest symptom of vitamin A deficiency is night blindness.
Prolonged deficiency results in drying of the conjunctiva (the mucous membrane
that lines the inner surface of the eyelids and extends over the forepart of the
eyeball). With continued vitamin A deficiency, the drying extends to the cornea
(xerophthalamia). The cornea eventually shrivels up and becomes ulcerated
(keratinomalacia). Superficial, foamy gray triangular spots may appear in the
white of the eye (Bitot's spots). Finally, inflammation and infection occur in
the interior of the eye, resulting in total and irreversible blindness.
Diagnosis
Vitamin A status is measured by tests for retinol. Blood-serum
retinol concentrations of 30-60 mg/dl are considered in the normal range. Levels
that fall below this range indicate vitamin A deficiency. Night blindness is
measured by a technique called electroretinography. Xerophthalamia,
keratinomalacia, and Bitot's spots are diagnosed visually by trained medical
personnel.
Treatment
Vitamin A deficiency can be prevented or treated by taking vitamin
supplements or by getting injections of the vitamin. The specific doses given
are oral retinyl palmitate (110 mg), retinyl acetate (66 mg), or injected
retinyl palmitate (55 mg) administered on each of two successive days, and once
a few weeks later if symptoms are not relieved.
Prognosis
The prognosis for correcting night blindness is excellent.
Xerophthalamia can be corrected with vitamin A therapy. Ulcerations, tissue
death, and total blindness, caused by severe vitamin A deficiency, cannot be
treated with vitamin A.
Prevention
Vitamin A deficiency can be prevented by including foods rich in
vitamin A or beta-carotene as a regular component of the diet; liver, meat,
eggs, milk, and dairy products are examples. Foods rich in beta-carotene include
red peppers, carrots, pumpkins, as well as those just mentioned. Margarine is
rich in beta-carotene, because this chemical is used as a coloring agent in
margarine production. In
Africa,
Indonesia, and the Philippines, vitamin A deficiency is prevented by public
health programs that supply children with injections of the vitamin.
Key Terms
Bitot's spots
Bitot's spots are superficial, foamy
gray, triangular spots on the white of the eyeball.
Carotenoids
Carotenoids are yellow to deep-red
pigments.
Conjunctiva
The conjunctiva is a clear layer of
cells that covers the eye and directly contacts the atmosphere. The conjunctiva
is about five-cells thick.
Cornea
The cornea is a clear layer of cells
that covers the eye, just under the conjunctiva. The cornea is about 50-cells
thick.
Fat-soluble vitamin
Fat-soluble vitamins can be dissolved
in oil or in melted fat. Water-soluble vitamins can be dissolved in water or
juice.
Keratinomalacia
Keratinomalacia is ulceration of the
cornea.
Recommended Dietary Allowance (RDA)
The Recommended Dietary Allowances are
quantities of nutrients in the diet that are required to maintain good health in
people. RDAs are established by the Food and Nutrition Board of the National
Academy of Sciences, and may be revised every few years. A separate RDA value
exists for each nutrient. The RDA values refer to the amount of nutrient
expected to maintain good health in people. The actual amounts of each nutrient
required to maintain good health in specific individuals differ from person to
person.
Xerophthalmia
Xerophthalmia is a dry, thickened,
lusterless condition of the eyeball resulting from vitamin A deficiency
