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THE IMPORTANCE OF VITAMIN A FOR CHILDREN'S HEALTH AND LIFE
Rano Nuraliyeva
Samarkand State Medical University
Assistant of the Department of Clinical Pharmacology
https://doi.org/10.5281/zenodo.10611393
Abstract.
Vitamin A is a group of substances close in chemical structure, they are retinol
(vitamin A1, axerophytol) and other retinoids with similar biological activity: dehydroretinol
(vitamin A2), retinal (retinen, vitamin A1 aldehyde) and retinoic acid. Provitamin A includes
carotenoids, the metabolic precursors of vitamin A; β-carotene is the most important among them.
Retinoids are found in animal-derived foods, while carotenoids are found in plant-derived foods.
All these substances are very soluble in non-polar organic solvents (for example, oils) and poorly
soluble in water. Vitamin A is stored in the liver and can be stored in tissues. It is toxic in cases of
overdose.
Key words:
vitamin A, polyvitamin A, products rich in vitamin A.
ЗНАЧЕНИЕ ВИТАМИНА А ДЛЯ ЗДОРОВЬЯ И ЖИЗНИ ДЕТЕЙ
Аннотация.
Витамин А представляет собой группу близких по химическому
строению веществ, к ним относятся ретинол (витамин А1, аксерофит) и другие
ретиноиды со схожей биологической активностью: дегидроретинол (витамин А2),
ретиналь (ретинен, альдегид витамина А1) и ретиноевая кислота. Провитамин А
включает каротиноиды, метаболические предшественники витамина А; Среди них β-
каротин является наиболее важным. Ретиноиды содержатся в продуктах животного
происхождения, а каротиноиды — в продуктах растительного происхождения. Все эти
вещества хорошо растворимы в неполярных органических растворителях (например,
маслах) и плохо растворимы в воде. Витамин А хранится в печени и может храниться в
тканях. Он токсичен при передозировке.
Ключевые слова:
витамин А, поливитамин А, продукты, богатые витамином А.
Vitamin A was discovered in 1913. Its structure was described in 1931, and it was
crystallized in 1937.
Vitamin A performs many important biochemical functions in the div of humans and
animals. Retinal, which is the main visual pigment, is a component of rhodopsin. The vitamin in
the form of retinoic acid stimulates growth and development. Retinol is a structural component of
cell membranes and provides antioxidant protection of the div.
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In the case of vitamin A deficiency, various damage to the epithelium develops, vision
deteriorates, and corneal hydration is disturbed. A decrease in immune function and retardation of
development are also observed.
Retinol breaks down under the influence of atmospheric oxygen and is very sensitive to
light. All compounds are prone to cis-trans isomerization, especially at bonds 11 and 13, but all
double bonds except 11-cis-retinal have a trans configuration.
STRUCTURE AND SHAPES
Vitamin A is a cyclic unsaturated alcohol consisting of a β-ionone ring and a side chain
consisting of two isoprene residues and a primary alcohol group. In the div, it is oxidized to
retinal (vitamin A-aldehyde) and retinoic acid. It is stored in the liver in the form of retinyl
palmitate, retinyl acetate and retinyl phosphate.
Vitamin A is present in all forms of animal products, but since pure retinol is unstable,
most of it is stored in the form of complex esters of retinol (produced in industry mainly in the
form of palmitate or acetate).
Plants contain provitamin A and some carotenoids. Precursors of the vitamin can be two
groups of substances that are similar in structure: carotenes (α-, β- and γ-carotenes) and
xanthophylls (β-cryptoxanthin). Carotenoids are also isoprenoid compounds, α- and γ-carotene
contain one β-ionone ring each, and in the process of oxidation, 1 molecule of retinol is formed
from them, while β-carotene has two ionone rings, so it has more biological activity. has and two
retinol molecules are formed from it.
Carnivores, such as cats, cannot convert carotenoids into retinal due to the lack of 15-15′-
monooxygenase (as a result, no carotenoid type is considered a form of vitamin A for these
animals).
VITAMIN A PRESERVING PRODUCTS
Retinol is found in large amounts in animal and plant-based products, especially marine
fish and mammalian liver. Carotenes can also be a source of vitamins for humans. They are not
toxic in high doses, but they cannot completely replace retinol, since only a limited amount can be
converted to vitamin A. The largest amount of β-carotene is found in different types of carrots, but
its concentration can vary dramatically depending on the variety (from 8 to 25 mg per 100 g).
Good sources are red peppers, green onions, lettuce, zucchini, and tomatoes.
Plant-based (carotenoids):
Green and yellow vegetables - carrots, pumpkin, sweet pepper, spinach, broccoli, green
onions, parsley;
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Legumes - soybeans, peas;
Fruits - peaches, apricots, apples, grapes, watermelons, melons, plums, chakanda, cherries.
Animal (retinoids) - fish oil, liver (especially beef liver), caviar, milk, butter, margarine,
sour cream, cottage cheese, cheese, egg yolk.
Genetically modified golden rice, rich in beta-carotene, is a potential solution to vitamin A
deficiency, but no variety of "golden rice" is available for consumption.
Synthetic retinol (in the form of complex esters) is obtained from β-ionone by gradually
growing a chain of double bonds.
INTERACTION
The synergist of vitamin A is vitamin E, which helps to maintain retinol in an active form,
absorption in the intestine and its anabolic effect. Vitamin A is often prescribed together with
vitamin D. In the treatment of hemeralopia, it should be used together with riboflavin, nicotinic
acid. It is impossible to prescribe cholestyramine, activated charcoal at the same time as vitamin
A, which disrupts its absorption.
TRANSCRIPTION OF GENES
Vitamin A and its derivatives affect special receptor proteins in the cell nucleus: Retinoic
acid receptor, beta (RARB); Retinoid-Related Orphan Receptor-gamma (RORC); Retinoid X
receptor, alpha (RXRA). The crystal structures of such nuclear receptors were studied by
crystallographic methods. Such a ligand-receptor complex binds to DNA fragments, causes gene
suppression, and thus regulates the synthesis of proteins, enzymes or tissue components, this action
is manifested in both embryogenesis and morphogenesis. The pharmacological effect of vitamin
A is determined by this property.
FUNCTIONS OF VITAMIN A
Vitamin A has the following pharmacological effects:
1. Synthesis of phosphoadenosine phosphosulfate-activating enzymes needed for the
synthesis of:
Mucopolysaccharides, hyaluronic acid, heparin;
Sulfocerbrosides;
Taurine;
Liver enzymes involved in the metabolism of endogenous and exogenous substances.
2. Synthesis of A1, A2, B and C somatomedins, which stimulate the synthesis of muscle
tissue proteins; incorporation of phosphate and thymidine into DNA, proline into collagen, and
uridine into RNA.
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3. Glycolysis of polypeptide chains:
Blood glycoproteins (α1 — macroglobulin, etc.);
Glycoproteins, which are components of cell and subcellular (mitochondrial and
lysosomal) membranes (this is of great importance for completing phagocytosis);
glycoprotein - fibronectin, which participates in intercellular interaction and inhibits cell
growth due to it.
4. Synthesis of sex hormones, shunindgek, interferon, immunoglobulin A, lysozyme.
5. Synthesis of epithelial tissue enzymes that prevent premature keratinization.
6. Activation of receptors for calcitriol (active metabolite of vitamin D).
7. Synthesis of rhodopsin in the rods of the retina, necessary for dawn vision.
Vitamin A compounds have different biological activities. Retinol is necessary for the
growth, differentiation and maintenance of functions of epithelial and bone tissues, as well as
reproduction. It is important in the mechanism of retinal vision. Retinoic acid is 10 times more
active than retinol in the processes of cellular differentiation, but less active in the processes of
reproduction.
PARTICIPATION OF VITAMIN A IN THE MECHANISM OF VISION
Vitamin A plays an important role in retinal vision. 11-cis-retinal opsin binds to proteins
to form the purple-red pigment rhodopsin, or one of three types of iodopsin, which are the primary
visual pigments involved in the generation of visual signals.
PARTICIPATION OF VITAMIN A IN ANTIOXIDANT PROTECTION OF THE
BODY
Due to the presence of two interconnected double bonds in the molecule, retinol is able to
interact with free radicals, including free oxygen radicals. This important property of the vitamin
allows it to be considered an effective antioxidant. Retinol also significantly improves the
antioxidant effect of vitamin E. However, vitamin A can also act as a poroxidant, as it is easily
oxidized by atmospheric oxygen to form highly toxic peroxide products. Vitamin E prevents
retinol oxidation.
HYPOVITAMINOSE A
In the world, hypovitaminosis A occurs in one third of children under the age of five. It
kills 670,000 children under the age of five each year. An estimated 250,000-500,000 children in
developing countries become blind each year due to vitamin A deficiency (mostly in Southeast
Asia and Africa).
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Vitamin A deficiency can be caused by primary or secondary deficiency. Primary vitamin
A deficiency is caused by not consuming enough carotenoids from fruits and vegetables or vitamin
A from animal and dairy products among children and adults. Stopping breastfeeding early can
also increase the risk of vitamin A deficiency in children.
Secondary vitamin A deficiency is associated with chronic exposure to oxidants such as
lipid absorption, biliary malabsorption, bile formation, cigarette smoke, and chronic alcoholism.
Vitamin A is a fat-soluble vitamin and depends on micellar solubilization for dispersion in the
small intestine, which results in poor absorption of vitamin A when fat content is low. Zinc
deficiency can also impair the absorption, transport and metabolism of vitamin A, as it is required
as a cofactor for the synthesis of transport proteins and the conversion of retinol to retinal. A study
in Burkina Faso found that a combination of vitamin A and zinc significantly reduced the incidence
of malaria among children.
The first symptom of hypovitaminosis A is squint (nyctalopia, hemeralopia) - a sharp
decrease in adaptation to the dark. Epithelial tissue damage is characteristic: skin (follicular
hyperkeratosis), intestinal mucosa (up to ulcer formation), bronchi (frequent bronchitis),
genitourinary system (mild infection). Dermatitis is accompanied by pathological proliferation,
keratinization and epithelial desquamation. Desquamation of the epithelium of the tear ducts can
lead to their blockage and a decrease in the hydration of the cornea, which begins to dry
(xerophthalmia) and soften (keratomalacia) with the formation of ulcers and leukemia. Damage to
the cornea can develop very quickly, because the breakdown of epithelial protective properties
leads to secondary infection. Growth retardation is also observed in vitamin deficiency.
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