Provitamin and Dietary Sources
The precursor to vitamin A is $\beta$-carotene. It exhibits pronounced antioxidant properties and exerts an anti-xerophthalmic effect, reliably protecting the cornea and conjunctiva of the eye from drying out.
In its natural form, this provitamin can be obtained exclusively from plant foods. It is most abundant in brightly colored vegetables and fruits: carrots, pumpkins, bell peppers, beets, apricots, and sea buckthorn. A significant amount of the substance is also found in greens, such as spinach and green onions. An important feature is that $\beta$-carotene is completely absent in animal-derived products.
Classification of Drugs (Retinoids)
Pharmacological preparations of vitamin A are divided into three main generations:
- 1st Generation: This includes Retinol itself, its ester forms (retinol acetate and retinol palmitate), and carotene, which is often included in complex multivitamin formulations.
- 2nd Generation (cyclic retinoids): This group is represented by acitretin (Neotigason).
- 3rd Generation (carotenoids): Includes drugs such as adapalene and tazarotene. It should be noted that some representatives of this group (specifically tazarotene) may not have official registration in the Russian Federation.
Pharmacodynamics and mechanism of action
The mechanism of action of retinoids is characterized by high specificity, as their effects are realized directly at the genetic level. Active substance molecules interact with specific nuclear receptors. In pharmacology, two main types of such structures are distinguished: RAR (retinoic acid receptors) and RXR (retinoid X receptors). Binding to them leads to the activation of transcription processes and fine regulation of gene expression.
At the organism level, this manifests in several major biological effects:
- Visual function: ensuring the synthesis of rhodopsin, without which normal vision under low-light conditions is impossible.
- Tissue metabolism: active participation in systemic oxidation-reduction reactions and the complex regulation of lipid metabolism.
- Protection of integuments: stimulation of the formation of glycosaminoglycans and glycoproteins, which guarantees the normal functioning and rapid regeneration of the skin epithelium and all mucous membranes.
Clinical Application and Pathological Conditions
In modern medical practice, indications for the use of vitamin A preparations are quite extensive. The main direction remains replacement therapy for the treatment and prevention of hypovitaminosis and avitaminosis. Due to its property of stimulating epithelization, retinoids are actively used in dermatology and traumatology (treatment of psoriasis, extensive burns, trophic ulcers). In gastroenterology, they are prescribed for inflammatory diseases of the gastrointestinal tract (chronic enteritis and enterocolitis). In addition, the drugs are used in the therapy of lung diseases, hepatitis, and ophthalmological disorders.
The clinical picture of deficiency (vitamin A hypovitaminosis) includes a characteristic triad of disorders:
- Ophthalmological: development of hemeralopia, a twilight vision disorder known as "night blindness".
- Dermatological: deep lesions of the skin and mucous membranes.
- Immunological: a sharp decrease in the content of the protective enzyme lysozyme in biological fluids and a drop in leukocyte activity.
With an overdose, a picture of acute poisoning (vitamin A hypervitaminosis) develops. From the central nervous system, lethargy, drowsiness, and severe headache occur. Dyspepsia joins in the form of nausea and vomiting. Specific signs of toxic action are intense skin desquamation and rapid hair loss (alopecia).
Vitamin A receptors are easy to remember by their abbreviations: RAR (Retinoic Acid Receptors) and RXR (where X is an unknown variable influencing gene expression). And the main triad of deficiency can be learned as "DIM": Dysfunction of vision/hemeralopia (eyes), Inflammation/dermatitis (skin and mucous membranes), Munodeficiency/immunodeficiency (decrease in lysozyme and leukocyte activity).
Class and exam questions
Which animal-derived products contain $\beta$-carotene?
None. $\beta$-carotene (provitamin A) is synthesized exclusively in plants. Its main sources are brightly colored vegetables, fruits, and greens.
Why does vision in the dark worsen with vitamin A deficiency?
Retinol is absolutely essential for the synthesis of the visual pigment rhodopsin. When it is lacking, the adaptation of the eyes to twilight is impaired, leading to the development of hemeralopia ("night blindness").
How does vitamin A affect cells at the molecular level?
It acts at the genetic level. By binding to specific nuclear RAR and RXR receptors, retinoids regulate gene expression through the activation of transcription.
What symptoms indicate acute poisoning with vitamin A preparations?
Acute hypervitaminosis manifests as lethargy, drowsiness, headache, nausea, and vomiting. Characteristic external dermatological signs are severe skin desquamation and hair loss (alopecia).
What else to study on this topic
- Detailed mechanism of visual pigment rhodopsin synthesis in the retina.
- Differences in the pharmacokinetics of retinol esters: acetate and palmitate.
- Features of the use of cyclic retinoids (acitretin) in the treatment of psoriasis.
- The role of vitamin A in the systemic regulation of lipid metabolism.
- Mechanisms of immunity reduction and lysozyme level drop in vitamin A hypovitaminosis.
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