Provitamin and Dietary Sources
The precursor to Vitamin A is $\beta$-carotene. It possesses pronounced antioxidant properties and exerts an anti-xerophthalmic effect, reliably protecting the cornea and conjunctiva of the eye from dryness.
Naturally, this provitamin is obtained exclusively from plant sources. It is most abundant in brightly colored vegetables and fruits, such as carrots, pumpkins, bell peppers, beets, apricots, and sea buckthorn. Significant amounts are also found in greens, including spinach and green onions. Notably, $\beta$-carotene is completely absent in animal-derived products.
Pharmacological Classification (Retinoids)
Pharmacological preparations of Vitamin A are divided into three main generations:
- First Generation: Includes Retinolum itself, its ester forms (retinol acetate and retinol palmitate), and carotene, which is frequently included in complex multivitamins.
- Second Generation (Aromatic Retinoids): This group includes acitretin.
- Third Generation (Arotinoids): Includes agents such as adapalene and tazarotene.
Pharmacodynamics and Mechanism of Action
The mechanism of action of retinoids is characterized by high specificity, as their effects are mediated directly at the genetic level. Active substance molecules interact with specific nuclear receptors. Pharmacology distinguishes two main types of such structures: RAR (retinoic acid receptors) and RXR (retinoid X receptors). Binding to these receptors leads to the activation of transcription processes and fine-tuned regulation of gene expression.
At the whole-organism level, this manifests in several major biological effects:
- Visual Function: Ensuring the synthesis of rhodopsin, without which normal vision in low-light conditions is impossible.
- Tissue Metabolism: Active participation in systemic oxidation-reduction reactions and complex regulation of lipid metabolism.
- Protection of Coverings: Stimulation of glycosaminoglycan and glycoprotein synthesis, guaranteeing normal functioning and rapid regeneration of skin epithelium and all mucous membranes.
Clinical Application and Pathological States
In modern medical practice, indications for Vitamin A preparations are extensive. The primary direction remains replacement therapy for the treatment and prevention of hypovitaminosis and avitaminosis. Due to its property of stimulating epithelialization, retinoids are actively used in dermatology and traumatology (treatment of psoriasis, extensive burns, trophic ulcers). In gastroenterology, they are prescribed for inflammatory gastrointestinal diseases (chronic enteritis and enterocolitis). Additionally, these agents are used in the therapy of pulmonary diseases, hepatitis, and ophthalmological disorders.
Clinical presentation of deficiency (Vitamin A hypovitaminosis) includes a characteristic triad of disturbances:
- Ophthalmological: Development of hemeralopia—impairment of scotopic vision, known as night blindness.
- Dermatological: Deep lesions of the skin and mucous membranes.
- Immunological: Sharp decrease in the protective enzyme lysozyme in biological fluids and a drop in leukocyte activity.
Overdose leads to acute poisoning (Vitamin A hypervitaminosis). Central nervous system symptoms include lethargy, drowsiness, and severe headache. Dyspepsia manifests as nausea and vomiting. Specific signs of toxic action include intense skin desquamation and rapid hair loss (alopecia).