Products
Vitamin A is commercially available in the form of pharmaceuticals, dietary supplements, foods, and cosmetics, among others. Dosage forms include, for example, capsules, tablets, effervescent tablets, syrups and eye ointments.
Structure and properties
Vitamin A is one of the fat-soluble vitamins and is practically insoluble in water. Vitamin A is the name given to a number of substances of very similar isoprenoid structure that are found in animal tissues. The most important and biologically active substance is all-()-retinol, a primary alcohol. Retinol can be oxidized to the corresponding aldehyde retinal and to the carboxylic acid retinoic acid (all all-trans shown here).Vitamin A is often used in the form of retinyl esters, for example as retinol palmitate, which is found in many pharmaceuticals. Vitamin A is very sensitive to air, oxidizing substances, acids, light and heat. Vitamin A is found, for example, in liver (high concentration! ), liver sausage, milk, butter, cheese, tuna and eggs. Certain carotenoids such as beta-carotene are provitamins that are metabolized to vitamin A in the body. Beta-carotene is found in carrots and hot peppers, for example. Vitamin A can be stored, unlike most water-soluble vitamins. Storage occurs primarily in the liver and in the form of fatty acid esters.
Effects
Vitamin A (ATC A11CA01) is involved in the following processes in the body (selection), among others:
- Visual process
- Cell growth and differentiation
- Reproduction, embryonic development
- Protein synthesis
- Bone formation
- Immune system
The effects are due to the activation of nuclear receptors that bind to DNA and affect gene expression. Beta-carotene is additionally antioxidant.
Indications
- For the prevention and treatment of vitamin A deficiency.
- For the treatment of skin diseases, mainly in the form of retinoids.
Dosage
According to the drug label. Vitamin A is administered perorally, parenterally, ocularly, and topically, among others. Dosage information is provided in international units (IU). The drug should not be overdosed (see below).
Contraindications
- Hypersensitivity
- Pregnancy, women of childbearing age without safe contraception: high doses of the vitamin are teratogenic (damaging to the fetus), especially in early pregnancy. In early pregnancy, therefore, the consumption of liver should be avoided.
- Hypervitaminosis A
- Severe renal insufficiency
- Increased intracranial pressure
- Therapy with retinoic acid or retinoids
For complete precautions, see the drug label.
Interactions
Some drugs such as colestyramine, colestipol, kerosene oil, neomycin, and orlistat may reduce the absorption of vitamin A and should not be taken concomitantly. When oral contraceptives are administered, blood levels of vitamin A may be increased. Other interactions have been described with vitamin K antagonists and tetracyclines.
Adverse effects
No adverse effects are expected at the recommended daily doses. Unlike other vitamins, acute and chronic overdose is possible, which may manifest, for example, as increased intracranial pressure.
