VitaminAFormen

Vitamin A refers to a family of fat-soluble compounds important for vision, growth and immune function. It comprises two main groups: preformed vitamin A, which includes retinol and its esters found in animal-derived foods, and provitamin A carotenoids, such as beta-carotene, alpha-carotene and beta-cryptoxanthin, found in colorful fruits and vegetables. Provitamin A carotenoids can be converted to retinol in the body, while retinol can be stored in the liver as retinyl esters.

In the visual cycle, retinal combines with opsin to form rhodopsin, enabling low-light vision. Retinoic acid, a metabolite, acts as a signaling molecule regulating gene expression through nuclear receptors. Absorption occurs in the small intestine with dietary fat, and the vitamin is transported in chylomicrons to the liver, where it is stored as retinyl esters and released as retinol when needed. In the bloodstream, retinol is carried by retinol-binding protein (RBP) and is delivered to tissues with the help of transthyretin.

Deficiency of vitamin A can cause night blindness and xerophthalmia and may impair growth and immune function, particularly in regions with limited dietary intake. Excess intake of preformed vitamin A can lead to toxicity, with acute or chronic symptoms and potential teratogenic effects during pregnancy; by contrast, high intake of intact provitamin A carotenoids from foods is generally considered safe, though high-dose supplements may cause skin hyperpigmentation and other effects.