vitaminsA

Vitamin A refers to a group of fat-soluble compounds essential for a variety of biological processes. It includes retinoids—retinol, retinal, and retinoic acid—and provitamin A carotenoids such as beta-carotene found in plants. Animal foods provide preformed vitamin A as retinol and retinyl esters, while plant foods supply carotenoids that the body can convert to retinol. Absorption requires dietary fat, and vitamin A is primarily stored in the liver. In the bloodstream, retinol is carried by retinol-binding protein and transthyretin. The efficiency of converting provitamin A carotenoids to retinol varies with nutrition and individual factors.

Functions of vitamin A are diverse. In vision, 11-cis-retinal forms the chromophore rhodopsin, enabling low-light perception and adaptation to changes in illumination. Retinoic acid regulates gene expression and cell differentiation, influencing growth, immune function, and tissue maintenance. Vitamin A also supports reproductive health and normal embryonic development.

Deficiency can cause night blindness, xerophthalmia, and Bitot’s spots, along with skin and mucous membrane changes and increased susceptibility to infections. It is more common in populations with inadequate dietary intake or fat malabsorption. Hypervitaminosis A can occur with excessive intake of preformed vitamin A, producing acute symptoms (nausea, headache) or chronic liver and bone effects and teratogenic risks during pregnancy.

Recommended intake varies by age and sex. Adults generally require about 700–900 μg/day of retinol activity equivalents, while the tolerable upper intake level for adults is about 3,000 μg/day of preformed vitamin A. Provitamin A carotenoids from foods pose little toxicity risk, though high-dose supplements, especially beta-carotene, may have adverse effects in certain populations.