Vitamin A in human nutrition.

Vitamin A, an unsaturated 20 carbon cyclic alcohol, has a variety of physiological functions including a role in vision, reproduction, growth and maintenance of epithelial and bone structures. The main sources of vitamin A are from preformed vitamin A in animal foods or from -carotene in green plants. Carotene is cleaved in the intestinal mucosa to retinol, which is transported in chylomicrons mainly to the liver which is the major storage site of vitamin A, where stores are mainly of retinyl palmitate. Utilization of vitamin A appears to be a highly regulated process; retinol is transported in the serum bound to a specific binding protein. There also may be some control of the level of retinol in cells possibly through membrane receptors or of excretion from the cell. Intracellular cytosolic retinol binding proteins transport retinol to the nucleus where specific receptors for retinol have been found. Intracellular binding proteins for retinoic acid and retinal, metabolites of retinol have also been found, and an interstitial protein transporting retinol is present in the eye. Vitamin A deficiency causes cessation of growth, night blindness, and renders the organism more susceptible to infection, and vitamin A supplementation has been shown to enhance immune response. Epidemiological studies have shown low vitamin A and carotene to be correlated with incidence of cancer. Excess vitamin A intake results in hypervitaminosis with severe detrimental effects. Vitamin A requirements appear to be met in most developed countries although in the populations at greatest risk, newborns and pregnant and nursing women, cases of deficiency are observed. However, in large areas of the world vitamin A deficiency is endemic, causing widespread blindness and mortality.