Retinol (Vitamin A1) Unit Conversion

Vitamin A is a subclass of a family of lipid-soluble compounds referred to as retinoic acids. There are essentially three forms of vitamin A: retinols, betacarotenes, and carotenoids. Retinol, also known as preformed vitamin A, is the most active form and is mostly found in animal sources of food. Betacarotene, also known as provitamin A, is the plant source of retinol from which mammals make two thirds of their vitamin A. Carotenoids, the largest group of the three, contain multiple conjugated double bonds and exist in a free alcohol or in a fatty acyl ester form. Vitamin A promotes normal vision and prevents night blindness; contributes to growth of bone, teeth, and soft tissues; supports thyroxine formation; maintains epithelial cell membranes, skin, and mucous membranes; and acts as an antiinfection agent.

A clearly defined physiologic role for retinol is in vision. Retinol is oxidized in the rods of the eye to retinal, which, when complexed with opsin, forms rhodopsin, allowing dim-light vision. This vitamin and vitamin D act through specific nuclear receptors in the regulation of cell proliferation. Vitamin A deficiency leads to night blindness (nyctalopia) and, when prolonged, may cause total blindness. In vitamin A deficiency states, epithelial cells (cells in the outer skin layers and cells in the lining of the gastrointestinal, respiratory, and urogenital tracts) become dry and keratinized. Fruits and vegetables contain carotene, which is a precursor of retinol. Carotenes provide more than one-half of the retinol requirements in the American diet. Vitamin A deficiency is most common among children living in nonindustrialized countries and is usually a result of insufficient dietary intake. Deficiency may also occur because of chronic fat malabsorption or impaired liver function or may be associated with severe stress and protein malnutrition. Premature infants are born with lower serum retinol and RBP levels, as well as lower hepatic stores of retinol; therefore, these newborns are treated with vitamin A as a preventive measure. When ingested in high doses, either chronically or acutely, vitamin A causes many toxic manifestations and may ultimately lead to liver damage due to hypervitaminosis. High doses of vitamin A may be obtained from excessive ingestion of vitamin supplements or large amounts of liver or fish oils, which are rich in vitamin A. Carotenoids, however, are not known to be toxic because of a reduced efficiency of carotene absorption at high doses and limited conversion to vitamin A. The RDA of vitamin A is 900 g per day for adult males and 700 g per day for adult females. Measurement of retinol is the most common means of assessing vitamin A status in the clinical setting. Retinol is most commonly measured by high-performance liquid chromatography (HPLC). Toxicity is usually assessed by measuring retinyl ester levels in serum rather than retinol, which is accomplished by HPLC.

Increased: Chronic kidney disease, idiopathic hypercalcemia in infants, vitamin A toxicity

Decreased: Abetalipoproteinemia, carcinoid syndrome, chronic infections, disseminated TB, hypothyroidism, infantile blindness, liver, GI, or pancreatic disease, night blindness, protein malnutrition, sterility and teratogenesis, zinc deficiency