These values, which vary by age and gender, include: DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people.
Intake recommendations for vitamin A and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences). In some studies, high plasma or serum concentrations of some provitamin A carotenoids have been associated with a lower risk of various health outcomes, but these studies have not definitively demonstrated that this relationship is causal. For clinical practice purposes, plasma retinol levels alone are sufficient for documenting significant deficiency.Ī plasma retinol concentration lower than 0.70 micromoles/L (or 20 micrograms /dL) reflects vitamin A inadequacy in a population, and concentrations of 0.70–1.05 micromoles/L could be marginal in some people. A plasma retinol level increase of at least 20% indicates an inadequate vitamin A level. Liver vitamin A reserves can be measured indirectly through the relative dose-response test, in which plasma retinol levels are measured before and after the administration of a small amount of vitamin A. However, their value for assessing marginal vitamin A status is limited because they do not decline until vitamin A levels in the liver are almost depleted.
Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy. Most of the body's vitamin A is stored in the liver in the form of retinyl esters. Both retinyl esters and provitamin A carotenoids are converted to retinol, which is oxidized to retinal and then to retinoic acid. The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A. Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin's important biological functions. The body converts these plant pigments into vitamin A. By far the most important provitamin A carotenoid is beta-carotene other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin.
Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids. Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs. Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea. Vitamin A is involved in immune function, vision, reproduction, and cellular communication. Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters. For a reader-friendly overview of Vitamin A, see our consumer fact sheet on Vitamin A. This is a fact sheet intended for health professionals.