However, these levels are not always reliable indicators of vitamin A status because they do not decline until vitamin A levels in the liver and other storage sites are almost depleted and because acute and chronic infections can decrease serum and plasma retinol concentrations.
Retinol and carotenoid levels are typically measured in plasma or serum because blood samples are easy to collect. Most of the body’s vitamin A is stored in the liver in the form of retinyl esters.
Retinol is then oxidized to retinal and retinoic acid, the two main active vitamin A metabolites in the body. Retinyl esters and provitamin A carotenoids are converted to retinol after uptake into the lumen (for retinyl esters) or absorption (for provitamin A carotenoids). The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells. Other carotenoids in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A and are referred to as non-provitamin A carotenoids they might have other important activities not involving vitamin A formation. The main provitamin A carotenoids in the human diet are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Provitamin A carotenoids are plant pigments that the body converts into vitamin A in the intestine. Preformed vitamin A is found in foods from animal sources, including dairy products, eggs, fish, and organ meats. The human diet contains two sources for vitamin A: preformed vitamin A (retinol and retinyl esters) and provitamin A carotenoids. Vitamin A is also critical for vision as an essential component of rhodopsin, the light-sensitive protein in the retina that responds to light entering the eye, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea. Vitamin A supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, eyes, and other organs. Vitamin A is involved in immune function, cellular communication, growth and development, and male and female reproduction. Vitamin A is the name of a group of fat-soluble retinoids, primarily retinol and retinyl esters. For a reader-friendly overview of Vitamin A and Carotenoids, see our consumer fact sheet on Vitamin A and Carotenoids. This is a fact sheet intended for health professionals.
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