Vitamin A describes a family of essential, fat-soluble compounds that are structurally related to retinol and share its biological activity. Provitamin A carotenoids are dietary precursers of retinol. Most of the vitamin A in the body is stored in the liver in the form of retinyl ester. A measure of liver stores would be the best index for vitamin A status, but such measurements are not practical. Moreover, the measure of plasma levels do not reflect body stores until they are severely depleted, as the plasma contains only about 1% of the total body reserve of vitamin A. Thus experts advise using a combination of measures from different techniques.
Serum vitamin A appears in the form of retinol and retinol-binding protein (RBP). Serum retinol levels remain constant until liver stores are severely depleted or contain an excess amount. Low serum levels are seen in patients with xerophthalmia. Normal serum vitamin A levels hit above 20 mcg/dL. Levels between 10 and 19 mcg/dL depict marginally low stores and below 10 mcg/dL indicate a deficient state. Excessive intakes of vitamin A can result in levels over 65 mcg/dL.
Factors that can decrease plasma vitamin A include stress, liver disease, infections, parasites, and zinc deficiency due to zinc's role in the synthesis of retinol-binding protein. Low-fat diets can impair the absorption of vitamin A because fat is needed for its absorption. Factors that increase plasma levels include renal disease and estrogens, which mobilize vitamin A from the liver. Ingestion of vitamin A does no effect serum levels of retinol and therefore fasting is not necessary before a test. However, serum samples should be protected from bright light and hemolysis after being obtained.
Less than 5% of vitamin A in the serum is in the form of retinyl esters. Levels increase when the capacity of the liver to store vitamin A is exceeded. Because ingestion of vitamin A immediately preceding a test can cause levels of these esters to rise, a patient must fast prior to being tested.
Levels of serum carotenoid reflect current intake. Serum carotenoid levels may be useful as a secondary measure of vitamin A in populations that consume carotenoids as their primary vitamin A source, but not very useful for populations consuming primarily preformed vitamin A.
The relative dose response measure is a functional test that estimates vitamin A in liver stores. In vitamin A deficiency, retinol-binding protein accumulates in the liver as apo-RBP, a form that is not bound to retinol. When a dose of vitamin A is administered, holo-RBP (protein bound to retinol) is released from the liver and an increase in serum retinol is rapidly seen. Plasma is taken at baseline, a dose of vitamin A is given, and a plasma sample is taken 5 hours later. The percentage change in serum retinol is then calculated. A percent- change of 20% and higher indicates a deficient liver store of vitamin A.