Vitamin A
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 Retinol
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.
Serum Retinyl Ester
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.
Serum Carotenoid
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.
Relative Dose Response
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.
Conjunctival Impression Cytology
The conjunctival impression cytology test is based on the lack of normal goblet
cells and the presence of enlarged epithelial cells in the conjunctiva of
vitamin-A deficient people. Cells are transferred from the conjunctiva to
filter paper, where they are stained and examined under a microscope.
Rapid Dark Adaptation Test
This test is based on measurements of the time of occurrence of the Purkinje
shift. This refers to the peak wavelength sensitivity of the retina shifting
from red to the blue end of the spectrum during the transition from day vision
to night vision. The test has high sensitivity and specificity.
