Inadequate vitamin A (VA) nutrition continues to be a major problem worldwide, and many
interventions being implemented to improve VA status in various populations need to be evaluated. The
interpretation of results after an intervention depends greatly on the method selected to assess VA status.
To evaluate the effect of an intervention on VA status, researchers in Cameroon, India, Indonesia, Mexico,
Senegal and Zambia have used serum retinol as an indicator, and have not always found improvement in
response to supplementation. One problem is that homeostatic control of serum retinol may mask positive
effects of treatment in that changes in concentration are observed only when status is either moderately
to severely depleted or excessive. Because VA is stored mainly in the liver, measurements of hepatic
VA stores are the “gold standard” for assessing VA status. Dose response tests such as the relative dose
response (RDR) and the modifi ed relative dose response (MRDR), allow a qualitative assessment of VA
liver stores. On the other hand, the use of the vitamin A-labeled isotope dilution (VALID) technique,
(using 13 C or 2
H-labeled retinyl acetate) serves as an indirect method to quantitatively estimate total body
and liver VA stores. Countries including Cameroon, China, Ghana, Mexico, Thailand and Zambia are
now applying the VALID method to sensitively assess changes in VA status during interventions, or to
estimate a population’s dietary requirement for VA. Transition to the use of more sensitive biochemical indicators of VA status such as the VALID technique is needed to effectively assess interventions in
populations where mild to moderate VA defi ciency is more prevalent than severe defi ciency.