Usefulness of Vitamin A Isotope Methods for Status Assessment: From Deficiency through Excess

Abstract

A variety of methods exist to assess vitamin A status of groups and populations. Vitamin A status is usually defi ned by the liver retinol concentration. Most indicators of status do not measure or estimate liver stores of retinol. Clinical signs only have utility when liver reserves are almost exhausted, and serum retinol concentrations have utility in the zone of overt defi ciency. Dose response tests offer more coverage, but cannot distinguish among liver vitamin A stores in the adequate through toxic range. Different countries continue, or are beginning, to add preformed vitamin A to a variety of staple foods through fortifi cation, and vitamin A supplements are still being distributed in many countries, especially to preschool children. Further, provitamin A biofortifi ed crops are currently being released in several countries. Assessing population vitamin A status in response to these interventions needs to move beyond serum retinol concentrations. Indicators that work in the excessive to toxic range of liver reserves are needed. To date, the only indirect indicator that has been validated in this range of liver reserves in animals and humans is the retinol isotope dilution test using deuterium or 13 C, which spans the entire liver reserve continuum from defi ciency through excess.

Host‐related factors explaining interindividual variability of carotenoid bioavailability and tissue concentrations in humans

“…This likely results in variable blood/tissue concentrations . However, blood plasma/serum alone may not constitute the best indicator to assess carotenoid status, and additional methods, such as isotopic labelling, similar as for retinoids or easy accessible compartments such as white blood cells or buccal cells , may allow for more insights regarding endogenous carotenoid levels, carotenoid compartments, and turnover .…”

Overlapping vitamin A interventions in the United States, Guatemala, Zambia, and South Africa: case studies

“…25 The fact that there was no difference in serum retinol between the two groups, and none had values >1.9 mol/L confirms that hypervitaminosis A cannot be detected by using serum retinol concentrations. 61…”

Use of Stable Isotopes to Evaluate Bioefficacy of Provitamin A Carotenoids, Vitamin A Status, and Bioavailability of Iron and Zinc

“…The dose response is based on the accumulation of apo -retinol-binding protein in the liver during times of vitamin A depletion and rapid release of the holo -retinol-binding protein after a challenge dose ( 23 ). The modified relative dose response (MRDR) test, which determines a serum ratio of orally consumed 3,4-didehydroretinol (vitamin A 2 ) to retinol, similar to tracer dilution, can detect the degree of vitamin A deficiency in a population ( 10 ) but does not quantify total body retinol stores in an individual or group ( 9 † ). For example, the MRDR test was used to evaluate high-dose supplementation ( 24 † ) and a green leafy vegetable intervention ( 25 † ) in Ghanaian lactating women.…”

“…Determination of these parameters is not trivial ( 8 † ). Stable isotope methods are the most powerful techniques available to scientists for evaluating vitamin A status and provitamin A bioefficacy ( 9 † , 10 , 11 † , 12 ) and iron and zinc bioavailability from meals ( 13 † ). The International Atomic Energy Agency (IAEA) has invested in capacity building of laboratories worldwide, and one of their main objectives is to foster the safe use of stable isotopes in human nutrition and health.…”