Why is the IAEA concerned with vitamin A related measurements?
Dietary causes of vitamin A deficiency
Consuming sufficient amounts of vitamin A from animal sources such as butter, liver, or egg yolks can prevent or reverse deficiency. Other sources such as leafy green and yellow vegetables and dark yellow fruits should, in principle, also be able to provide the provitamin A carotenoids from which vitamin A is synthesized.
But many factors affect the bioavailability (the proportion of a nutrient that is absorbed and utilized) of food-based provitamin A carotenoids including plant structure, diet composition, nutritional status and other physiological factors. Effective interventions for populations that rely on plant-based sources depend on better information about the efficacy of provitamin A-rich foods.
Assessing vitamin A status: conventional indicators and isotope techniques
Approximately 90% of the body's vitamin A is stored in the liver. Because direct measurements of liver vitamin A content would require liver biopsies, more practical indicators including dietary, physiological, biochemical, histological and clinical procedures have been adopted.
While such conventional indicators can help to identify individuals with critical vitamin A deficiency, they are not reliable measures of the size of the total vitamin A in the body-measures that are critically needed in global efforts to enhance nutrition.
Pioneering work begun nearly 20 years ago using radiotracers in animals demonstrated the scientific validity of isotopic tracers to measure vitamin A stores. Several new, more accurate isotopic tracer methods are now being developed for assessing human vitamin A status and assessing the bioavailability of provitamin A carotenoids. IAEA is also helping to pioneer these methods.
|Provitamin A Carotenoid Bioavailability
IAEA initiatives for eliminating vitamin A deficiency
Vitamin A deficiency has many causes; preventing or reversing it requires the integration of a variety of nutritional and public health interventions. The flow chart illustrates which interventions have comparative advantages and the roles to which isotopic techniques are uniquely well-suited.
An IAEA programme was launched in 1995 to develop and modify isotopic techniques for measuring whole body vitamin A under conditions of supplementation (Ghana, Peru), food fortification (Peru, Israel) and dietary improvement (China, Thailand, Philippines, India). The projects all address problems of vitamin A nutrition in children and pregnant or lactating women.
In Peru and Ghana, the IAEA is collaborating with WHO in evaluating the efficacy of WHO vitamin A supplementation trials. In South Africa, the maternal to infant transmission of HIV is being evaluated with regard to maternal vitamin A status.
In addition, the IAEA is helping with the production of isotopically labelled foods. Once consumed, the isotopically labeled foods provide information about the bioavailability of carotenoids, which can lead to better strategies for improving vitamin A nutrition.
Vitamin A investigations in Ghana and Peru
WHO and other expert groups have long been concerned that methods are not adequate to measure biologically relevant changes in vitamin A status. But now the IAEA is collaborating in a major WHO project to evaluate a vitamin A supplementation trial in some 5,500 mother-infant pairs in two Member States, Ghana and Peru.
WHO is seeking to determine whether a particular supplementation scheme is more efficient in protecting the vitamin A status of infants who are otherwise at risk of deficiency and the associated risks of increased morbidity and mortality. The IAEA is providing isotopes, experts in isotopic techniques, and isotopic analysis for measurement vitamin A status in infants. The inter-agency collaboration is leading to improved isotopic methods for evaluating vitamin A nutrition in Member States that desperately need better tools for monitoring and controlling programme impact.