Document reference: http://namls.info/Zinc/Gibson.html (August 2003)
THE APPLICATION OF NUCLEAR METHODS TO RESEARCH ON ZINC IN HUMAN NUTRITION
Department of Human Nutrition, University of Otago, Dunedin 9015, New Zealand
The United Nations has only recently recognized the public health consequences of zinc deficiency in developing countries (DCs). The presence of stable and long-lived radioactive isotopes of zinc has allowed nuclear techniques to play a critical role in highlighting the widespread occurrence of zinc deficiency in populations consuming plant-based diets by providing data on: (i) the zinc content of soils, plant-based staples, foods, and whole diets; (ii) the bioavailability of dietary zinc and how this is modified by the simultaneous presence of other miconutrients and antinutrients (especially phytate) in the diet; and (iii) zinc content of biological tissues and fluids used to identify population subgroups at risk of zinc deficiency.
The contribution of isotopic techniques to our understanding of factors affecting zinc bioavailability has played a critical role in identifying four factors associated with the etiology of zinc deficiency in DCs: low intakes of dietary zinc; excessive intakes of dietary components (e.g., phytate) that inhibit zinc absorption; environmental and disease states such as recurrent diarrhea, malaria and schistosomiasis that induce excessive losses of zinc; and physiological states (e.g. infancy, pregnancy and lactation, and catch-up growth in low birth weight infants) that increase zinc requirements. The extent to which these increased needs are met by homeostatic adjustments in zinc absorption or endogenous excretion, in individuals consuming marginal-zinc diets can also be quantified using stable isotope techniques.
Results of randomized controlled trials in DCs have confirmed a causal relationship between zinc deficiency and childhood stunting, as well as infectious diseases such as diarrhea and pneumonia, two leading causes of death in DCs. Individuals with zinc deficiency may also be more susceptible to malaria and have impaired neuropsychologic performance during childhood. In some pregnancy studies, zinc deficiency has been shown to contribute to complications and low birthweight, and disturbances in fetal neurobehavioural development.
With the recognition of the impact of zinc deficiency on human health has come the need to develop intervention programmes to alleviate this deficiency in DCs, preferably by incorporating zinc into pre-existing micronutrient intervention strategies. Stable isotope methods have important applications in these intervention programmes. Supplementation, fortification, dietary diversification/modification can be used Isotopic methods can be used to quantify the bioavailability of various forms of zinc supplements and fortificants, either singly or in combination with multiple-micronutrients. This is critical: there may be significant differences in the bioavailability of zinc supplements as the oxide or sulphate, both forms often used as components of iron prenatal supplements. In the longer term, isotope methods can be used to measure zinc absorption from staple diets before and after modification to reduce their phytate content. This can be achieved by genetic mutations which block the synthesis of phytate in cereals such as corn, or by household processing methods such as soaking to remove water soluble phytates in corn by diffusion. In all such cases, changes in fractional zinc absorption can be monitored by in vivo human isotope studies. Only when the dietary zinc is utilized more efficiently, can an improvement in zinc status be expected.
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