Document reference: http://namls.info/Zinc/Teucher.html (August 2003)
RECENT ADVANCES IN ISOTOPE METHODS FOR THE ASSESSMENT OF BIOAVAILABILITY OF ZINC IN FOODS, DIETS AND FORTIFICANTS
B. Teucher, J.R. Dainty, and S.J. Fairweather-Tait
Institute of Food Research, Norwich NR4 7UA, Norfolk, UK
Zinc bioavailability is a combination of fractional absorption (diet-related) and intestinal endogenous excretion (host-related); both can be measured using radio-or stable isotope approaches. Whole body counting can be employed to measure zinc bioavailability from foods labelled with the gamma-emitting 65Zn radioisotope but is restricted to centres with the necessary infrastructure and can only be used on low risk volunteers. Three out of the five zinc stable isotopes have low natural abundance (67Zn 4.1%; 68Zn 18.8%; 70Zn 0.6%) and in recent years these have been the main tools for zinc metabolism studies in children and women of child-bearing age. Non-isotope techniques such as the measurement of plasma zinc concentration and total urinary zinc excretion may be used as an approximate method of assessing bioavailability of zinc supplements.
Single and double stable isotope faecal monitoring techniques for 10-12 days can be used to estimate true absorption, but are subject to errors. In the single isotope method, correction is made for re-excreted absorbed tracer by plotting cumulative isotope excretion against time; true absorption is then calculated from the y intercept. In the double isotope technique, true absorption from oral zinc is calculated using appearance data of a second zinc stable isotope given intravenously. Compliance can be monitored by giving a non-absorbable rare earth element with the test meal e.g. dysprosium chloride; this follows the same excretory pattern as unabsorbed zinc.
Fractional zinc absorption from a single food/meal can be measured in urine after simultaneous oral and intravenous administration of different stable isotopes of zinc. Either 24-h or spot urine samples are collected 24 to 48 hours post-dosing. The oral stable isotope dose may become crucial when absorption and urinary excretion are low; under these conditions the isotope enrichment in urine may be below the levels of quantification of some mass spectrometers. Further research is required to confirm that the intravenous infusion of zinc causes no perturbation to systemic zinc metabolism and that zinc given intravenously is handled kinetically in the same way as orally absorbed zinc.
The appearance of zinc isotopes in plasma following oral and intravenous administration can be used to measure fractional absorption. Assuming that the two isotopes undergo the same rate of removal from the plasma, the fractional absorption of the oral dose can be calculated from the dose-corrected oral/intravenous ratio of the areas under the plasma concentration versus time curve. The accuracy of this method depends strongly on how long the zinc isotope appearance/disappearance in plasma is monitored for.
Urinary monitoring is the least invasive and most convenient stable isotope technique but the most important assumption concerning the validity of the method (i.e. same rate of clearance for the oral and intravenous dose) is still subject to debate. Furthermore, this technique is limited when the efficiency of absorption is low since isotope enrichment in the urine may be below the levels of quantification for some mass spectrometers
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