ABSTRACT
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4.1.1 Use of Isotopes
Measuring trace-element absorption from the diet requires the use of isotopes to label the trace-element source. This allows differentiation between the proportion of trace element derived from the test food(s) and that derived from other sources (dietary or endogenous origin) in body fluids and tissues, as illustrated in Figure 4.1. The isotope used to label the food(s) must be present in the same chemical form as the native trace element. This can be achieved through biosynthetic (intrinsic) labelling, but it is an expensive and time-consuming approach; therefore, extrinsic labelling is used wherever possible. The latter technique was developed for iron using radioisotopes and is valid under conditions where complete isotopic exchange takes place prior to absorption.
