Several approaches for estimation of fractional zinc absorption (FZA) by calculating the ratio of oral to intravenous stable isotopic tracer concentrations (at an appropriate time) in urine or plasma following their simultaneous administration have been proposed in the last decade. These simple to implement approaches, often referred to as the double isotopic tracer ratio (DITR) method, are more attractive than the classical method termed "deconvolution" and the more commonly used single tracer methods based on fecal monitoring and indicator dilution following oral or intravenous tracer administration, respectively. However, the domain of validity of DITR for measuring FZA has recently been questioned. In this paper we provide a theoretical justification of the validity of four "approximate" different formulations of the DITR technique by mathematically demonstrating that their accuracy is a consequence of the particular properties of zinc kinetics.