Isotope-dilution technique for determination of endogenous faecal excretion and true absorption of iodine in 125I labelled rats

Introduction In studies on the quantitative mineral metabolism the separation of total faecal mineral excretion into the fraction of dietary and of endogenous origin is often a methodical barrier. Both components cannot be distinguished by standard chemical procedures. But their separation is essential to the quantification of the mineral flux from the diet into the organism and back from tissues into the faecal excretion as it is necessary for example to quantify the bioavailability of dietary mineral sources (K irchgessner et al. 1993). For this purpose, the use of isotopes is an appropriate means. During the last decades, several techniques employing radioactive or stable isotopes have been developed, e.g. the ‘comparative balance method’, the ‘dual tracer’ or ‘double isotope’ technique, methods based on computerized ‘compartment analysis’ and the ‘isotope-dilution technique’ (e.g. A ubert et al. 1963; T hompson 1965; B elshaw et al. 1974; G ibson et al. 1988). Among these methods, the isotope-dilution technique, in particular, comprises direct and quantitative measurements of mineral fluxes and provides robust estimates of endogenous faecal excretions as has been shown for a series of macrominerals and trace elements (W eigand and K irchgessner 1976a,b; W eigand et al. 1986a,b; K reuzer and K irchgessner 1991; R euber et al. 1993; K irchgessner et al. 1994; W indisch and K irchgessner 1994; W indisch et al. 1997; G abler et al. 1997). For iodine however, there is no appropriate isotope method available. Therefore, the present experiment was designed to establish the isotope-dilution technique for iodine. The isotope-dilution technique is based on a single parenteral injection or a long-term oral administration of the tracer (W indisch and K irchgessner 1994). After the labelling procedure, all tracer that appears in the faeces is of endogenous origin. The calculation from the quantity of tracer recovered in the faeces to the total amount of endogenous faecal excretion of the respective element is performed by the use of a reference tissue from which the endogenous excretion originates or which is at least in very close physiological relationship to the endogenous excretion. Using ¹²⁵I as tracer the total amount of endogenous faecal iodine is calculated as follows: Endogenous faecal iodine (ng/day) = A_faeces/SA_{reference tissue}A_faeces = ¹²⁵I activity in the faeces (Bq/day)SA_{reference tissue} = specific ¹²⁵I activity of the reference tissue (Bq ¹²⁵I per ng of total iodine)True absorption of dietary iodine (ng/day) = Iodine intake – iodine in faeces (total – endogenous)In total, the present methodological study had to focus on two major aspects. Since the administered tracer needs time to reach steady-state kinetics within the excretory pool of iodine it was to be clarified at first, from which day after a single ¹²⁵I administration does the faecal ¹²⁵I excretion correctly represent the total endogenous excretion quantitatively. Secondly, it had to be clarified which tissue or body fluid may be used as a proper reference source to quantify the endogenous faecal excretion and thus to calculate true absorption of iodine.