Determination of Intestinal Uptake of Essential Trace Elements Using Stable Isotopic Tracers and Rare Earth Markers

Previous studies for determination of intestinal uptake of trace elements by faecal monitoring using enriched stable isotopic tracers have required extended collection of faecal output to ensure quantitative recovery of excreted tracer. This requirement can lead to underestimation of 'absorption' since the contribution of 're-excreted' isotopic tracer initially taken up can not be identified. The use of non-absorbable dietary markers can enable account to be taken of faecal recoveries achieved in a reduced period of collection, removing the inconvenience of long-term collection of faeces and extending the potential applications of such investigations. Faecal recoveries of rare earths and their excretion kinetics have been investigated to confirm that they meet the required criteria for nutritional markers. Particular attention is given to the extent to which markers duplicate the behaviour of tracers. Determination of intestinal uptake of iron and zinc by combined use of enriched stable isotopic tracers and rare earths as markers is then considered. La, Sm, Eu, Tb, and Yb were administered to two healthy subjects with their lunch. Faecal samples were collected for 1 day before (baseline) and their concentrations in faecal output were determined by instrumental neutron activation analysis. The recovery investigation was then extended to six subjects with the rare earth contents of samples being separated by ion- exchange before irradiation, to minimize radiation exposure during analysis. This reduced the time required for analysis and improved limits of detection. Faecal recoveries of rare earths are interpreted in terms of profiles of excretion as a function of time and kinetic parameters. Having established that recoveries of rare earths in faeces for a week after consumption were nearly quantitative, their application to determine intestinal uptake of iron and zinc is considered. Procedures for determination of intestinal uptake from a standard drink containing ferrous sulphate labelled with 57Fe tracer and Sm marker, and from a wheat flour based meal containing ferric chloride and zinc labelled with 58Fe and 70Zn tracers and Yb marker were established. Seven healthy subjects consumed the standard solution and meal at a 3 day interval. Faecal samples were collected for 10 days starting from day before the first administration. Urine samples were collected for 24 h before the first and for 24 h after each administrations. Blood samples also were taken at the end of collection period to establish if there is any relation between intestinal uptake of iron and ferritin concentrations in blood serum. Following ion exchange column separation, concentrations of Sm and Yb markers, and 58Fe and 70Zn tracers were determined by neutron activation analysis. 57Fe and natural background levels of rare earths in faeces and urine were determined by inductively coupled plasma mass spectrometry. Total iron and zinc contents of faecal samples were determined by atomic absorption spectrophotometry, to enable account to be taken of the isotopic contribution of iron and zinc of natural composition to enrichment in the faecal pool. Faecal recoveries are interpreted in terms of the intestinal kinetic behaviour of markers and tracers and Intestinal uptake of tracers are derived from recoveries of markers and tracers for individual samples and for composites of sequential faecal outputs. Recovery profiles of the rare earths show that they appear within 24 hours after consumption, and reach a maximum in 48 hours. The major fraction of recovery is with the first two samples, and recoveries with the three samples are not significantly different from recoveries with the first four or five samples. For iron consumed with a standard solution, intestinal uptakes obtained for composites of the first two and three samples are significantly different from those for composites of the first four samples and total collection. Correlation between serum ferritin levels and intestinal uptakes obtained for composites of the first two samples, and total collection suggest that iron is bioavailable in the ferrous state, taken up by the intestine and transferred into circulating blood at a slow rate, whilst a significant fraction is excreted with exfoliated epithelial cells. Intestinal uptakes of iron consumed with the farina meal are more consistent and there is no significant differences between composites of the first two, three, and four samples, but not for composites of the following samples. This can be explained by the contribution of retarded excretion of iron intially taken up becoming significant in the later collections. Intestinal uptakes of zinc, obtained from composites of the first to five samples are not significantly different. Excretion of endogenous zinc is approximately equivalent to its daily absorption. For determination of intestinal uptake of essential trace elements with reduced faecal collection by the combined use of enriched stable isotopic tracers and rare earths as markers has been established for iron and zinc for specified conditions. It has been shown that rare earths enable such investigations with multi-isotopes/elements. It is anticipated that the method developed should be widely applicable to unconfined situations and vulnerable groups of subjects such as children and pregnant women