RARE-EARTH ELEMENTS AS NONABSORBABLE FECAL MARKERS IN STUDIES OF IRON-ABSORPTION

The use of rare earth elements as nonabsorbable fecal markers for stud ies of iron absorption from sources labeled extrinsically with stable isotopes was evaluated. On 3 successive days 13 healthy fasting adults were given different stable isotopes of iron with samarium, ytterbium , or dysprosium. On day 1, three meals were given with Fe-57 (1 mg per meal) plus samarium (0.33 mg per meal); on day 2, identical meals (ta ken with a calcium supplement to reduce iron bioavailability) were giv en with equivalent amounts of Fe-58-labeled iron and ytterbium; on day 3, a well-absorbed reference dose of Fe-54 (3 mg) was given with 1 mg Dy. A complete fecal collection was carried out for 5-9 d and each st ool was analyzed for rare earth elements by inductively coupled plasma -mass spectrometry and iron isotopes by thermal ionization quadrupole mass spectrometry. Mean recovery of rare earth elements was 101%, indi cating that they are totally unabsorbed. The excretory pattern of the iron isotopes and the rare earth elements was very similar; the correl ation coefficients between samarium and Fe-57, ytterbium and Fe-58, an d dysprosium and Fe-54 were 0.992, 0.989, and 0.988, respectively (P < 0.001). Iron absorption was calculated as the difference between isot ope dose and fecal excretion. Mean (+/- SEM) iron absorption was 16.7 +/- 2.4%, 4.3 +/- 1.6%, and 40.3 +/- 3.1% on days 1-3, respectively. P redicted values estimated from the first 4 d of pooled feces, using th e rare earth element recovery data to produce corrected figures for un absorbed isotope, were in close agreement: 19.1 +/- 2.1%, 4.6 +/- 1.7% , and 40.8 +/- 3.1%, respectively (P < 0.001). With the diet of medium iron bioavailability and with the highly bioavailable reference dose it was possible to predict iron absorption accurately from only one or two stools, provided that they were sufficiently enriched with isotop e and a rare earth element.