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The Validity of Extrinsic Stable Isotopic Labeling for Mineral Absorption Studies in Rats¹

Department of Nutrition Diet and Health, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Norwich NR4 7UA, United Kingdom

The use of extrinsic stable and radioisotopic labels (Fe, Zn, Cu and Se) was compared with the use of intrinsic labels by measuring label retention in rats. Saccharomyces cerevisiae (Hansen strain CBS 1171) was prepared intrinsically enriched with a stable isotope of iron, zinc, copper or selenium, and unenriched freeze-dried yeast was extrinsically labeled with the appropriate stable and/or radioisotope. Male Wistar rats, weighing 80–100 g and fed a purified diet, were given a test meal of one of the above labeled yeasts. Isotopic retention was determined by fecal monitoring. Retention of the stable isotopes was determined by thermal ionization quadruple mass spectrometry (TIQMS) and retention of the radioisotopes by counting feces in a whole-body counter. The results indicated that the behavior of the labels differed among the minerals, with copper as the only one in which the intrinsic and extrinsic stable isotopes were comparably retained. With zinc, retention of the extrinsic radiolabel and intrinsic label was similar, but retention of the extrinsic stable isotope label was higher. With iron, the intrinsic label had a significantly lower retention than the two extrinsic labels; with selenium, retention of all three labels was different, but these differences were not of a sufficient magnitude to conclude that extrinsic stable isotopic labeling is not valid. These results demonstrate that an extrinsic stable isotope label can be used for copper, selenium and inorganic iron, but that such a label is not valid for studies on zinc.

KEY WORDS: • retention • minerals • rats • intrinsic extrinsic labeling • stable isotopes

¹ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

² Julio Boza was funded by the Ministerio de Educacion y Ciencia, Spain.

³ To whom correspondence should be addressed.

Manuscript received 1 June 1994. Revision accepted 30 November 1994.