APPARENT ZINC-ABSORPTION BY RATS FROM FOODS LABELED INTRINSICALLY AND EXTRINSICALLY WITH ZN-67

A variety of foods (peas (Pisum sativum), chicken meat, eggs, goat's milk, human milk) enriched with the stable isotope Zn-67 were prepared by means of intrinsic- and extrinsic-labelling procedures. They were fed to rats and apparent absorption of Zn-67 determined from faecal excretion measurements using thermal ionization mass spectrometry. There were significant differences in the absorption of the extrinsic and intrinsic label which differed in magnitude between the foods tested. The extrinsic Zn-67 was less well absorbed in peas, chicken meat, eggs, and human milk than intrinsic Zn-67, but in goat's milk the extrinsic Zn-67 was better absorbed than the intrinsic label. These results demonstrate that extrinsically-added stable Zn isotopes do not fully exchange with endogenous Zn in many foods, and illustrate the need for caution when using extrinsic labels for Zn bioavailability studies.