A DOUBLE STABLE-ISOTOPE TECHNIQUE FOR MEASURING IRON-ABSORPTION IN INFANTS

A stable isotope technique has been developed which uses Fe-57 and Fe-58 as labels and which enables the simultaneous measurement of Fe absorption from two test meals in infants. The method was evaluated by measuring Fe absorption from a commercial whey-adjusted infant formula in nine healthy infants aged 13-25 weeks. Each infant was fed 210 ml formula, labelled with either Fe-57 or Fe-58, on four consecutive mornings, in random order. The total Fe content in each feed was 2.5 mg Fe; either as 2.5 mg Fe-57, or 0.6 mg Fe-58 plus 1.9 mg Fe with normal isotopic composition. Isotopic enrichment of Fe in erythrocytes was measured by thermal ionization mass spectrometry 14 d after the last administration, and Fe absorption was calculated based on isotope ratio shifts, total circulating Fe and intake of each isotope. Geometric mean absorption for the Fe-57 and Fe-58 labels was 6.72 and 6-58 % respectively, and the absorption of the two isotopes was not significantly different (Student's paired t test). By this technique, paired comparisons of Fe absorption can be obtained and systematic studies of the influence of dietary factors on Fe absorption during infancy can be conducted.