Determination of iron absorption using erythrocyte iron incorporation of two stable isotopes of iron (57Fe and 58Fe) in very low birthweight premature infants.

Due to limited erythropoiesis, iron needs for the premature infant during the first 2 months of life are low. With the potential for increased use of erythropoietin in the preterm infant, iron requirements may become markedly higher. The ability of the preterm infant to absorb iron, therefore, becomes crucial. Previous studies using single stable isotopes of iron without metabolic balances were unable to quantitatively determine iron absorption since the percentage of absorbed iron appearing in the erythrocyte could not be measured. The objective of the current study was to determine iron absorption using the method of erythrocyte iron incorporation of two stable isotopes of iron given by the enteral and parenteral route to very low birth weight (VLBW) infants. Results obtained were compared to iron absorption values from studies using single isotopes and metabolic balance techniques. Six VLBW premature infants (gestational age 26.8 +/- 1.7 weeks, postnatal age 3.6 +/- 1.9 weeks, birth weight 863 +/- 117 g) were studied. Iron dosages were (i.v.) 0.15 mg/kg (57FeSO4) and (enteral) 1.5 mg/kg (58FeSO4). Erythrocyte isotope ratios, 57Fe/54Fe and 58Fe/54Fe, were determined by inductively coupled plasma mass spectrometry (ICP/MS) from single blood samples (100 microliters) collected before and after concurrent enteral (58Fe) and parenteral (57Fe) administration of isotopes. Only 17.8% of the i.v.-infused iron dose was incorporated into hemoglobin on day 15. Using a correction factor based on the percentage of i.v. iron (57Fe) incorporated into erythrocytes, the corrected incorporation of 58Fe was calculated to be 26.3 +/- 13.0% of the enteral dose of 58FeSO4.