IRON-ABSORPTION AND RED-BLOOD-CELL INCORPORATION IN PREMATURE-INFANTSFED AN IRON-FORTIFIED INFANT FORMULA

This study was designed to identify differences in red blood cell (RBC ) incorporation and iron absorption in premature infants between iron provided in a premature infant formula compared with iron provided as a supplement between feedings. We used a triple stable isotope techniq ue in which 13 infants received Fe-57 mixed with Enfamil Premature For mula on d 1 of the study, and Fe-54 with a multivitamin supplement bet ween meals on d 2. Two weeks later, blood was drawn for isotope analys is and Fe-58 was given i.v. The percentage RBC incorporation of the Fe -54 and Fe-57 was calculated, and the percent absorption of these trac ers was estimated by dividing by the percentage of Fe-58 identified in RBCs 14 d after its infusion. We found a small, but significantly gre ater, percentage of RBC incorporation of the Fe-54 given as a suppleme nt compared with the Fe-57 given in the formula (9.7 +/- 3.8% versus 7 .8 +/- 3.1%, p = 0.02). The RBC Fe-57 incorporation was closely correl ated with the reticulocyte count (r = 0.80, p = 0.001), but not the se rum ferritin or the Hb concentration. Approximately 68% of an i.v. dos e of Fe-58 was incorporated into RBCs. These findings indicate 1) iron is incorporated well into RBCs from preterm infant formula, with only a small increase in incorporation when given as a supplement, and 2) the reticulocyte count, but not the Hb concentration, is a good measur e of RBC iron-incorporating capacity.