Iron absorption and red blood cell incorporation in premature infants fed 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 technique in which 13 infants received Fe-57 mixed with Enfamil Premature Formula on d 1 of the study, and Fe-54 with a multivitamin supplement between meals on d 2. Two weeks later, blood was drawn for isotope analysis 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 tracers was estimated by dividing by the percentage of Fe-58 identified in RBCs 14 d after its infusion. We found a small, but significantly greater, percentage of RBC incorporation of the Fe-54 given as a supplement 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 correlated with the reticulocyte count (r = 0.80, p = 0.001), but not the serum ferritin or the Hb concentration. Approximately 68% of an i.v. dose 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 measure of RBC iron-incorporating capacity.