Iron Balance and Iron Nutritional Status in Preterm Infants During the First Four Months of Life

Objectives: To determine whether iron absorption occurs in a dose-dependent fashion and/or is a function of iron nutritional status (INS) in preterm infants during the first 4 months of life. Methods: Preterm very-low-birth-weight infants (VLBWI) were fed an iron-fortified (0.7 mg/dL) infant formula. Three 48 h balance studies were performed on each infant. INS was determined by serially measuring hemoglobin, mean corpuscular volume (MCV), hematocrit, ferritin, transferrin and transferrin saturation levels. The data were analyzed using ANOVA and stepwise regression. Results: Fifty-four balance studies were performed in 18 infants (birth weight, 1347 +/- 201 g; gestation, 30 +/- 1.3 weeks; mean +/- standard deviation) at 33 +/- 1.3, 34 +/- 1.2, and 48 +/- 0.5 weeks corrected age and weights of 1768 +/- 260, 2298 +/- 314, 5127 +/- 939 g. No relationship was detected between iron intake and absorption. Intake decreased during the study (1.17 +/- .08, 1.24 +/- 0.11 > 1.1 +/- 0.15 mg center dot kg(-1) center dot day(-1)) but net (0.32 +/- 0.26, 0.36 +/- 31 < 0.49 +/- .23 mg center dot kg(-1) center dot day(-1)) and % (27 +/- 22, 29 +/- 23 < 46 +/- 21) absorption increased (P < 0.01). Serum ferritin, transferrin saturation and MCV fell, while hematocrit and hemoglobin remained stable. No relationship was noted between serum ferritin and iron absorption but transferrin saturation (54%), MCV (7%), and hematocrit (6%) accounted for 67% of the variation in iron absorption (P < 0.001). Conclusions: At intakes of 0.8-1.4 mg center dot kg(-1) center dot day(-1), iron absorption is not dose-dependent nor affected by iron stores. Only when iron delivery to the tissues decreases does absorption increase to meet needs in these otherwise normal and rapidly growing infants.