2.2 Study design of iron absorption study Infants enrolled in the study will receive a combination of parenteral and enteral feeds. The 57FeSO4 solution will be provided parenterally while the 58FeSO4 will be given via the enteral route. Erythrocyte incorporation of 57Fe and 58Fe will then be measured from blood samples taken two weeks after initial infusions. The percentage of the I.V administered dose presents in the circulating erythrocytes will be used as a reference for the percentage of absorbed iron (from the orally administered dose) present in the circulating erythrocytes. The “corrected” percent erythrocyte 58Fe and 57Fe incorporation (58 Fe inc-cor/ 57Fe inc-cor) will be calculated mathematically as described below.