Abstract: Disclosed are human milk fortifier compositions, in either powder or liquid forms, comprising nutrients and selected iron-containing materials, wherein the fortifiers when added to human milk do not significantly inhibit or otherwise eliminate the inherent, in-vitro antimicrobial properties of the milk. This is accomplished by formulating the compositions with iron-containing insoluble iron, soluble bound iron, or combinations thereof, with little or no soluble unbound iron. Also disclosed are methods of providing nutrition to infants, especially preterm infants, by adding the human milk fortifier described herein with human milk to form a fortified human milk, and then administering the fortified human milk to the infant. The fortifier can also be used to fortify other infant formulas.