While breast milk provides complete nutrition for full term infants, supplementation with human milk fortifiers (HMF) is required to achieve optimal weight gain in very low birthweight (VLBW) preterm neonates. Traditionally, HMF have been derived from bovine milk. Bovine-based infant formula has been shown to cause dysbiosis of the infant gut microbiome (Azad et al 2013) and increased oxidative stress in preterm neonates (Friel et al 2011). Microbiome dysbiosis and oxidative stress have been implicated in numerous inflammatory conditions, including both acute (eg. necrotizing enterocolitis, NEC) and long-term (eg. asthma, metabolic syndrome) sequela of preterm birth (Torrazza et al 2013, Goulet et al 2015, Flora et al 2007, Perrone et al 2014). Recent studies show that new human-derived HMF (H2MF) are superior to standard bovine HMF for nourishing VLBW preterm infants and preventing NEC (Sullivan et al 2010, Cristofalo et al 2013). However, the biological basis for these clinical benefits is unknown, which limits our ability to inform and improve feeding strategies for VLBW preterm infants. This will be the first study to evaluate the impact of H2MF on gut microbiota and oxidative stress in preterm infants.