Most very low birth weight infants accumulate a nutrient deficit in hospital due to minimal nutrient reserves and elevated nutritional requirements which may contribute to poor outcome. Adding nutrients to human milk improves their nutritional status and growth, but it is unclear if adding bovine protein-based fortifiers as is the current standard of care has some unintended negative consequences to neonates. Infants will be randomized to have their feeds (mother's own milk or pasteurized donor breastmilk) nutrient enriched with a human milk-based fortifier or a bovine protein-based fortifier and will be followed in hospital to assess feeding tolerance, growth, gut inflammation, mother's milk and infant gut microbiome, and neonatal morbidity and mortality.

Inclusion Criteria:

• <1250g birth weight
• Parent(s) or guardian(s) agree to use donor breastmilk if own mother's milk supply is insufficient

Exclusion Criteria:

• Infant receives formula or a nutrient fortifier prior to randomization
• >day 14 at the time of enrollment and enteral feeds have not commenced
• Infants with major congenital or chromosomal anomalies that could impact growth
• Enrollment in another research study affecting nutritional management during the feeding intervention
• Reasonable potential that the infant will be transferred to a neonatal intensive care unit where the study protocol will not be continued