Impaired growth and nutrition after preterm birth is associated with morbidities and poor neurodevelopmental outcome, but it is not clear whether this reflects causal relationships. To improve growth, human milk is often fortified with protein, energy and minerals, providing small, but significant, fortification-induced gains in body weight, length and/or head circumference. the effect of Bovine Colostrum fortification on AAs and proteins in plasma, and how fortification-induced AA changes during the first two weeks of fortification associated with changes in anthropometric measures, gestational age (GA) at birth, birth weight, key morbidities (BPD, ROP, LOS) and IGF-1 levels. Collectively, the analyses shed light on the complex interplay between fortification-induced changes to plasma AA/protein levels and the growth and morbidities in very preterm infants fed fortified human milk.