ASPIRE is a nutrition study focusing on the effect of arginine supplementation on immune function in postoperative infants. The investigators will explore the effect of current intravenous feeding (parenteral nutrition (PN)) formulations and oral arginine supplementation on blood arginine levels and the genes that are involved in body nutrition and fighting infection in babies who have had major bowel surgery or been diagnosed with necrotising enterocolitis. The investigators will undertake an exploratory physiological study across two sites under which are part of a single neonatal partnership. 48 infants will be recruited; 24 preterm infants and 24 term/near term infants. 16 of these infants (8 preterm and 8 term/near term) will be supplemented with arginine in both oral and parenteral form, 16 infants will receive arginine supplementation in oral form alone and 16 infants will receive standard nutrition with no arginine supplement. The investigators will record nutritional intake and routine biochemical testing data (which includes amino acid levels) collected over the first 30 days post surgery or post NEC diagnosis. The investigators will take blood for analysis at prespecified intervals for RNA sequencing, ammonia and metabolomics. RNA sequencing findings will allow the investigators to describe the effect of arginine on gene activity in postoperative infants The investigators hypothesise that arginine supplementation will result in changes in gene expression that are consistent with changes in T-cell function and associated inflammatory pathways.