The investigators will explore the effect of current intravenous feeding (parenteral nutrition (PN)) formulations on blood arginine levels and the genes that are involved in body nutrition and fighting infection in premature babies. They will also investigate the effect of supplementing arginine on these genes. The investigators will undertake a single centre exploratory physiological study in 12 very premature infants receiving PN. 4 of these infants will be supplemented with arginine. The investigators will record nutritional intake and routine biochemical testing data (which includes amino acid levels) collected over the first 10 days of life. They will take blood for analysis at prespecified intervals for microarray, ammonia and IGF-1 levels. Microarray findings will allow the investigators to describe the effect of arginine on gene activity in preterm infants.