Phenylalanine, Methionine and Histidine Requirements in TPN Fed Neonates

This is a single center project assessing the requirements for three essential amino acids in TPN fed neonates. Using the Carbon Oxidation method (indicator amino oxidation and direct amino acid oxidation method), the investigators will determine the requirement of each of the 3 amino acids. The investigators will first determine the requirement for Phenylalanine, then Methionine and finally Histidine. The investigators will recruit 18 - 20 babies per amino acid study. Breath and urine samples will be collected to determine the oxidation of the indicator amino acid. The response of the indicator amino acid to changes in intake of the test amino acid (phenylalanine, methionine, and histidine) will be analyzed by bi-phase linear mixed effect model to determine the breakpoint or mean requirement for each amino acid. It is hypothesized that the requirement for phenylalanine, methionine and histidine will be at least 50% lower than what is currently available in commercial amino acids solutions used for TPN feeding of neonates.