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.

The study is based on the carbon oxidation protocol. Each baby will receive one or two levels of either phenylalanine or methionine or histidine. Each level will be assessed over a two-day period.

On day 1: each baby will receive a standard pediatric amino acid solution, Primene used in the NICU and L-[1-13C]Na Bicarbonate will be given to measure carbon dioxide production. On day 2: he/she will receive the appropriate TPN Test Solution made up of the appropriate Bulk Amino Acid solution (for Phenylalanine/ Methionine/ Histidine Studies) with a different amount of the amino acid being investigated. To see how these amino acids are used in the body the investigators will add a small amount (calculated based on the body weight of the baby) of the amino acid L-[1-13C]phenylalanine to the amino acid solution. Breath and urine will be collected from the baby to study the oxidation of the L -[1-13C]phenylalanine. The investigators will take 2 x 0.75 ml blood to measure plasma amino acid concentration. The investigators will aim to coordinate blood taking with regular blood work done for clinical monitoring.

Inclusion Criteria:

1. Stable preterm babies that are growing and fully TPN fed (at least 90% of calories and protein).
2. TPN providing adequate calories and protein as determined by attending physician and dietitian.
3. Babies born ≥ 28 weeks gestation,
4. ≤ 28 days chronological age at the time of the study,
5. Birth weight and length appropriate for gestational age,
6. Medically stable as determined by normal blood results and lack of a fever or infection,
7. At least 3 days after surgery, if the baby had a surgery.

Exclusion Criteria:

1. Babies on mechanical ventilation, on low flow oxygen and CPAP.
2. Small for gestational age,
3. On medications known to affect protein and amino acid metabolism,
4. Documented infection, fever
5. Unstable medical condition
6. Receiving enteral feeding providing > 10% of protein intake.