Amino Acid and Acylcarnitine Profiles in Premature Neonates

Malnutrition is a common problem in the neonatal intensive care unit. Recent studies indicate that prematurely born neonates commonly develop a severe nutritional deficit during the first weeks after birth, referred to as extrauterine growth restriction. Despite an increase in growth during the second month of hospitalization, many neonates are ultimately discharged home having grown inadequately. The early nutritional deficit affects weight gain as well as growth in length and head circumference. Aggressive administration of parenteral amino acids to improve protein accretion rates in very preterm neonates has been supported in the literature. Although tolerance of high dose amino acids has been described, researchers acknowledge that sensitive tests to monitor amino acid toxicity are not readily available in the clinical setting. The goals of this study are: * To better define normal amino acid and acylcarnitine values and how they change in premature neonates * To measure the effect nutritional support has (human breastmilk vs. formula) on amino acid and acylcarnitines profiles * To measure the effect of illness (parenteral nutrition associated cholestasis) on amino acid and acylcarnitine profiles * To better define abnormal metabolic profiles (low tyrosine levels) in neonates that have hypothyroidism.