This protocol will use common parenteral feeding regimens to study protein kinetics and the effects of the quality of infused energy on protein metabolism in the ELBW infant in the first 48 hours of life using stable isotope methodology combined with indirect calorimetry. Stable isotopic tracers of leucine, glucose and glycerol will be infused simultaneously to determine protein synthesis and oxidation, glucose production and utilization, and the rate of lipolysis.