Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine.

The definition of amino acid requirements for neonates receiving total parenteral nutrition (TPN) is critical for the further improvement of this nutritional regimen. In the present study we investigated the kinetics and requirements of phenylalanine and tyrosine in neonatal piglets receiving TPN. Twenty-four 3-d-old male Yorkshire piglets were fitted with external jugular and femoral catheters and maintained on identical TPN formulations for 5 d. Total amino acid, phenylalanine, tyrosine, and energy intakes were 15, 0.61, and 0.51 g. kg-1 . d-1 and 1.1 MJ . kg-1 . d-1, respectively. On day 5, piglets (three per level) were randomly assigned to receive one of eight phenylalanine intakes: 0.2. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 1.2 g. kg-1 . d-1. On day 6, phenylalanine kinetics were measured during a 4-h primed, continuous infusion of L-[1-14C]phenylalanine. Plasma phenylalanine and phenylalanine oxidation were statistically similar for the three lowest phenylalanine intakes and increased thereafter. Crossover regression analysis yielded estimates for the mean requirement and safe phenylalanine intake of 0.45 and 0.48 g . kg-1 . d-1, respectively (equivalent to 30 and 32 mg/g amino acids, respectively), in the presence of excess tyrosine. An inability of piglets to maintain a linear oxidative response at phenylalanine intakes > 0.8 g . kg-1 . d-1 (equivalent to 53 mg/g amino acids) was found. These data represent the first direct estimates of phenylalanine requirements in neonates receiving TPN and demonstrate the use of oxidation techniques for the estimation of amino acid requirements during parenteral nutrition.