The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosinerequirement

Although tyrosine is considered indispensable during the neonatal period, i ts poor solubility has limited its inclusion in parenteral amino acid solut ions to less than 1% of total amino acids. Dipeptides of tyrosine are highl y soluble, have been shown to be well used and safe in animal models and hu mans, and, therefore, may be used as an effective means of providing tyrosi ne in the parenterally fed neonate. The goal of the present study was to de termine the tyrosine requirement of the parenterally fed neonate receiving graded intakes of glycyl-L-tyrosine as a source of tyrosine. Thirteen infan ts receiving adequate energy (340 +/- 38 kJ.kg(-1).d(-1)) and protein (2.4 +/- 0.4 g kg(-1).d(-1)) were randomized to receive parenteral nutrition wit h one of five graded levels of glycyl-L-tyrosine. The mean requirement and safe level of intake were estimated using a 1-C-13-phenylalanine tracer and linear regression cross-over analysis that identified a break point in the response of label appearance in breath CO2 ((FCO2)-C-13), and phenylalanin e oxidation to graded tyrosine intake. Based on the mean estimates of whole -body phenylalanine oxidation, the tyrosine mean requirement and safe level of intake were found to be 74 mg.kg(-1).d(-1) and 94 mg.kg(-1).d(-1), resp ectively. This represents 3.1 and 3.9% of total amino acids, respectively, considerably higher than levels found in present commercially available ped iatric amino acid solutions. These data raise concern regarding the adequac y of aromatic amino acid intake in the parenterally fed neonate.