Amino acids in pediatric and neonatal nutrition

Modern infant formulas contain sufficient indispensable as well as dispensable amino acids to support normal growth of both term and preterm infants. However, current parenteral and specialized enteral amino acid preparations do not appear to be optimal. Many of the problems with parenteral preparations reflect the fact that ingested amino acids (protein) undergo more extensive enteric and hepatic metabolism than parenterally administered amino acids, including conversion to other amino acids (e.g. arginine) that reach the plasma for support of ongoing protein synthesis. Because this important source of amino acids is bypassed when nutrients are delivered parenterally, parenteral requirements of these amino acids are increased. In addition, while ingested phenylalanine and methionine appear to be converted to tyrosine and cysteine, respectively, it seems that parenterally administered phenylalanine and methionine are not. While glutamine, the branched-chain amino acids and arginine appear to be important in stressed infants and infants with compromised gastrointestinal function, specific roles have not been defined. Finally, because some amino acids are insoluble (e.g. tyrosine) and others are unstable in aqueous solution (e.g. glutamine and cysteine), suitable ways to provide these amino acids are needed. Soluble dipeptides of all types are available and have been shown to be both efficacious and safe in adults. This is also likely to be true for pediatric patients but data concerning their efficacy and safety in this population are lacking. Although recent research has been helpful, further research is necessary to define the optimal amino acid requirements of infants who are dependent on specialized nutrition regimens and to investigate whether these needs may differ depending upon the reasons why parenteral nutrition is required (e.g. growth versus response to stress).