Essential and Non-essential Amino Acids in Neonatal Nutrition

There has been an evolution in the appreciation of the functional roles that amino acids play in neonatal nutrition over the last 30 years. Up to the early 1960s primary interest was in the identification of those amino acids that had to be supplied in the diet in order to maintain growth and positive nitrogen balance (Table 1). Most of the experiments that were performed to identify these amino acids were conducted in animals or human adults; thus they often failed to take into account the special needs of the biochemically immature neonate, particularly the preterm neonate. Appreciation of the special biochemical needs of the developing neonate resulted in the designation of a second group of amino acids as semiessential, conditionally essential, or developmentally essential (Table 2). Studies that have defined the role that the latter compounds play in early development have reflected on properties such as infant growth (histidine, cysteine) (1,2), the developmental pattern of enzymes that catalyze synthesis (cysteine, tyrosine) (3,4), stable isotope metabolism (glycine) (5), presence in human milk coupled with animal and human studies (taurine) (6-8), and prevention of adverse metabolic effects (arginine) (9). The data to support the "essentiality" of these amino acids have often been conflicting, as for cysteine (3,10) and glycine (5,11), making a final determination difficult at best. The next stage in the understanding of the role that amino acids play in early nutrition evolved from the many functions that they were identified with, other than as precursors for protein synthesis. Some of this impetus came from a growing appreciation of the nutritional importance of the amino acid taurine, despite the fact that it could not be incorporated into protein as it is a sulfonic amino acid. Other functions of amino acids include supporting the synthesis of hormones, neurotransmitters, and bile acids. In addition, many amino acids may themselves function as neurotransmitters (Table 3). Thus, monitoring the amino acid responses of neonates to different nutritional regimens may be of more functional importance than just determining the sufficiency of the protein synthesis precursor pool. Intakes of individual enteral formulas and total parenteral nutrition solutions are reflected by specific amino acid patterns in