50 Years Ago in The Journal of Pediatrics:

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Andrews BF, Bruton OC, de Barre L. J Pediatr 1962;60:201-5

Andrews et al published normal values for total amino acid nitrogen in 57 infants and children and compared these values with data from 52 children with a variety of pathological conditions, including infection, fevers, neurologic abnormalities, hepatic failure, and intoxications, but none with genetic disorders of specific amino acids.

Today, concentrations of individual amino acids are measured instead of total amino acid nitrogen. Genetic amino acid disorders are diagnosed when markedly elevated concentrations of specific amino acids are identified by newborn screening laboratories and in children with encephalopathy, metabolic acidosis, and hyperammonemia using tandem mass spectrometry and ion-exchange chromatography. Mildly elevated, nontoxic concentrations of amino acids in patients receiving intravenous (IV) nutrition reflect a balance of intake and urinary disposal of amino acids, as well as protein synthesis, breakdown, and oxidation, rather than specific pathological conditions. Such metabolic rates have been measured using stable isotopic tracers, now a fundamental approach to quantifying human metabolism.

Most recently, normal amino acid profiles in preterm and term newborns have been measured from IV amino acid solution trials in which amino acid mixtures designed for adults were modified for neonates to match serum amino acid concentrations from healthy, breast-fed, term newborns. Human cordocentesis studies determined normal fetal amino acid concentrations, guiding rates of IV amino acid infusions in preterm infants needed to achieve rates of protein accretion consistent with normal in utero growth. This revolutionized neonatal practice from seldom providing IV amino acids, often for days after birth, to starting IV amino acid infusions after birth using gestational age-specific rates ranging from 2 g/kg/day (term) to 4 g/kg/day (24-28 weeks).

The most consistent observation from such studies over the past 50 years has been that protein balance is directly related to amino acid supply and plasma amino acid concentrations. This is true under normal conditions, as well as in sick infants who experience considerable stress. Current research continues to define the comparative values of enteral and IV amino acid nutrition and to examine how amino acid metabolism is further affected by clinical disorders. Amino acid metabolism now is central to all nutritional management in preterm and term newborns, infants, and children.