Effect of parenteral amino acids on leucine and urea kinetics in preterm infants

To examine the effect of two commonly used parenteral alimentation amino acid mixtures on whole-body leucine and urea kinetics.

Ten stable preterm infants were studied during the first 4 weeks after birth. Six infants received a mixture containing higher branched-chain amino acids, lower glycine plus added dicarboxylic acids formulation (Trophamine), and four received a standard amino acid mixture (Aminosyn). Whole-body protein turnover was measured with (1-13C)leucine tracer, and the rate of oxidation of protein was calculated by quantifying the appearance of carbon 13 from leucine in carbon dioxide as well as from rates of urea synthesis estimated by using (15N2)urea tracer. Energy consumption and substrate oxidation were quantified by indirect respiratory calorimetry. Infants were given similar quantities of energy (approximately 61 kcal/kg per day), glucose (approximately 10.7 mg/kg per minute) and protein (approximately 2.1 gm/kg per day).

There was no significant difference in the rate of appearance of leucine in the two groups. However, the fraction of leucine oxidized (p = 0.002) and total rte of oxidation of leucine was higher in the Trophamine group. Thus additional branched-chain amino acids resulted in an increased contribution of C-1 of leucine to expired CO2. The rate of urea N synthesis was also similar in the two groups (Trophamine: 2.92 +/- 0.87 mg N/kg per hour; Aminosyn: 2.70 +/- 1.18 mg N/kg per hour).

Although the use of Trophamine normalizes the blood amino acid pattern, it does not appear to improve nitrogen/protein kinetics. Furthermore, the additional branched-chain amino acids are disposed of by increased oxidation.