Adequate amino acid availability is crucial in achieving positive protein balance in parenterally fed ELBW infants; maximal accretion of protein may additionally depend on a critical level of energy intake. We hypothesized that additional nonprotein energy intake would promote protein accretion in these infants. To assess this, we measured the endogenous rates of appearance (Endog Ra) of the essential amino acids leucine (LEU) and phenylalanine (PHE) (each reflecting proteolysis), utilization of PHE for protein synthesis (PS), PHE hydroxylation (OH) (reflecting irreversible loss of PHE), and the rate of urea production in seven clinically stable, ventilated ELBW infants in the first few days of life (27±0.8 wks gestation, 1006±81 gms birth wt, 980±70 gms study wt, 2±0.3 days of age) during a graded increase in nonprotein substrate (70 and 90 kcal/kg/d). Protein intake was held constant at 3.5 gm/kg/d (Aminosyn PF) and a balanced ratio of caloric intake from glucose and lipid was maintained throughout the 5.5 hour study. Kinetics are shown below in mean±SE in μmol/kg/h; *p≤0.05.

CONCLUSIONS: In response to provision of additional nonprotein energy: 1) Proteolysis was reduced by ≈10%. 2) Utilization of PHE for protein synthesis decreased by a similar magnitude. 3) Rates of PHE hydroxylation were unchanged; therefore PHE balance was unaffected (8±1 μmol/kg/h during both periods). 4) No net conservation of nitrogen (as reflected by no change in urea Ra). At 3.5 gm amino acids/kg/d, increasing nonprotein energy intake from 70 to 90 kcal/kg/d reduces overall protein turnover but does not appear to acutely effect increases in protein anabolism in ELBW infants. Table

Table 1 No caption available.
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