Protein balance in the first week of life in ventilated neonates receiving parenteral nutrition

Background: Protein intake is frequently delayed in ill neonates because of concerns about their ability to metabolize substrates. Objective: We aimed to determine the factors affecting protein balance in ventilated, parenterally fed newborns during the first week of life. Design: Leucine kinetic studies were performed in 19 neonates by using the [1-C-13]leucine tracer technique after 24 h of a stable total parenteral nutrition (TPN) regimen. TPN intakes were prescribed by rotating attending physicians, enabling assessment of protein metabolism over a range of clinically used nutrient intakes. Results: Mean (+/-SD) birth weight was 1.497 +/- 0.779 kg, gestational age at birth was 30.3 +/- 4.0 wk, and age at study was 3.9 +/- 1.4 d. Amino acid intakes (AAIs) ranged from 0.0 to 2.9 g . kg(-1) . d(-1). Based on leucine kinetic data, protein balance was calculated as the difference between protein synthesis and catabolism. By multiple regression analysis, AAI was the only predictor associated independently with protein balance (P < 0.01); energy intake, lipid intake, glucose intake, birth weight, and gestational age were not. Both leucine oxidation and nonoxidative leucine disposal rates were significantly correlated with leucine intake (P < 0.0005 and P < 0.01, respectively). Of the 12 infants with AAIs >1 g . kg(-1) . d(-1), only 1 infant was significantly catabolic (protein balance < -1 g . kg(-1) . d(-1)). There was no evidence of protein intolerance as determined by elevated creatinine (69 +/- 31 mu mol/L), plasma urea nitrogen (6.7 +/- 2.53 mmol/L), or metabolic acidosis (pH: 7.36 +/- 0.05). Conclusions: Ill neonates can achieve a positive protein balance in the first days of life without laboratory evidence of protein toxicity.