A supplemental intravenous amino acid infusion sustains a positive protein balance for 24 hours in critically ill patients

Background: Providing supplemental amino acids to ICU patients during a 3-h period results in improved whole-body net protein balance, without an increase in amino acid oxidation. The primary objective was to investigate if a 24-h intravenous amino acid infusion in critically ill patients has a sustained effect on whole-body protein balance as was seen after 3 h. Secondary objectives were monitoring of amino acid oxidation rate, urea and free amino acid plasma concentrations. Methods: An infusion of [1-C-13]-phenylalanine was added to ongoing enteral nutrition to quantify the enteral uptake of amino acids. Primed intravenous infusions of [ring-H-2(5)]-phenylalanine and [3,3-H-2(2)]-tyrosine were used to assess whole-body protein synthesis and breakdown, to calculate net protein balance and to assess amino acid oxidation at baseline and at 3 and 24 hours. An intravenous amino acid infusion was added to nutrition at a rate of 1 g/kg/day and continued for 24 h. Results: Eight patients were studied. The amino acid infusion resulted in improved net protein balance over time, from -1.6 +/- 7.9 mu mol phe/kg/h at 0 h to 6.0 +/- 8.8 at 3 h and 7.5 +/- 5.1 at 24 h (p = 0.0016). The sum of free amino acids in plasma increased from 3.1 +/- 0.6 mmol/L at 0 h to 3.2 +/- 0.3 at 3 h and 3.6 +/- 0.5 at 24 h (p = 0.038). Amino acid oxidation and plasma urea were not altered significantly. Conclusion: We demonstrated that the improvement in whole-body net protein balance from a supplemental intravenous amino acid infusion seen after 3 h was sustained after 24 h in critically ill patients.