• Olav Rooyackers, Ramin Kouchek-Zadeh, Inga Tjäder, Åke Norberg, Maria Klaude, and Jan Wernerman.
• Department of Anesthesiology and Intensive Care Medicine at Karolinska University Hospital Huddinge and Karolinska Institutet, Stockholm, Sweden. Electronic address: olav.rooyackers@ki.se.
• Clin Nutr. 2015 Feb 1;34(1):95-100.

Background & AimsTo evaluate the effect of nutrition therapy on protein turnover in critically ill patients isotopically labeled amino acids can be used. Here parallel measurements using (13)C-leucine and (2)H5-phenylalanine were performed to evaluate if one tracer was to be preferred.MethodsAs a reference group, healthy volunteers (n = 8) were studied in the postabsorptive state and during parenteral nutrition delivery. ICU patients with multiple organ failure (n = 8) were studied during parenteral nutrition delivery only.ResultsFor the volunteers, the net protein balances changed from negative to positive during parenteral nutrition delivery (compared to the postabsorptive state) when evaluated with leucine and phenylalanine (P < 0.0001). For phenylalanine this change was attributable to an increased protein synthesis (P < 0.0001), while for leucine the change was attributable to a decreased protein degradation (P < 0.0001). For the patients, only measured during parenteral nutrition delivery, the estimates by the two amino acid tracers agreed, showing a protein balance not statistically significantly different from zero. The whole body protein turnover was higher than that of the healthy volunteers during parenteral nutrition delivery. In the patients, the net protein balance correlated positively to the amount of amino acids given.ConclusionsCritically ill patients with multiple organ failure have an increased protein turnover. The findings in the healthy volunteers indicate that the use of the two different amino acid tracers in parallel in future studies should be considered.Copyright © 2014 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

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