Appearance of Individual Amino Acid Concentrations in Arterial Blood During Steady-State Infusions of Different Amino Acid Formulations to ICU Patients in Support of Whole-Body Protein Metabolism

Abstract

Background: Previous work has demonstrated a relationship between arterial amino acid concentrations and uptake of amino acids across peripheral tissues in healthy volunteers, as well as in chronically and acutely ill patients. The aim of the present study was to evaluate whether different amino acid profiles in commercially available amino acid formulations are translated into significantly different arterial amino acid concentrations presumably high enough to promote protein metabolism in intensive care unit (ICU) patients. Methods: Nonprotein calories (60% glucose: 40% lipid) were simultaneously and constantly infused over 72 hours. Different free amino acid solutions were infused at random to each patient for 24 hours in order to determine the appearance of steady-state arterial concentrations of individual amino acids. Basal metabolic and nutrition states were defined after a 12-hour infusion period with glucose in each patient. Healthy volunteers receiving a standardized oral meal served as reference subjects in measurements of venous amino acid concentrations after normal oral food intake. Results: The sum of all amino acids in arterial plasma increased significantly during steady-state infusions of all the free amino acid solutions vs basal state in ICU patients. Only glutamine, taurine, and tyrosine did not increase at all vs basal state during steady-state infusions of the 3 formulations. Alanine, arginine, citrulline, glycine, histidine, serine, methionine, phenylalanine, valine, and ornithine showed different concentration among the amino acid solutions during infusions. Healthy volunteers had significantly higher overall concentrations of amino acids in both fasted and fed state compared with ICU patients, which indicates that free amino acid solutions remain a limiting component in artificial nutrition to patients to promote arterial amino acid concentrations in the artificially fed state. Conclusions: It appears important to continue further improvement of composition profile in solutions of free amino acids to promote adequate uptake across organ beds in promotion of protein balance in artificially nourished patients.

This study evaluates whether the amino acid composition in nutrition formulations makes a difference in determination of arterial amino acid concentrations, which are important to promote protein balance in skeletal muscles of ICU patients. The results suggest that available amino acid formulations are not optimized for promotion of protein synthesis in peripheral tissues.