Glutamine supplementation fails to affect muscle protein kinetics in critically ill patients

Background: In vitro work suggests that glutamine availability may be an important factor in controlling the rate of muscle protein synthesis. The objective of this study was to determine if enteral administration of glutamine affects muscle protein metabolism in critically ill patients. Methods: Six postsurgical patients requiring prolonged mechanical ventilation for pneumonia (age, 51 ± 12 years, Acute Physiology and Chronic Health Evaluation [APACHE] 22 ± 6, mean ± SEM) and 6 normal healthy volunteers (age, 33 ± 4 years) underwent evaluation of whole body and muscle protein metabolism using an 8-hour infusion of d₅-phenylalanine, 5¹⁵N glutamine, and d₃-alanine with serial blood sampling from the femoral artery and vein and biopsies from the vastus lateralis muscle. Metabolic measurements were obtained while subjects received Peptamen enterally (Basal Period) and with glutamine supplementation (24 g/3 h; Glutamine Period). Results: Glutamine concentration in muscle was significantly less in the critically ill patients. Glutamine supplementation increased the arterial plasma concentration of glutamine, yet with no demonstratable effect on muscle glutamine concentration or on the rate of muscle protein synthesis in either volunteers or patients. Furthermore, muscle glutamine kinetics (incorporation into muscle, release from muscle, and rate of de novo glutamine synthesis in muscle) were not affected by glutamine supplementation in the critically ill patients. In contrast, there was a significant decrease in these kinetic parameters with glutamine supplementation within the muscle of healthy subjects. Metabolism of alanine was unaffected by administration of glutamine in either group. Conclusions: Enteral glutamine supplementation to critically ill patients fails to alter muscle glutamine metabolism or muscle protein synthesis. This suggests a possible restriction in transport of glutamine into muscle of critically ill patients.