Nutrition for critically ill patients: The role of glutamine in nutritional immunomodulation

Glutamine is a nonessential amino acid, conditionally essential in stressful conditions (catabolism/hyper catabolism). The lungs, liver, brain, and skeletal muscles take part in specific glutamine synthesis pathways. Plasma concentration of this amino acid is about 50% of total amino acid concentration and about 60% of free amino acids in the body. It is a significant source of energy for all cells, including immune cells. Glutamine can be used as a substrate for nucleotide synthesis. In the last few decades, advanced technology significantly changed the treatment of critically ill patients. Mechanical ventilation, blood products transfusion, renal replacement therapy, invasive monitoring, and many other technical procedures prolong the life of patients by changing and modulating homeostasis and developing new pathways and mechanisms of adaptation - allostasis. Systemic inflammatory response and immunomodulatory activity are part of complex underlying mechanisms involved in allostasis. Nutrition is an important part of the strategy for the treatment of critically ill patients. Based on recently published results, few nutrients (omega-3 fatty acids, arginine, glutamine), when added to the standard formula for enteral and parenteral nutrition, reduce intensive care unit (ICU) stay, infection rate, and the duration of mechanical ventilation in critically ill patients. Glutamine has a high immunomodulatory capacity, as fuel for muscles and a "shuttle" for nitrogen, protecting lung and gut function as well as the function of immunocompetent cells. The most vulnerable systems in COVID-19 patients are respiratory and renal. Despite no universally accepted strategy, treatment with glutamine could have an important role in protecting the cellular integrity of immune cells, alveolar-capillary, and enteral membrane.