Glutamine Supplementation in Clinical Practice

Introduction to Glutamine

Glutamine is a conditionally essential amino acid that serves as a substrate for gluconeogenesis, increases intestinal cellularity, and is commonly supplied in total parenteral nutrition for critically ill patients 1, 2

Glutamine is the most abundant free amino acid in the body and has been increasingly incorporated into total parenteral nutrition formulations, particularly for critically ill patients 1, 3
When PN is indicated in ICU patients, parenteral glutamine supplementation (0.2-0.4 g/kg/day of L-glutamine or 0.3-0.6 g/kg/day of alanyl-glutamine dipeptide) is recommended 2
Traditionally, L-glutamine was omitted from standard TPN formulations because crystalline L-glutamine is poorly soluble and degrades during heat sterilization, but glutamine-containing dipeptides (alanyl-glutamine or glycyl-glutamine) now provide stable and soluble alternatives 1

Under normal conditions, glutamine is not an essential amino acid as it has an endogenous production rate (predominantly in skeletal muscle) of approximately 50-80 g/24h for an adult 2
However, during critical illness, glutamine becomes "conditionally essential" because the increased demand for its utilization (for immune function and tissue repair) exceeds the body's capacity to produce it 1, 2
In critically ill patients, plasma glutamine levels often fall, and this hypoglutaminemia is associated with worse clinical outcomes 2

Glutamine is a major substrate for gluconeogenesis, serving as a carrier for nitrogen and carbon between organs 1, 2
It participates in protein and glucose metabolism and is involved in the regulation of ammonia and acid-base balance 1
During critical illness, there is increased dependence on glutamine as a gluconeogenic substrate due to increased protein catabolism and the inability of exogenous glucose to inhibit gluconeogenesis 4

Glutamine supplementation is particularly beneficial in specific clinical scenarios, such as acute pancreatitis, where parenteral glutamine supplementation (>0.30 g/kg Ala-Gln dipeptide) should be considered when PN is indicated 5, 6
In critically ill ICU patients, glutamine supplementation has been associated with reduced mortality and improved morbidity with reduced infections 2
For surgical patients requiring exclusive PN, parenteral glutamine supplementation may be considered, though evidence is less strong than for critical illness 7
Caution should be exercised with indiscriminate use of glutamine supplementation for all critically ill patients, as recent evidence suggests potential harm in certain subgroups 8