Glutamine: Properties and Role in Total Parenteral Nutrition
Glutamine is a substrate for gluconeogenesis and is not an essential amino acid under normal conditions, but it is now recommended to be supplemented in total parenteral nutrition for critically ill patients. 1
Key Properties of Glutamine
Non-essential amino acid: Under normal conditions, glutamine is not an essential amino acid as it has an endogenous production rate of 50-80g/24h in adults, primarily in skeletal muscle 1
Substrate for gluconeogenesis: Glutamine participates in glucose metabolism and serves as a carrier for nitrogen and carbon between organs 1
Intestinal function: Glutamine increases intestinal cellularity and helps maintain intestinal structure and function 2
Supplementation in TPN: Glutamine is now supplied in total parenteral nutrition, particularly for critically ill patients 1
Role in Critical Illness
Glutamine becomes conditionally essential during critical illness for several reasons:
During critical illness, increased demand for glutamine utilization (for immune activity and repair) is not adequately met by endogenous production 1
Plasma glutamine levels fall during critical illness, and low levels are associated with worse outcomes 1
Glutamine depletion occurs in muscle tissue during catabolic stress, which cannot be reversed by conventional nutritional means 3
Glutamine Supplementation in TPN
The ESPEN guidelines recommend:
When parenteral nutrition is indicated in ICU patients, the amino acid solution should contain 0.2-0.4 g/kg/day of L-glutamine 1
Glutamine-containing dipeptides (alanyl-glutamine or glycyl-glutamine) are used because crystalline L-glutamine is poorly soluble and degrades during heat sterilization 1
Clinical outcome studies have shown reduced mortality risk with PN containing glutamine in critically ill patients (RR 0.67, CI 0.48-0.92) 1
Clinical Benefits of Glutamine Supplementation
- Helps maintain intestinal structure and function 2
- May reduce infections and hospital length of stay 4
- Supports immune function 1
- Acts as a precursor for nucleotides and cellular protection through glutathione 1
Important Considerations and Caveats
More recent guidelines have become more cautious about glutamine supplementation, downgrading recommendations from Grade B to Grade 0 (may be considered) 1
A large multicenter RCT in well-nourished patients undergoing major gastrointestinal surgery found no significant benefit for postoperative complication rates 1
High-dose glutamine administration in critically ill patients with organ dysfunction has been associated with increased mortality 1
The route of administration matters: parenteral glutamine produces higher plasma concentrations compared to enteral administration 1
In summary, glutamine is a non-essential amino acid under normal conditions that becomes conditionally essential during critical illness. It is a substrate for gluconeogenesis, increases intestinal cellularity, and is now recommended as a supplement in total parenteral nutrition for critically ill patients who cannot be adequately fed enterally.