Some nutrients have specific biological effects other than their use as energy sources or substrates for protein synthesis. Glutamine
Glutamine is a major metabolic fuel for the enterocytes of the gut mucosa, lymphocytes, and macrophages. It is the most abundant amino acid in plasma and is the principle interorgan transporter of nitrogen between sites of synthesis (muscle and liver) and sites of utilization (gut, lymphocytes, and lung). Glutamine is a precursor of nucleotides and glutathione. While not usually regarded as an 'essential' amino acid, it becomes conditionally essential during critical illness. In most enteral formulations glutamine is present in only low concentrations. When added to parenteral nutrition it improves nitrogen balance and gut barrier function, and reduces infection and duration of mechanical ventilation. Glutamine supplementation of enteral feeds has still to be shown to improve patient outcome ( Souba 1997), although experimental studies suggest that it should be beneficial.
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