Summary Enteral and parenteral feedings supplemented with GLN or GLN precursors have been well tolerated in the studies published to date, and beneficial effects have been documented in a number of patient groups. The mechanisms of GLN actions in nutrition are yet unclear. It is likely that GLN supplementation exerts a number of potentially beneficial effects which may be interrelated. Thus, a combination of enhanced protein synthesis and reduced protein-catabolic responses improve nitrogen retention. The important role of GLN in DNA and RNA synthesis and its ability to be utilized as a metabolic fuel may explain, in part, its effect on skeletal muscle protein metabolism. Improved protein sparing combined with enhanced function of immune cells, maintenance of mucosal barrier defenses, and/or maintenance of tissue antioxidant stores may contribute to improved resistance to infection and microbial clearance (58). Further studies on the clinical efficacy and the molecular basis for GLN effects are necessary, as are additional controlled, blinded trials to determine the clinical efficacy and cost effectiveness of enteral or parenteral GLN supplementation in catabolic patients (68). However, based on the available data in experimental animals and in humans GLN should be considered a potentially important dietary amino acid in a number of clinical settings (Table 4).