The energy substrate for neutrophils has been believed to be glucose. However, a recent investigation has demonstrated that neutrophils use glutamine (Gln) as well as glucose. Nevertheless, little is known about the effects of Gln on neutrophil function. Thus, this study was designed to investigate the effects of Gln on phagocytosis and reactive oxygen intermediate (ROI) production by neutrophils from postoperative patients in vitro. Eleven patients who had undergone major gastrointestinal surgery were randomly selected. Peripheral blood was drawn before surgery and on postoperative days (PODs) 1, 3, and 7. The blood was washed with medium to remove plasma. Washed whole blood was incubated in RPMI 1640 medium containing neither Gln nor glucose for 24 h at 37 degrees C. The medium was supplemented with Gln at a concentration of 0, 500, 1000, or 2000 microM. Whole blood was then assessed for phagocytosis by flow cytometry using fluorescent beads. ROI production by phagocytes was measured by flow cytometry using dihydrorhodamine 123. In each assay, the neutrophil population was gated and analyzed. Serum amino acids were also measured. Postoperative serum Gln level decreased significantly until POD 7. Phagocytosis by neutrophils on PODs 3 and 7 was significantly greater at 2000 microM Gln than at other Gln concentrations. Neutrophil ROI production was significantly greater at 2000 microM Gln than at 0 microM Gln at each time point. In conclusion, supplemental Gln enhances both phagocytosis and ROI production by neutrophils from postoperative patients in vitro.