ObjectiveTo obtain accurate values for the components of energy expenditure in critically ill patients with sepsis or trauma during the first 2 wks after admission to the intensive care unit.DesignProspective study.SettingCritical care unit and university department of surgery in a single tertiary care center.PatientsTwelve severely septic (median Acute Physiology and Chronic Health Evaluation II Score, 23; range, 15 to 34) and 12 major trauma patients (median Injury Severity Score, 33.5; range, 26 to 50).InterventionsTotal body fat, total body protein, and total body glycogen were measured as soon as hemodynamic stability had been reached and repeated 5 and 10 days later. Resting energy expenditure (REE) was measured daily by indirect calorimetry.Measurements and Main ResultsChanges in total body fat, total body protein, and total body glycogen in critically ill patients provide data for the accurate construction of an energy balance. Energy intake minus energy balance gives a direct measurement of total energy expenditure (TEE) and, when combined with measurements of REE, activity energy expenditure can be obtained. TEE, REE, and activity energy expenditure were calculated for two sequential 5-day study periods. REE progressively increased during the first week after the onset of severe sepsis or major trauma, peaking during the second week at 37 +/- 6% (SEM) and 60 +/- 13% greater than predicted, respectively. For both the sepsis and trauma patients, TEE was significantly higher during the second week than during the first week (3257 +/- 370 vs. 1927 +/- 370 kcal/day, p < .05, in sepsis; 4123 +/- 518 vs. 2380 +/- 422 kcal/day, p < .05, in trauma). During the first week after admission to the hospital, TEE in sepsis and trauma patients, respectively, averaged 25 +/- 5 and 31 +/- 6 kcal/kg of body weight/day, and during the second week, 47 +/- 6 and 59 +/- 7 kcal/kg/day (p < .03, for comparison of first and second weeks). For the first week, the ratio of TEE to REE was 1.0 +/- 0.2 and 1.1 +/- 0.2 but during the second week rose to 1.7 +/- 0.2 and 1.8 +/- 0.2 in patients with sepsis (p < .05, for comparison of weeks) and trauma (p = .09), respectively.ConclusionsTotal energy expenditure is maximal during the second week after admission to the critical care unit, reaching 50 to 60 kcal/kg/day. (Crit Care Med 1999; 27:1295-1302)