Patients who spend significant time in critical care may lose a staggering amount of weight, particularly lean body mass. In early critical illness glucose is used as the principle energy source in the stress response; glycogen is rapidly exhausted and glycogenic amino acids are mobilized from muscular protein to generate glucose via gluconeogenesis, to maintain plasma glucose levels to feed, principally red blood cells. This has a major impact on muscle mass and in particular muscular strength, that may take years, perhaps a decade to restore. The most effective mechanism of preventing the development of critical illness cachexia is to curtail the duration of the stress response, by rapid source control, deresuscitation and early mobilization. In general, patients should be receiving full nutrition and be mobilized by day 8 following injury.