This review covers current knowledge related to the initiation of enteral or parenteral feeding among critically ill patients in the ICU. Critically ill patients requiring vital organ support in the intensive care unit (ICU) commonly have anorexia and may be unable to feed volitionally by mouth for periods ranging from days to months. Unless such patients are provided with macronutrients in the form of enteral or parenteral nutrition, they accumulate an energy deficit that rapidly reaches proportions that contribute to lean-tissue wasting and that are associated with adverse outcomes. 1 The catabolic response to acute critical illness is much more pronounced than that evoked by fasting in healthy persons, since the energy deficit in acutely ill patients is often superimposed on immobilization . . .