The critically ill patient in intensive care often has to face significant metabolic alterations caused directly by the illness or secondary by complications (i.e. infections, organ failure or sepsis). Situations of rapidly evolving altered metabolism are maintained by or can trigger complex hormonal reactions which in turn promote further metabolic derangements. Surgery or trauma patients and those afflicted by septic complications can present significant nitrogen loss with consequent altered protein, carbohydrate or lipid metabolism. Further knowledge regarding the pathophysiology of metabolism and how abnormal stimulation of pyhsiological mechanisms can alter the consumption of principal energy substrates is necessary for optimal care of patients with altered metabolic functions.