Understanding of the physiologic and metabolic changes during acute illness. Energy requirements and macronutrient utilization. Protein metabolism and its importance. Screening and assessment of malnutrition. Specific situations related to previous nutrition status (severe undernutrition, obesity, sarcopenia), comorbidity (cancer, age, renal or liver failure, ...), and previous stay in the hospital. Determination of nutritional prescription (calories, protein), choice of the route, decision regarding timing (early enteral or parenteral nutrition), requirement of supplemental parenteral nutrition. Choice of special nutrients and in which conditions: glutamine, omega 3 fatty acids, various lipid emulsions, high protein feeding, probiotics, vitamins and trace elements in special conditions (CRRT, liver failure, ARDS, ECMO). Adaption of the nutritional regimen according to the progress of the patient in the course of his therapy. Prevention of frailty during stabilization and preparing for best rehabilitation.