Critically ill patients are often unable to eat by themselves over a long period of time, sometimes for weeks. In the acute phase, serious protein-energy malnutrition may develop with progressive muscle weakness, which may result in assisted respiration of longer duration as well as longer stay in intensive care unit and hospital. In view of the metabolic processes, energy and protein intake targets should be defined and the performance of metabolism should be monitored. Enteral nutrition is primarily recommended. However, parenteral supplementation is often necessary because of the disrupted tolerance levels of the gastrointestinal system. Apparently, an early parenteral supplementation started within a week would be of no benefit. Some experts believe that muscle loss can be reduced by increased target levels of protein. Further studies are needed on the effect of immune system feeding, fatty acids and micronutrients. Orv. Hetil., 2014, 155(51), 2048–2053.