PURPOSE OF REVIEW: In the setting of ICU, the characteristics of patients have changed during the last decade. Patients are older, frequently overweight or obese, present with more chronic diseases and undernutrition. These conditions are characterized by reduced muscle mass and vulnerable homeostasis. This review sustains the hypothesis that an early and optimal nutritional support, combining enteral and parenteral nutrition, could improve the clinical outcome of ICU patients. RECENT FINDINGS: The combination of stress and undernutrition observed in the ICUs is associated with negative energy balance, which leads to lean body mass loss. Catabolism of lean body mass has been repeatedly associated with a worsening of the clinical outcome, increased length of hospital stay, recovery and healthcare costs. Early enteral nutrition is the recommended feeding route in ICU patients, but it is often unable to fully cover the nutritional needs. Parenteral nutrition is recommended if enteral nutrition is not feasible. SUMMARY: It is hypothesized that supplemental parenteral nutrition, together with insufficient enteral nutrition, could optimize the nutritional therapy by preventing the onset of early energy deficiency, and thus, could allow to reduce the side-effects of undernutrition and promote better chances of recovery after the ICU stay.