## 1 Introduction  
The critical importance, complexity, and challenge of clinical nutrition for critically ill patients is best understood when recapitulating the evolution of this complex patient group and, consequently, the proposed guidelines and clinical practice strategies during the last decades, as follows: (1) Adult critically ill patients present as a heterogeneous group with regard to diagnosis, severity of illness, and the number of (pre-)existing comorbidities. (2) Critically ill patients are treated not only on intensive care units (ICU), but increasingly in smooth transition on so-called step-down or intermediate care units (IMC-U) depending on the severity of illness, the available facilities, and hospital internal regulations. Still, these patients should not be systematically divided into separate categories. From a metabolic point of view, all patients requiring intensive or intermediate care suffer from a relevant catabolic stress response, which parallels the severity of injury or illness. From an institutional point of view, IMC-Us are relatively new facilities treating patients who require less care than standard intensive care, but more than that which is available from ward care [1]. Taking these two points into account, an overlap of IMC-U and ICU treatment is owed to the organization and available facilities of each individual hospital, as follows: Patients with the same severity of illness may be treated in one hospital on the ICU and in another hospital on the IMC-U. Since IMC-Us are an emerging type of treatment facility, IMC-U specific randomized clinical trials (RCTs) and guidelines are still missing and should be a field of future research. Therefore, patients in both of these groups are considered to be critically ill patients throughout this review article as their nutritional needs, first and foremost, parallel the course of illness and are not defined by the treating facility per se. (3) The ICU population suffers from a high prevalence of malnutrition when admitted to hospital (up to 60%) and during the intensive care treatment phase [2; 3; 4; 5]. (4) The diagnosis of malnutrition and the assessment of nutritional needs are complex and ask for systematic re-evaluation and adaptation. (5) Some study data from recent years have highlighted the association between protein and caloric deficiency and higher morbidity and mortality rates [2; 6; 7; 8; 9]. These findings have introduced the conversion from \"nutritional support\" as an adjunctive care to \"nutrition therapy\", which not only covers micro- and macronutrient needs, but also intends to blunt the metabolic response, cellular injury, and immunological alteration. However, several further studies showed no clinical benefit [10; 11; 12; 13] for higher protein and calorie administration in the ICU or reported harmful results, such as increased adjusted mortality at 6 months for mechanically ventilated patients [10] and less gastrointestinal tolerance in patients with acute lung injury [13]. Additionally, the idea of pharmaco-nutrition was abandoned after several neutral and one lethal trial [14; 15; 16]. On these grounds, the current European Society for Clinical Nutrition and Metabolism (ESPEN) guideline defines the term \"medical nutrition therapy\" (MNT), which encompasses oral nutritional supplements, enteral, and parenteral nutrition. Furthermore, ESPEN underlines the need for well conducted trials on optimal protein intake since currently available studies are not comparable in terms of patient selection and calorie and protein intake, as well as timing and route of administration. Yet, ESPEN evokes the possibility that it is possible that, similar to caloric targets, optimal protein targets change over time in the ICU and that a high protein intake is only beneficial if not associated with overfeeding [17]. However, so far this hypothesis is not yet confirmed by RCTs.