Critically ill patients are known to develop serious nutritional deterioration during the course of their disease. They develop, from the beginning, a multifactorial protein malnutrition that relates to a poor clinical course and the development of weakness. Due to the increased protein catabolism in this type of patient, there is a rapid degradation of muscle mass and loss of functional proteins, and therefore nutritional support is mandatory. Indeed, achieving a high protein intake may promote a better evolution of the critically ill patient, i.e., maintenance of muscle protein, less deterioration of muscle strength, lower Intensive care unit-acquired weakness (ICUAW), lower mortality, decrease in the number of infections, decrease in days on mechanical ventilation, and days of hospital stay and in ICU.

The goal of this clinical trial is to compare the appearance and degree of ICUAW in critically ill patients receiving invasive mechanical ventilation treated with two different doses of protein (1.5 g/kg/day vs.1.0 g/kg/day).

It is known that protein metabolism is altered in critically ill patients due to metabolic alterations derived from stress. This critical situation is manifested by a severe catabolic alteration, especially in the first week, which is fundamentally characterized by severe glucose intolerance and the use of the protein itself as a metabolic substrate.

Despite protein synthesis is increased, this is insufficient to compensate for the high protein degradation rate, which leads, among others, to muscle deterioration resulting in increased morbidity and mortality. This muscle destruction has been implicated in the early appearance of Intensive care unit-acquired weakness (ICUAW). Although the pathophysiology of ICUAW is multifactorial, protein intake may play an key role in its treatment. However, protein intake cannot reduce muscle destruction, but it can stimulate protein synthesis.

Current evidence supports that the administration of early artificial nutritional support with a high protein intake can improve the clinical course of critically ill patients. However, there is still no consensus on the exact amount of protein needed to be administered to these patients in order to reduce adverse outcomes and prevent ICUAW.

Thus the aim of this study is to evaluate the effect of a nutritional supplementation containing 1.5 g of protein/kg/day vs 1.0 g of protein /kg/day in critically ill patients receiving mechanical ventilation on the development and degree of ICUAW.

Inclusion Criteria:

• Critically ill patient
• ICU admission during the previous 48h
• Patients on expected invasive mechanical ventilation for three days
• Patients with a minimum expected duration of clinical nutrition of at least seven days
• Written informed consent signed by the patient or the patient's legally authorized representative.
• Available central venous access for continuous infusion of the study drugs.

Exclusion Criteria:

• Denied informed consent
• Acute renal failure (renal injury stage 3)
• Liver failure (cirrhosis or Child-Pugh Scale > 5)
• Severe liver failure with International Normalized Ratio (INR) > 1.7 (prothrombin time > 50%) and encephalopathy
• Patients with COVID-19-derived pneumonia
• Body Mass Index (BMI) > 40 or < 18.5 (morbid obesity or previous caloric malnutrition)
• Pregnant patients
• Central Nervous System pathologies (Glasgow < 6)
• Peripheral Nervous System pathologies interfering with study evaluations
• Patients with cognitive dysfunction/dementia or unable to follow instructions regarding MRC tests
• Severe muscular pathology
• Already participating in another clinical trial
• Impossibility to contact after ICU discharge to carry out the follow-up visit on day 90
• Known hypersensitivity to milk protein or any of the components of the nutritional supplement
• Inborn errors in the amino acid metabolism
• Previous inclusion in the present study