Inclusion Criteria:  Age >18 years Mechanically ventilated patients (for three days or more )  Exclusion Criteria:  Age < 18 years old Contraindication for enteral nutrition (gut ischemia, obstruction, or perforation) Expected intolerance for enteral nutrition (paralytic ileus) Inability to start enteral nutrition within 24-hours(i.e. surgery or other interventions) Short bowel syndrome Hemodynamic instability ( i.e shock state, need for vasopressors ) Child C liver cirrhosis or acute liver failure Dialysis dependency