Optimizing Protein Intake and Nitrogen Balance in Adult Critically Ill Patients

The purposes of the study are: * To strictly ensure the appropriate level of protein and caloric provision in critical illness, according to the latest literature recommendation; * To determine if an association exists between protein intake and nitrogen balance in the context of a nutrition with appropriate protein provision based on the latest literature recommendations, rather than in a standard diet with a caloric target weight-based. Creatinine clearance will also be evaluated to determine if there is any harmful effect to the kidney secondary to an elevated BUN. * To determine if there is a difference in ICU mortality rate between the current nutrition standard and a hyperproteic nutrition protocol.