Substantial body of evidence indicates that medical and elective surgical patients who have evidence of malnutrition suffer greater levels of morbidity and mortality with those who have adequate nutritional reserve. The metabolic status of many critically ill patients is fundamentally different from that detectable in the malnutrition that may tragically be manifest in the community, or in the extreme circumstances that are seen in famine victims. Thus, in the critically ill, basal metabolic rate (BMR) is usually elevated, whereas in malnutrition, it is depressed. The increase in BMR is attributed to a complex catabolic process that teleologically represents an attempt by the body to aid in the healing process. Although initially beneficial, prolonged catabolism ultimately causes severe protein loss and probably contributes to the high mortality and morbidity that are seen in critically ill patients who required a prolonged intensive care unit (ICU) stay.
Identifying the differences between malnutrition and the catabolism of critical illness, therefore, is highly significant in evaluating the efficacy of nutritional support. Malnutrition is reversible, given careful and adequate refeeding, unless a preterminal stage of illness has been reached. By contrast, the catabolic response to critical illness is not reversed by simple nutrition. This fact is now well understood, but in the early days of critical care, attempts were made to completely replace the marked nutritional losses that were observed. This led to overfeeding, an inability by the patients to adequately metabolize the nutrient load, and resulted in a variety of metabolic and clinical complications. The naıve view that feeding the critically ill must be beneficial also led to the widespread use of total parenteral nutrition (TPN).
Enthusiasm for TPN in the ICU setting has diminished in the last decade, as a result of mounting evidence that it is associated with an increased incidence of infection and immune-related problems in certain patients.
https://doi.org/10.1016/S0272-5231(03)00101-1 - article here