Critical illness creates a state of severe metabolic stress characterised by massively elevated catabolism. Inflammatory mediators and counter-regulatory hormones (cortisol, catecholamines, glucagon) drive gluconeogenesis, proteolysis, and lipolysis at rates that cannot be prevented by any form of nutritional support. Critically ill patients are in a catabolic state by definition — the goal of nutrition support is not to reverse catabolism but to provide substrates that limit the degree of muscle wasting, support immune function, maintain gut mucosal integrity, and provide energy for healing.

Severe malnutrition in ICU is associated with increased susceptibility to infection, impaired wound healing, weaning failure from mechanical ventilation (respiratory muscles are lost along with other skeletal muscle), prolonged ICU stay, and worse long-term functional recovery. Conversely, early over-feeding is also harmful — it causes hyperglycaemia, hyperlipidaemia, increased CO2 production (worsening respiratory failure), and hepatic steatosis.