Parenteral nutrition (PN) — intravenous delivery of nutrients bypassing the gut — should be reserved for patients in whom the gut cannot be used: short gut syndrome, high-output proximal fistula, complete gut obstruction, or severe ileus preventing any enteral absorption. PN must be delivered via a dedicated central venous catheter (the high osmolality of PN solutions causes thrombophlebitis in peripheral veins). The most significant complications of PN are line-related bloodstream infection, metabolic abnormalities (hyperglycaemia, hyperlipidaemia, electrolyte disturbances), and hepatic steatosis with prolonged use.

A pragmatic approach used increasingly in ICU is to start enteral nutrition and supplement with PN only if enteral intake remains inadequate (below 60% of target) after 3–5 days. This ‘supplemental PN’ approach avoids the risks of exclusive PN while addressing nutritional deficits in patients with established feeding intolerance.