Enteral and parenteral nutrition

Contents

Overview

Critical illness generates a catabolic state driven by inflammation, catecholamine excess, and corticosteroid release. Muscle protein is broken down to fuel gluconeogenesis, and without nutritional support, lean body mass is lost rapidly. Malnutrition worsens outcomes across ICU populations — increasing infection, impaired wound healing, prolonged ventilator dependence, and delayed recovery. Nutritional support is a core component of ICU care, and the route, timing, and targets matter.

Nutritional Assessment

All ICU patients should be nutritionally assessed at admission.

Nutritional Risk Screening (NRS-2002) and NUTRIC score are validated for use in ICU patients. The NUTRIC score incorporates APACHE II, SOFA, number of comorbidities, days from hospital to ICU admission, age, and IL-6 (if available). High NUTRIC score (≥5) identifies patients who benefit most from aggressive nutritional support.

Body weight: Use actual body weight when below or near ideal; use adjusted body weight (IBW + 0.25 × [actual − IBW]) in obesity for calculating targets.

Baseline nutritional status: Recent weight loss, reduced intake pre-admission, presence of muscle wasting, and diagnosis (oncology patients, chronic illness) inform risk.

Energy and Protein Targets

Energy: 25–30 kcal/kg/day is the target for most ICU patients. Overfeeding is harmful — it increases CO2 production, worsens hyperglycaemia, and promotes hepatic steatosis.

During the acute phase of critical illness (first 48–72 hours), some evidence supports permissive underfeeding (hypocaloric feeding), targeting 50–70% of energy requirements, particularly in those who are well-nourished at admission. Full feeding can be reached progressively over the following days.

Indirect calorimetry is the gold standard for measuring energy expenditure but requires specialist equipment. Predictive equations (Harris-Benedict, Mifflin-St Jeor, Penn State) are commonly used but have significant inaccuracy in critical illness.

Protein: 1.2–2.0 g/kg/day. Protein requirements are high in critical illness, often higher than energy can drive — protein should be targeted independently. In burns, ARDS, and severe illness, targets may exceed 2 g/kg/day.

Glucose: Target blood glucose 6–10 mmol/L. Tight glycaemic control (4.5–6 mmol/L) was associated with increased hypoglycaemia and mortality in the NICE-SUGAR trial and is not recommended. See the NICE-SUGAR journal club entry for more detail.

Enteral Nutrition

Enteral nutrition (EN) is preferred over parenteral nutrition when the gastrointestinal tract is functional. It maintains gut mucosal integrity and barrier function, supports the gut-associated immune system, and reduces bacterial translocation and infection risk.

Starting Enteral Nutrition

EN should be started within 24–48 hours of ICU admission in patients who are haemodynamically stable and have a functioning gut. A standard polymeric formula providing 1–1.5 kcal/mL is appropriate for most patients.

Gastric route (nasogastric tube, NGT): preferred first-line. Confirm tube position by chest X-ray before starting feeds.

Post-pyloric route (nasojejunal or nasoduodenal tube, or jejunostomy): used when gastric feeding is not tolerated — high gastric residual volumes, gastroparesis, high aspiration risk, recurrent vomiting. Post-pyloric feeding bypasses impaired gastric motility.

Gastric Residual Volumes

The clinical significance of high gastric residual volumes (GRV) has been challenged by the REGANE trial. GRVs of up to 500 mL are generally acceptable in the absence of vomiting or clinical signs of aspiration; routine GRV monitoring is no longer advocated in most guidelines. Stopping feeds unnecessarily on the basis of moderate GRV causes undernutrition.

Enteral Feeding in Specific Conditions

Prone positioning: EN can continue during prone positioning, though the rate should be reduced and the risk of aspiration assessed.

Vasopressor support: EN can usually be started at low dose vasopressor requirements. Withhold or reduce if there is evidence of intestinal ischaemia (rising lactate, bloody stool, abdominal distension).

Post-operative (following bowel surgery): Early post-operative EN (within 24 hours) is safe and beneficial in most cases.

Prokinetics

Metoclopramide 10 mg TDS IV, or erythromycin 250 mg TDS IV, improves gastric motility and EN tolerance. Erythromycin has a more rapid onset. Combined prokinetic therapy increases the response rate.

Parenteral Nutrition

Parenteral nutrition (PN) is indicated when the gastrointestinal tract cannot be used or EN is insufficient to meet nutritional targets after adequate attempts at enteral feeding.

Indications

  • Gastrointestinal failure (ileus, high-output fistula, short bowel syndrome)
  • Intestinal obstruction or ischaemia
  • Inability to establish enteral access
  • Supplemental PN when EN alone is insufficient (after 5–7 days of attempted EN)

PN should not be started within the first 48 hours in patients who were well-nourished pre-admission unless there is a specific indication. The EPaNIC trial showed that early PN (before day 3) in patients who could not tolerate full EN increased infection rates and prolonged ICU stay compared with late initiation.

Composition

A standard all-in-one PN bag (total nutrient admixture, TNA) provides dextrose, amino acids, and lipid in one container. Electrolytes, vitamins, and trace elements are added. Most ICUs use commercially prepared standardised bags, with individualised compounding for patients with specific requirements.

Central vs Peripheral PN

Central PN: Delivered via a central venous catheter. Standard PN is hyperosmolar and causes thrombophlebitis in peripheral veins. The CVC must be dedicated to PN (or have a designated lumen).

Peripheral PN: Uses a lower-osmolality formulation. Suitable for short-term supplemental PN or when central access is not available. Thrombophlebitis is still a risk.

Complications of PN

  • Line complications: Infection (CLABSI), thrombosis, pneumothorax (at insertion)
  • Metabolic: Hyperglycaemia, refeeding syndrome, hyperlipidaemia, electrolyte disturbances
  • Hepatic: PN-associated liver disease — hepatic steatosis, cholestasis (especially with prolonged use or in neonates). Reducing dextrose load, cycling PN, and transitioning to EN reduce hepatic complications.

Monitoring and Complications

Daily: Blood glucose, electrolytes (sodium, potassium, phosphate, magnesium), fluid balance. Adjust formula as needed.

Twice weekly: Renal function, liver function, triglycerides (if lipid infusion), FBC.

Weekly: Zinc, selenium, copper in prolonged ICU stay.

Refeeding Syndrome

Refeeding syndrome occurs when nutrition is reintroduced to a severely malnourished or prolonged-fast patient. Insulin secretion rises in response to glucose, driving intracellular uptake of phosphate, potassium, and magnesium. Rapid falls in these electrolytes — particularly phosphate — cause cardiac arrhythmias, respiratory failure, haemolytic anaemia, neuromuscular dysfunction, and seizures.

Risk factors: prolonged starvation (>5 days), chronic alcoholism, anorexia nervosa, malignancy, prolonged NPO post-operatively.

Prevention: identify at-risk patients, supplement thiamine before starting nutrition (Pabrinex 1–2 pairs IV TDS for 3 days), start nutrition at 10–20 kcal/kg/day and increase gradually, monitor and replace phosphate, potassium, and magnesium aggressively.

Special Situations

Renal failure and RRT: Protein losses occur through the CRRT circuit (amino acids) and are substantial. Protein targets should be increased to 1.5–2.5 g/kg/day in patients on CRRT. Water-soluble vitamins (B, C) are also lost and require supplementation.

Hepatic failure: Protein restriction is not recommended in standard acute liver failure — adequate protein intake supports hepatocyte regeneration and reduces muscle wasting. Branched-chain amino acid (BCAA) enriched formulae are used in some centres for patients with encephalopathy, though the evidence of benefit over standard formulae is limited.

Obesity: Use adjusted body weight for calculation. Hypocaloric high-protein strategies (permissive underfeeding with enhanced protein) are often used in obese ICU patients to allow mobilisation of fat stores while preserving lean mass.

Viva Questions

What are the advantages of enteral over parenteral nutrition in critical illness?

Enteral nutrition preserves gastrointestinal mucosal integrity through the direct trophic effect of luminal nutrients on enterocytes and colonocytes. This maintains the gut barrier against bacterial translocation — a key mechanism of secondary infection and systemic inflammation in critical illness. EN supports the gut-associated lymphoid tissue (GALT) and maintains IgA secretion, contributing to mucosal immune defence. It is associated with lower rates of nosocomial infection, particularly pneumonia and CLABSI, compared with parenteral nutrition in multiple trials and meta-analyses. EN is also more physiological in terms of metabolic handling of nutrients, less prone to causing hyperglycaemia and hepatic complications, and substantially cheaper. Complications of EN — aspiration, diarrhoea, tube misplacement — are generally less serious than those of PN (central line infection, sepsis). For these reasons, EN is always preferred when the gastrointestinal tract is functional, and PN is considered only when adequate EN is not achievable.

What is refeeding syndrome and how do you prevent it?

Refeeding syndrome is the metabolic complication of reintroducing nutrition to a severely malnourished or prolonged-starvation patient. During starvation, the body depletes intracellular stores of phosphate, potassium, and magnesium while maintaining near-normal serum levels. When carbohydrate is reintroduced, insulin secretion rises sharply and drives these electrolytes intracellularly, causing rapid falls in serum levels. Hypophosphataemia is the most clinically significant consequence: it causes cardiac arrhythmias, respiratory muscle weakness, haemolysis, seizures, and neurological dysfunction. Prevention requires identifying at-risk patients — those with very low BMI, minimal intake for more than five days, chronic alcoholism, cancer, or malabsorption. Thiamine (Pabrinex) must be given before starting nutrition to prevent Wernicke's encephalopathy. Nutrition should be introduced slowly, at around 10–20 kcal/kg/day, increasing over several days. Phosphate, potassium, and magnesium should be supplemented aggressively, guided by frequent monitoring.

When is parenteral nutrition indicated and what are its main complications?

PN is indicated when the gastrointestinal tract cannot be used — severe ileus, intestinal obstruction, high-output proximal fistula, short bowel syndrome, or bowel ischaemia — or when adequate nutrition cannot be delivered enterally after at least five to seven days of attempted EN. In well-nourished patients, PN should not be started within the first 48–72 hours when EN is being attempted; the EPaNIC trial demonstrated that early PN in this context increased infection and ICU stay. Complications of PN fall into three categories. Line-related complications include central line-associated bloodstream infection (CLABSI), which can be reduced by a strict aseptic CVC insertion and maintenance bundle, and venous thrombosis. Metabolic complications include hyperglycaemia (common and requires insulin infusion), refeeding syndrome, hypertriglyceridaemia, electrolyte disturbances, and trace element deficiencies. Hepatic complications include hepatic steatosis and cholestasis, particularly with prolonged PN, excess dextrose load, or in the absence of any enteral stimulation. Where possible, even small volumes of enteral feed alongside PN help maintain mucosal integrity and reduce hepatic dysfunction.