Nutrition Care in the ICU and Anaesthesiology - A Detailed Evidence-Based Review

1. Why Nutrition Matters in Critical Illness

• Progressive loss of lean body mass (muscle wasting)
• Impaired wound healing and immune function
• Increased infection rates and ICU length of stay
• Higher mortality

Barash Clinical Anesthesia, 9e, p. 4855: "Poor nutritional status is associated with increased mortality and morbidity among critically ill patients. Therefore, appropriate nutrition is an important aspect of critical care and adequate nutritional support should be considered a standard of care."

2. Phases of Critical Illness and Nutritional Goals

3. Nutritional Risk Screening and Assessment

3.1 Validated Screening Tools

• NUTRIC score - incorporates age, APACHE II/SOFA, days from hospital to ICU admission, number of comorbidities, and IL-6 (if available). Scores ≥5 (without IL-6) or ≥6 (with IL-6) identify high nutritional risk.
• NRS-2002 - evaluates nutritional status and disease severity; widely used for surgical patients.

Mulholland & Greenfield's Surgery, 7e, p. 135: "Upon admission to the ICU, some assessment of nutritional risk should be performed. There are several validated tools, including the NUTRIC and NRS 2002, both of which evaluate nutritional status and disease severity."

3.2 Serum Markers - What NOT to Use

Sabiston Textbook of Surgery, p. (Ch 11): "ASPEN recommends against using such markers as proxies for nutritional status, and their values should be interpreted in conjunction with a patient's hospital course and clinical status."

3.3 Energy Expenditure Measurement

4. Energy Requirements in the ICU

4.1 Current Caloric Targets

12 to 25 kcal/kg/day

Sabiston Textbook of Surgery: "The most recent (2021) ASPEN guidelines for the critically ill patient recommend feeding 12 to 25 kcal/kg/day. This represents a change from prior recommendations for higher-calorie feeding and is reflective of more recent evidence suggesting, at a minimum, a lack of harm of lower-calorie feeding."

4.2 Early Permissive Underfeeding vs. Full Feeding

• Early "full nutrition" is likely harmful and associated with higher infection rates in the first days of critical illness.
• The goal in the early acute phase is a caloric target below actual energy expenditure, with the aim of providing >80% of estimated total energy goals gradually by Day 3 to 4.
• Early enteral feeding (even at trophic rates) protects the intestinal epithelial barrier function and maintains microbiome diversity.
• A meta-analysis showed no significant difference in overall mortality between underfeeding and full feeding groups, but a subgroup fed more than one-third of caloric requirements showed lower mortality.

4.3 Hypocaloric (Permissive Underfeeding) in Obese Patients

• 11-14 kcal/kg actual body weight (or 22-25 kcal/kg ideal body weight)
• Protein: 2.0-2.5 g/kg ideal body weight/day

5. Protein Requirements

• Wound healing
• Maintenance of lean body mass
• Immune function support

Current Recommendations

• Burns (up to 2.5 g/kg/day or more)
• Polytrauma
• Patients on continuous renal replacement therapy (CRRT), which removes amino acids

The High-Protein Controversy - Latest Evidence

No significant benefit of higher protein dosing (≥1.2 g/kg/day) vs. lower dosing on mortality, ICU LOS, hospital LOS, duration of mechanical ventilation, or incidence of acute kidney injury.

6. Route of Nutrition: Enteral vs. Parenteral

6.1 Enteral Nutrition (EN) - First Choice

• Lower cost than parenteral nutrition
• Avoids complications of IV access (infection, thrombosis)
• Maintains gut barrier integrity - the gut mucosa depends on luminal nutrients for trophic effects; disuse leads to diminished sIgA production, bacterial overgrowth, altered mucosal immune defenses, and potential bacterial translocation
• Reduces infectious complications (meta-analyses confirm significant reductions vs. PN)

Schwartz's Principles of Surgery, 11e, p. 94: "While EN is recommended as the first choice for nutritional support in patients who can tolerate it, a recent large trial from Europe comparing early isocaloric EN vs. PN in adult critically ill patients with shock did not reduce mortality or the risk of secondary infections but was associated with a greater risk of digestive complications including intestinal ischemia."

6.2 Timing of Enteral Nutrition

• Enteral feeding should begin as soon as the patient is hemodynamically stable (adequate urine output is a practical guide).
• Presence of bowel sounds and passage of flatus/stool are NOT absolute prerequisites.
• High gastric residual volumes (>200 mL over 4-6 hours) or abdominal distension may warrant cessation and rate adjustment.

6.3 Gastric vs. Post-Pyloric Feeding

• Most critically ill patients can be fed safely into the stomach (gastric route).
• If gastric feeding intolerance persists despite prokinetic agents, convert to post-pyloric (nasojejunal) feeding.
• Despite early enthusiasm for post-pyloric feeds, studies show no clinically significant reduction in pneumonia, ICU length of stay, or mortality compared to gastric feeding.
• Routine monitoring of gastric residual volumes is no longer recommended (SCCM/ASPEN 2016 update).

6.4 Continuous vs. Intermittent Enteral Feeding

6.5 Parenteral Nutrition (PN) - When and How

• GI tract non-functional (obstruction, high-output fistula, GI ischemia, severe ileus)
• EN is not feasible or well-tolerated
• As supplement when EN is meeting <60% of energy/protein requirements after 1 week

• In low nutritional risk patients: delay PN initiation until Day 7-10. Earlier initiation was historically associated with higher infectious complications, though more recent data suggest this was driven by excessive caloric delivery rather than PN per se.
• In high nutritional risk patients (NUTRIC ≥5, severe malnutrition): start PN early if EN is not feasible.

Schwartz's Surgery, 11e, p. 100: "PN use is recommended for those critically ill or injured patients who are at high nutritional risk, when EN is not possible. Alternately, PN can also be used to supplement EN after 1 week of use if use of EN is unable to meet >60% of energy and protein requirements."

• Requires central venous access (high dextrose concentration: 15-25%)
• All macronutrients and micronutrients delivered via this route

• Lower osmolality (dextrose 5-10%, protein 3%)
• Not suitable for severely malnourished patients or long-term use (>2 weeks)
• Use when central access unavailable or supplemental support needed short-term

• Line sepsis (catheter-related bloodstream infections)
• Hyperglycemia
• Hepatic steatosis/cholestasis
• Electrolyte imbalances
• Essential fatty acid deficiency (with fat-free solutions - manifests as dry scaly dermatitis, hair loss; prevent with periodic lipid emulsion infusions)
• Vitamin K deficiency (not in standard IV vitamin preparations - supplement weekly)

7. SCCM/ASPEN Nutrition Bundle - Summary of Core Recommendations

1. Assess all ICU patients on admission for nutritional risk; calculate energy and protein requirements.
2. Initiate EN within 24-48 hours of onset of critical illness and ICU admission; increase to goals over first week.
3. Reduce aspiration risk and improve tolerance: prokinetic agents, continuous infusion, chlorhexidine mouthwash, elevate head of bed 30-45°, consider diverting feeding level in GI tract.
4. Implement feeding protocols with institution-specific strategies to promote EN delivery.
5. Do NOT routinely monitor gastric residual volumes in ICU patients on EN.
6. Start PN early when EN is not feasible/sufficient in high-risk or poorly nourished patients.

8. Enteral Formula Selection

9. Immunonutrition

9.1 Glutamine

• Best-studied immunonutrient
• Conditionally essential in critical illness; serves as a fuel for enterocytes and immune cells
• Strongest data in burns and trauma patients
• High-concentration glutamine formulas have the strongest evidence base among immunonutrients

Barash Clinical Anesthesia, 9e, p. 4856: "Specific enteral formulations, particularly those with high concentrations of glutamine, have the strongest data to support their use."

9.2 Arginine

• Precursor to nitric oxide; roles in wound healing, immune cell proliferation
• Benefit primarily in elective surgical patients and trauma
• Use remains controversial in sepsis (theoretical risk of excess nitric oxide production)

9.3 Omega-3 Polyunsaturated Fatty Acids (PUFAs)

• Anti-inflammatory mechanism (compete with arachidonic acid pathway)
• Some data suggest reduction in ICU length of stay; no mortality benefit demonstrated
• Current guidelines do not support the use of various fatty acid formulations as routine immunonutrition in critically ill patients.

Barash Clinical Anesthesia, 9e: "The use of various fatty acids is not supported by current guidelines."

9.4 Micronutrients (Trace Elements and Vitamins)

• Selenium, zinc, vitamins A, C, E - deficiencies common in critical illness
• Supplementation is generally considered safe
• Evidence supporting their use is of overall low quality and high risk of bias; should be interpreted with caution
• Vitamin K is not present in standard IV vitamin preparations; supplement weekly in PN patients

10. Glycemic Control and Nutrition

Historical Context

• Van den Berghe's landmark 2001 RCT showed intensive insulin therapy (glucose target <110 mg/dL) reduced ICU mortality by ~50% in surgical ICU patients.
• This led to widespread adoption of "tight glycemic control" protocols.

Current Practice

• Multiple subsequent trials failed to reproduce these benefits. The NICE-SUGAR trial (large multicenter RCT) showed increased mortality with tight glucose control (80-110 mg/dL) due to severe hypoglycemia.
• Tight glucose control (80-110 mg/dL) was widely abandoned.

Barash Clinical Anesthesia, 9e, p. 4850: "After an additional large, multicenter, multinational randomized controlled trial of intensive insulin therapy showed increased mortality in the tight glucose control group, the practice of maintaining blood glucose between 80 and 110 mg/dL was widely abandoned. Current serum glucose targets are somewhat variable, but most agree that 140 to 180 mg/dL is an acceptable goal in most patients."

11. Refeeding Syndrome

Pathophysiology

• Insulin release drives glucose, phosphate, potassium, and magnesium intracellularly
• Rapid drop in serum phosphate (hypophosphatemia), hypokalemia, hypomagnesemia
• Thiamine depletion may also occur (thiamine is required for glucose metabolism)
• The resultant electrolyte shifts can cause: fatal cardiac dysrhythmias, respiratory failure, seizures, hemolytic anemia, and encephalopathy

Risk Identification

Prevention

• Identify at-risk patients before initiating nutrition (prolonged starvation >5-7 days, low BMI, chronic alcohol use, long-term diuretics, etc.)
• Start feeding at low rates (10-20 kcal/kg/day) and escalate slowly over 4-7 days
• Supplement phosphate, potassium, magnesium, and thiamine proactively before and during early refeeding
• Monitor electrolytes closely (daily in the first week)

12. Feeding During Vasopressor Support

• A randomized trial comparing EN to PN in patients requiring vasoactive support showed EN was not associated with significantly worse outcomes.
• Cautious early EN can be attempted once vasopressors are stable or being weaned, but should be withheld in patients with signs of bowel ischemia (abdominal distension, rising lactate, absent bowel sounds with clinical concern).
• In the shock setting, early EN was associated with increased risk of intestinal ischemia in one large trial (see Section 6.1).

13. Special Populations

Burns

• Among the highest caloric and protein requirements
• Protein targets up to 2.5 g/kg/day or more
• Immunonutrition (glutamine) has the strongest evidence base here
• Early EN within 6-12 hours of burn injury is recommended

Obese ICU Patients (BMI >30)

• Hypocaloric high-protein strategy: 11-14 kcal/kg actual body weight (or 22-25 kcal/kg ideal body weight)
• Protein: 2.0-2.5 g/kg ideal body weight/day

Renal Failure / CRRT

• Standard protein targets (1.5-2.5 g/kg/day) - CRRT removes amino acids, so protein requirements are higher
• Restrict potassium and phosphorus in formula selection
• Avoid excessive fluid from PN

Perioperative Nutrition (Anaesthesiology Context)

14. Overfeeding - A Significant Hazard

• Increased CO2 production - prolonged need for ventilatory support
• Increased oxygen consumption
• Hepatic steatosis / fatty liver
• Suppression of leukocyte function
• Hyperglycemia and increased infection risk

Schwartz's Surgery, 11e, p. 95: "Overfeeding may contribute to clinical deterioration via increased oxygen consumption, increased carbon dioxide production and prolonged need for ventilatory support, fatty liver, suppression of leukocyte function, hyperglycemia, and increased risk of infection."

15. Prolonged Critical Illness - PICS

Summary Table: Key Nutritional Parameters

Key References

1. McClave SA et al. SCCM/ASPEN Guidelines. JPEN 2016;40(2):159-211
2. ASPEN Critical Care Guidelines, 2021 update
3. ESPEN Guideline on Clinical Nutrition in the ICU (Singer P et al.)
4. Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - "Nutrition in the Critically Ill Patient" (p. 4855-4856)
5. Mulholland & Greenfield's Surgery, 7e - "Nutritional Support in Critical Care" (p. 133-135)
6. Schwartz's Principles of Surgery, 11e - "Enteral Nutrition" (p. 94-101)
7. Sabiston Textbook of Surgery - "Nutritional Requirements of the Critically Ill Patient"
8. Qin Y et al. No benefit of higher protein dosing in critically ill patients: systematic review and meta-analysis. PeerJ 2024. PMID 38799065
9. Alencar LO et al. Nutritional therapy in ICU inpatients at risk for refeeding syndrome: systematic review. Nutrition 2024 Dec. PMID 39317131
10. Zhang S et al. Intermittent vs. continuous enteral nutrition in critically ill patients: updated systematic review and meta-analysis, 2026. Front Nutr 2026. PMID 42095216