Nutrition in Surgical Patients - Enteral vs Parenteral Nutrition and Nutritional Assessment

RACS Generic Surgical Sciences Examination (GSSE) Learning Objective: GSSE_PHYS_MN_1_001

Definition / Overview

Nutritional support in surgical patients aims to meet metabolic demands, preserve lean body mass, support wound healing, and modulate the inflammatory and immune response to injury or illness. Malnutrition - whether pre-existing or acquired during the perioperative period - is an independent predictor of postoperative morbidity, delayed wound healing, immunosuppression, and mortality.

Key principles: - Nutritional assessment identifies patients at risk before deficits become clinically apparent - Enteral nutrition (EN) is preferred over parenteral nutrition (PN) in patients who can tolerate it - The timing, route, and composition of nutritional support must be tailored to the clinical context - Overfeeding carries its own set of metabolic harms and must be avoided

Metabolic Response to Surgical Illness and Starvation

Fasting Physiology

During short-term fasting: - Glycogen stores are depleted within approximately 24 hours - Gluconeogenesis is upregulated (substrates: alanine, glutamine, glycerol, lactate) - Lipolysis provides free fatty acids as the primary energy source; ketone bodies serve brain and cardiac muscle - Protein catabolism is initially minimal

During prolonged starvation (days to weeks): - Progressive skeletal muscle proteolysis to fuel gluconeogenesis - Visceral protein depletion follows somatic depletion - Adaptation: increased reliance on ketone bodies reduces glucose demand and partially spares protein

The Surgical/Catabolic State

Surgical stress, sepsis, and trauma superimpose a hypermetabolic response on top of starvation physiology: - Neuroendocrine activation: cortisol, glucagon, catecholamines drive gluconeogenesis and protein catabolism - Hyperglycaemia: insulin resistance is characteristic; exogenous glucose does not suppress gluconeogenesis effectively - Negative nitrogen balance: urinary nitrogen losses can exceed 20 g/day in severe illness; young muscular males lose the most - Lean mass loss: 200-300 g/day of skeletal muscle catabolism in severe critical illness - Adipose tissue is mobilised but oxidation may be impaired if carbohydrate overload occurs

This catabolic state cannot be fully reversed by nutritional support alone, but adequate nutrition limits further depletion and supports recovery.

Nutritional Assessment

Identifying malnutrition or nutritional risk is a prerequisite for targeted support. No single test is adequate; a composite approach is standard.

Screening Tools

Clinical Parameters

• Body weight: unintentional loss >10% over 6 months, or >5% over 3 months, is clinically significant
• BMI: $< 18.5\,\text{kg/m}^2$ indicates underweight; however, BMI may overestimate nutritional status in oedematous patients
• Dietary history: reduced intake over preceding days/weeks

Biochemical Markers

• Serum albumin $< 30\,\text{g/L}$ is strongly associated with surgical complications independent of nutritional state
• Nitrogen balance: $$\text{N balance} = \text{N intake (g/day)} - [\text{Urinary urea nitrogen (g/day)} + 4]$$ where the constant (~4 g) accounts for faecal and insensible losses; a balance of $\geq 0$ is the target in anabolism

Anthropometry and Functional Tests

• Mid-arm muscle circumference and triceps skinfold thickness estimate somatic and fat stores respectively
• Handgrip dynamometry correlates with functional status and clinical outcomes; a simple, reproducible measure of muscle function

Estimating Nutritional Requirements

Energy Requirements

$$\text{Total Energy Expenditure (TEE)} = \text{Resting Energy Expenditure (REE)} \times \text{Stress Factor}$$

• REE can be estimated by the Harris-Benedict equation (sex, weight, height, age) or derived from indirect calorimetry (gold standard)
• Indirect calorimetry measures $\dot{V}O_2$ and $\dot{V}CO_2$ directly; the respiratory quotient $$RQ = \frac{\dot{V}CO_2}{\dot{V}O_2}$$ reflects substrate utilisation: RQ of 0.7 = fat, 0.85 = mixed, 1.0 = carbohydrate, >1.0 = lipogenesis (overfeeding)
• In the absence of calorimetry, a pragmatic estimate of $25\text{-}30\,\text{kcal/kg/day}$ is widely used for critically ill adults

Protein Requirements

Micronutrients and Electrolytes

• Vitamins C, A, zinc, and copper are essential for wound healing; deficiencies impair collagen synthesis
• Thiamine (B1) supplementation is critical in malnourished patients before/during dextrose infusion to prevent Wernicke's encephalopathy
• Electrolytes - particularly phosphate, potassium, and magnesium - require careful monitoring and repletion, especially in refeeding syndrome

Enteral Nutrition

Rationale and Physiological Benefits

Enteral feeding preserves gut mucosal integrity by maintaining enterocyte nutrition (predominantly from luminal nutrients), supporting gut-associated lymphoid tissue (GALT), and sustaining secretory IgA production. Gut disuse leads to: - Villous atrophy and increased intestinal permeability - Bacterial translocation across the mucosal barrier - Impaired immune surveillance and hepatic Kupffer cell function

EN is associated with reduced infectious complications and ICU length of stay compared to PN in critically ill patients, though no survival benefit has been consistently demonstrated.

Timing of Enteral Nutrition

• Early EN (within 24-48 hours of ICU admission or surgery) is recommended when haemodynamically stable
• Early feeding attenuates the inflammatory response, preserves gut architecture, and reduces septic morbidity
• Haemodynamically unstable patients (requiring escalating vasopressors, evidence of gut hypoperfusion): EN should be withheld or given at trophic rates until resuscitation is established - bowel ischaemia is a recognised risk
• In elective surgery without preoperative malnutrition, oral intake can usually resume within the first 1-2 postoperative days; formal EN is rarely required

Delivery Routes

Enteral Formulas

• Standard polymeric formulas: intact protein, complex carbohydrates, long-chain fats - suitable for the majority
• High-protein formulas: protein ≥20% of total energy - preferred in critical illness
• Semi-elemental/elemental: pre-digested peptides and/or free amino acids; for patients with significant malabsorption - higher osmolarity, more expensive
• Disease-specific formulas: modified for renal failure (low electrolytes), hepatic failure (branched-chain amino acid enriched), diabetes (lower glycaemic index)
• Immune-modulating formulas: supplemented with arginine, glutamine, omega-3 fatty acids, nucleotides - evidence of benefit primarily in elective surgery and burns; results inconsistent in critical illness and sepsis

Intermittent vs Continuous Feeding

• Continuous infusion: more predictable delivery; preferred in ICU and post-pyloric feeding
• Intermittent/bolus: more physiological; suits nasogastric feeding; facilitates mobility
• Monitoring for gastric residual volumes (>200-500 mL is concerning) and signs of intolerance (distension, vomiting, diarrhoea)

Parenteral Nutrition

Indications for Parenteral Nutrition

PN is indicated when the gastrointestinal tract cannot be used safely or when EN is insufficient to meet nutritional targets: - Intestinal failure (short bowel syndrome, high-output enterocutaneous fistula) - Prolonged ileus or intestinal obstruction - Severe GI haemorrhage - Active bowel obstruction or ischaemia - EN intolerance despite optimisation - Supplement to EN when target rates cannot be reached (supplemental PN)

Total vs Peripheral Parenteral Nutrition

Components of TPN

A standard TPN bag contains: - Carbohydrate (dextrose): primary energy source; maximum oxidation rate ~$5\,\text{mg/kg/min}$; excess → lipogenesis, hyperglycaemia, elevated $CO_2$ production (increased ventilatory demand) - Lipid emulsion: soybean/olive/fish oil; provides essential fatty acids and fat-soluble vitamins; should not exceed $1.5\,\text{g/kg/day}$ - Amino acids: complete essential + non-essential amino acid solutions; glutamine may be added separately - Electrolytes: sodium, potassium, chloride, phosphate, calcium, magnesium - adjusted daily - Trace elements and vitamins: added to all PN; thiamine supplemented first in malnourished patients

Initiation and Monitoring of PN

1. Confirm central venous access (tip position at cavoatrial junction on CXR)
2. Start at 50% of calculated requirements on day 1; advance over 24-48 hours
3. Monitor blood glucose 4-6 hourly initially; target $6\text{-}10\,\text{mmol/L}$
4. Daily U&E, LFTs, phosphate, magnesium; weekly LFTs and trace elements in long-term PN
5. Weigh patient 3 times weekly; reassess nitrogen balance weekly

Complications of Parenteral Nutrition

Refeeding Syndrome

A potentially fatal metabolic complication occurring when nutritional support - enteral or parenteral - is introduced too rapidly in chronically malnourished or starved patients.

Pathophysiology

• Prolonged starvation depletes intracellular phosphate, potassium, and magnesium stores, though serum levels may appear normal
• Reintroduction of carbohydrate → insulin surge → rapid cellular uptake of phosphate, potassium, and magnesium → profound hypophosphataemia ($PO_4 < 0.5\,\text{mmol/L}$)
• Thiamine is consumed as a cofactor in carbohydrate metabolism → Wernicke's encephalopathy if not pre-supplemented

Clinical Manifestations

• Cardiac arrhythmias, heart failure
• Respiratory muscle weakness, ventilatory failure
• Neuromuscular dysfunction (weakness, tetany, seizures)
• Haemolytic anaemia; thrombocytopenia

Prevention

1. Identify high-risk patients: prolonged fasting $> 5\,\text{days}$, BMI $< 16\,\text{kg/m}^2$, chronic alcohol use, anorexia nervosa, malabsorption syndromes
2. Supplement thiamine $200\text{-}300\,\text{mg}$ IV before commencing nutrition
3. Correct electrolyte deficiencies before starting nutritional support
4. Begin feeding at 10 kcal/kg/day; increase slowly over 4-7 days
5. Monitor serum phosphate, potassium, magnesium, and glucose daily for the first week

Perioperative Nutritional Management

Preoperative Nutrition

• Well-nourished patients undergoing elective surgery: standard carbohydrate loading (oral or IV) up to 2 hours before induction; reduces insulin resistance
• Malnourished patients (SGA class C, weight loss >10%, albumin $< 30\,\text{g/L}$): preoperative nutritional support for at least 2-4 weeks improves outcomes, particularly in elective gastrointestinal cancer surgery
• TPN preoperatively is reserved for severely malnourished patients who cannot absorb adequate enteral nutrition; the benefit is most pronounced in those with severe malnutrition

Postoperative Nutrition

• Enhanced Recovery After Surgery (ERAS) protocols: early oral intake within hours of surgery is safe and reduces length of stay; ileus is no longer a contraindication to oral feeding
• Bowel sounds and flatus are not required before commencing oral diet in most circumstances
• Transition from IV fluids to oral/enteral feeding should be proactive; prolonged 5% dextrose administration without nutritional support is appropriate for only 5-7 days in previously well-nourished patients

Special Clinical Contexts

Key Examination Points

• EN vs PN: always prefer EN if the gut works; evidence supports lower septic complication rates with EN
• Route selection: match to clinical context, expected duration, and aspiration risk
• Assessment tools: SGA for surgical patients; NUTRIC score for ICU; no single biochemical marker is reliable in isolation
• Refeeding syndrome: high-yield viva topic - identify risk factors, supplement thiamine first, start slowly
• Overfeeding is harmful: excess carbohydrate → lipogenesis, hepatic steatosis, $CO_2$ retention, hyperglycaemia, and immunosuppression
• Glucose control: target $6\text{-}10\,\text{mmol/L}$ in ICU; hyperglycaemia independently increases nosocomial infection rates
• Protein requirements in critical illness ($1.2\text{-}2.0\,\text{g/kg/day}$) are substantially higher than standard estimates
• Preoperative nutritional optimisation requires a minimum of 2-4 weeks to deliver measurable clinical benefit