What are the components and mechanisms of action of total parenteral nutrition (TPN) for patients unable to receive enteral feeding?

Components and Mechanisms of Total Parenteral Nutrition

Total parenteral nutrition delivers all essential nutrients intravenously through amino acids, glucose, lipids, electrolytes, vitamins, and trace elements, bypassing the gastrointestinal tract entirely to maintain life in patients with intestinal failure. 1

Core Macronutrient Components and Their Mechanisms

Amino Acids (Protein Source)

• Dosing: 1.2–2.0 g/kg ideal body weight per day for critically ill patients 2
• Mechanism: Provides nitrogen for protein synthesis, prevents muscle catabolism, supports wound healing, and maintains immune function 3, 4
• Target: Approximately 20% of total energy should come from protein 2
• In liver disease, reduce to 1.2–1.5 g/kg/day to prevent hepatic encephalopathy 1, 2

Glucose (Carbohydrate Source)

• Dosing: Start at 2–3 g/kg/day, may increase to 4–5 g/kg/day if tolerated 2
• Maximum rate: Do not exceed 7 mg/kg/min to prevent hyperglycemia 5
• Mechanism: Primary energy source for cellular metabolism, spares protein from gluconeogenesis, maintains blood glucose homeostasis 3, 4
• Target: 50–60% of non-protein energy (approximately 50–70% of total calories) 2
• Critical safety: Administer vitamin B1 (thiamine) before glucose infusion to prevent Wernicke's encephalopathy, especially in alcoholic patients 1, 2

Lipid Emulsions (Fat Source)

• Dosing: 30–40% of non-protein calories, optimal infusion rate 80 mg/kg/hr 2
• Mechanism: Provides essential fatty acids (linoleic and α-linolenic acid), delivers concentrated calories (9 kcal/g), serves as carrier for fat-soluble vitamins, prevents essential fatty acid deficiency 3, 4
• Preferred formulation: SMOFlipid (mixed-oil emulsion) contains lower n-6 unsaturated fatty acids than pure soybean oil, reducing hepatotoxicity risk 2
• Safety limit: Maintain triglycerides <400 mg/dL (<12 mmol/L) 2

Micronutrient Components and Their Mechanisms

Water-Soluble Vitamins

• Administration: Daily from day 1 of TPN 1, 2
• Key vitamins: B-complex (B1, B2, B6, B12, niacin, pantothenic acid, biotin, folic acid) and vitamin C 3
• Mechanism: Cofactors for enzymatic reactions in energy metabolism, DNA synthesis, antioxidant defense, and neurological function 3

Fat-Soluble Vitamins

• Components: Vitamins A, D, E, and K 3
• Mechanism: Support vision, bone metabolism, antioxidant protection, and coagulation cascade 3

Trace Elements

• Administration: Daily from day 1 of TPN 1, 2
• Essential elements: Zinc, copper, manganese, chromium, selenium 3
• Mechanism: Cofactors for metalloenzymes, support immune function, wound healing, glucose metabolism, and antioxidant systems 3
• Special consideration: Zinc and magnesium may require increased dosing with gastrointestinal losses 2

Electrolytes

• Components: Sodium, potassium, chloride, calcium, magnesium, phosphate 1, 3
• Mechanism: Maintain fluid balance, acid-base homeostasis, neuromuscular function, and cellular membrane potential 3
• Individualization: Composition should reflect fluid losses and metabolic state 1
• Critical monitoring: Check phosphate, potassium, and magnesium daily in first 72 hours to prevent refeeding syndrome 2

Energy Targets and Distribution

Total Energy Requirements

• Standard patients: 25–30 kcal/kg ideal body weight per day 1, 2
• Severe stress/critical illness: Up to 30 kcal/kg/day 2
• Liver disease: 1.3 × resting energy expenditure 1, 2

Recommended Caloric Distribution

• Traditional ratio: Protein 20% : Fat 30% : Glucose 50% 2
• Modern trend: Increasing glucose-to-fat ratio to 60:40 or 70:30 reduces hyperlipidemia and fatty liver risk 2

Administration Protocol and Mechanisms

Infusion Method

• Continuous 24-hour infusion provides optimal nitrogen sparing and metabolic stability 2
• Cyclic infusion (10–14 hours) for home TPN improves quality of life, allows daytime freedom from pump 5
• Mechanism: Continuous delivery prevents metabolic fluctuations in glucose and electrolytes, maintains steady anabolic state 2

Route of Administration

• Central venous access required for standard TPN due to high osmolarity (>850 mOsm/L) 2
• Mechanism: High-flow central veins rapidly dilute hypertonic solution, preventing phlebitis and thrombosis 3

Common Pitfalls and Safety Considerations

Refeeding Syndrome Prevention

• Start with 20–25 kcal/kg/day for first 72–96 hours in malnourished patients 2
• Monitor phosphate, potassium, magnesium, and calcium daily during initiation 2
• Mechanism: Rapid refeeding shifts metabolism from catabolic to anabolic, causing intracellular electrolyte shifts and severe deficiencies 2

Hyperglycemia Management

• Maintain blood glucose ≤10 mmol/L (≈180 mg/dL) 2
• Insulin infusion should not exceed 4–6 units per hour 2
• Mechanism: Hyperglycemia increases infection risk, impairs wound healing, and worsens outcomes 2

Overfeeding Avoidance

• Never exceed 30 kcal/kg/day—this increases complications without benefit 2
• Mechanism: Excess calories cause hyperglycemia, hepatic steatosis, increased CO₂ production, and immune dysfunction 2

Contraindications

• Never initiate TPN when enteral nutrition is feasible—it increases catheter-related sepsis without improving outcomes 2
• TPN is indicated only when enteral feeding cannot provide >60% of energy needs 1, 2