Regarding parenteral nutrition in surgical patients, which substrate is preferred as the primary energy source, and what is the recommended caloric ratio of non-protein to protein calories?

• A Fat emulsions (lipids) as the exclusive calorie source; 100% non-protein calories as fat
• B Mixed glucose + lipid; non-protein calories: protein nitrogen ratio approximately 150 kcal:1g nitrogen ✓
• C Glucose only as calorie source; 200 kcal per gram of nitrogen provided
• D Amino acids as the primary calorie source; protein provides >50% of total calories

Explanation

In total parenteral nutrition (TPN), calorie delivery uses a mixed substrate of glucose (dextrose) and lipid emulsions. The non-protein calorie to nitrogen ratio (NPC:N) should be approximately 150 kcal per gram of nitrogen, allowing protein (amino acids) to be used for anabolism rather than burned for energy. Typical TPN provides 25-30 kcal/kg/day total, with protein at 1.2-1.5 g/kg/day (critically ill). Excessive glucose alone causes hyperglycemia, fatty liver, and CO2 overproduction — lipids provide 30-50% of non-protein calories to avoid these complications.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.