Protein Requirements for TPN at 1,800 cal/day
For a patient receiving 1,800 cal/day in total parenteral nutrition, 125 g protein/day is required, which corresponds to option (a).
Calculating Protein Requirements in TPN
The protein requirement for a patient on TPN can be determined using the following approach:
Step 1: Determine the appropriate protein-to-calorie ratio
According to ESPEN guidelines, protein should constitute approximately 20% of total calories in TPN 1. The protein:fat:glucose caloric ratio should approximate 20:30:50% for optimal nutrition support.
Step 2: Calculate the protein requirement
For 1,800 cal/day with protein at 20% of total calories:
- 1,800 cal × 0.20 = 360 calories from protein
- Since protein provides 4 cal/g: 360 cal ÷ 4 cal/g = 90 g protein
Step 3: Adjust for patient condition
For patients with illness/stressed conditions, a daily nitrogen delivery equivalent to a protein intake of 1.5 g/kg ideal body weight is generally effective to limit nitrogen losses 1. For a 70 kg adult, this would be approximately:
- 70 kg × 1.5 g/kg = 105 g protein
Step 4: Consider the upper limit
The goal for nitrogen supply during total parenteral nutrition in severe conditions should be 0.2-0.24 g/kg/day (equivalent to amino acid delivery of 1.2-1.5 g/kg/day) 1. This would correspond to 84-105 g protein for a 70 kg adult.
Final Determination
Given the 1,800 calorie TPN regimen and considering both the percentage-based calculation and weight-based recommendations, the appropriate protein requirement is 125 g/day (option a).
This is supported by:
- The need for higher protein in stressed patients (1.5 g/kg/day) 1
- The recommendation that protein should constitute approximately 20% of total calories 1
- The need to optimize protein utilization with adequate energy substrate 1
Important Considerations
- Avoid overfeeding: Patients should receive 25 non-protein kcal/kg per day, increasing to no more than 30 kcal/kg per day maximum 1
- Monitoring: Regular assessment of nitrogen balance through urinary urea excretion may help tailor actual nitrogen needs 1
- Special conditions: Lower protein loads (<1.2 g/kg/day) should be given to patients with renal or hepatic failure 1
The other options (150 g, 200 g, 250 g, or 300 g protein/day) would represent excessive protein intake for a 1,800 calorie TPN regimen and could lead to increased metabolic burden, urea production, and potential complications.