Nitrogen Balance

Definition

Calculation of the difference between nitrogen intake and nitrogen excretion, expressed as: Nitrogen balance = (Protein intake ÷ 6.25) - (Urinary urea nitrogen + 4). Used to assess adequacy of protein support.

How it works

Nitrogen balance is a fundamental tool for assessing protein nutrition status. Positive nitrogen balance (more nitrogen in than out) indicates protein anabolism and tissue building, desirable during recovery and growth. Negative nitrogen balance indicates net protein loss and catabolism, seen in starvation and critical illness. The calculation uses the fact that 1 gram of nitrogen approximately equals 6.25 grams of protein. The formula includes urinary urea nitrogen (measured) plus estimated non-urea nitrogen losses (approximately 4 grams daily). Serial nitrogen balance assessments guide adjustment of protein intake to optimize recovery and minimize muscle wasting.

Role

Quantifies protein adequacy and guides adjustment of protein support to achieve anabolism and minimize muscle loss during critical illness and recovery.

Examples

• Nitrogen balance +5 g/day indicates adequate protein support
• Nitrogen balance -10 g/day indicates need for more protein
• Serial measurements show trend toward positive balance with improved nutrition

Recommendations

Calculate nitrogen balance twice weekly in critical care or during refeeding. Maintain positive nitrogen balance of +2 to +5 g/day during recovery. Increase protein intake if balance persistently negative. Use 24-hour urine collection for accurate urea nitrogen measurement. Target 1.2-2.0 g protein/kg/day in critical illness.

Key takeaway

Nitrogen balance assessment guides protein provision to ensure adequate nutritional support and minimize catabolic loss during critical illness and recovery.

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