Nitrogen Balance Calculator
Nitrogen Balance Calculator
Calculate 24-hour nitrogen balance from protein intake and urine urea nitrogen, and see what makes the answer unreliable.
Nitrogen balance
Intake − lossesProtein intake 90 g/24 h, urine urea nitrogen 12 g/24 h
Formula
- 6.25
- dietary protein is 16% nitrogen, so protein grams divided by 6.25 gives nitrogen intake
- 4
- grams of insensible and non-urinary nitrogen loss — stool, skin and non-urea urinary nitrogen
- UUN
- urine urea nitrogen from a complete 24-hour collection; the completeness of that collection is the main limitation of the whole calculation
Worked example
Protein intake 90 g/24 h, urine urea nitrogen 12 g/24 h
Nitrogen intake = 90 ÷ 6.25 = 14.40 g
Nitrogen losses = 12 + 4 = 16.00 g
14.40 − 16.00 = −1.60 g N/24 h
A modest negative balance — the deficit is equivalent to about 10 g of protein a day
Reading the result
| g N/24 h | Interpretation |
|---|---|
| < −4 | Markedly catabolic — review delivery of intake and treat the driver of catabolism |
| −4 to 0 | Negative balance; expected in acute illness, where limiting the deficit is the realistic goal |
| 0 to 4 | Balance or mild anabolism |
| > 4 | Strongly anabolic — confirm the collection was complete and exclude overfeeding |
What invalidates the calculation
| Problem | Effect on the result |
|---|---|
| Incomplete 24-hour collection | Falsely positive balance — the commonest error |
| Burns, high-output fistula, enteropathy, large wounds | The 4 g constant understates losses, so the balance is overstated |
| Renal impairment | Urea is retained rather than excreted, so urine urea nitrogen understates production |
| Dialysis | Nitrogen is removed in the dialysate and is not captured by a urine collection |
Reading nitrogen balance without over-reading it
Nitrogen balance is nitrogen in minus nitrogen out over 24 hours. Dietary protein is 16% nitrogen, so protein grams divided by 6.25 gives nitrogen intake; urine urea nitrogen from a complete 24-hour collection gives the measured loss, and a constant of 4 g is added to account for insensible and non-urinary nitrogen lost in stool, from skin and as non-urea urinary nitrogen. One gram of nitrogen corresponds to 6.25 g of protein and roughly 30 g of lean tissue, which is the arithmetic that makes a deficit of a few grams a day matter over a fortnight.
The calculation is only as good as the 24-hour urine collection. A missed void or a collection started and finished at the wrong times lowers the measured urea nitrogen and produces a falsely positive balance — telling the team the patient is anabolic when they are not. This is the commonest error by a wide margin. A urine creatinine measured on the same collection is the usual check, since daily creatinine excretion is relatively constant for a given patient and an implausibly low figure exposes an incomplete collection.
The insensible-loss constant is an approximation and it is optimistic in exactly the patients most likely to be measured. Losses through burns, high-output fistulae, protein-losing enteropathy and large open wounds can be far higher than 4 g, and in extensive burns substantially so, which means the true balance is worse than the calculated one. Renal impairment invalidates the method altogether, because urea is retained rather than excreted and the urine figure no longer reflects production; the same applies on dialysis, where nitrogen leaves in the dialysate.
Finally, the target needs stating carefully. A positive nitrogen balance during the acute phase of critical illness is not usually achievable — the catabolic drive is hormonal and inflammatory rather than nutritional — and pushing protein and energy to force one leads to overfeeding, hyperglycaemia and hepatic steatosis without preserving lean mass. Early on, the sensible goal is to limit the deficit; reversing it belongs to the recovery phase, once the inflammatory drive has settled.
Frequently asked questions
How is nitrogen balance calculated?
Nitrogen balance = protein intake in g/24 h ÷ 6.25 − (urine urea nitrogen in g/24 h + 4). With 90 g of protein and 12 g of urine urea nitrogen, the balance is 14.40 − 16.00 = −1.60 g N/24 h.
Why divide protein by 6.25?
Dietary protein is about 16% nitrogen by weight, and 1 ÷ 0.16 is 6.25. So 6.25 g of protein contains 1 g of nitrogen, and dividing the 24-hour protein intake by 6.25 converts it to nitrogen intake.
What does the constant of 4 represent?
Insensible and non-urinary nitrogen losses in stool, from skin and as non-urea urinary nitrogen. It is an approximation and understates losses in burns, high-output fistulae, protein-losing enteropathy and large open wounds.
What is the commonest source of error?
An incomplete 24-hour urine collection. It lowers the measured urea nitrogen and produces a falsely positive balance, suggesting the patient is anabolic when they are not. Checking the urine creatinine on the same collection helps expose it.
Should we aim for a positive balance in critical illness?
Not in the acute phase. The catabolic drive is hormonal and inflammatory, a positive balance is rarely achievable, and forcing one risks overfeeding. The early goal is to limit the deficit, with reversal belonging to the recovery phase.
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References
- Singer P, Blaser AR, Berger MM, et al. ESPEN practical and partially revised guideline: clinical nutrition in the intensive care unit. Clin Nutr. 2023;42(9):1671–1689.
- McClave SA, Taylor BE, Martindale RG, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient (SCCM/ASPEN). JPEN J Parenter Enteral Nutr. 2016;40(2):159–211.
- Dickerson RN. Nitrogen balance and protein requirements for critically ill older patients. Nutrients. 2016;8(4):226.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.