Physiology · Temperature, exercise and metabolism
Nitrogen balance and body protein !
- Measurement
- For completeness
Comparing the nitrogen that comes in with food with the nitrogen lost in urine shows whether the body is gaining or losing protein.
Updated 2026-11-01
Also called: Nitrogen balance
What it is
Protein is the only major dietary source of nitrogen, and the body has no protein store: every protein has a job. Nitrogen balance compares intake with losses (mainly urinary urea). Positive means tissue is being built; negative means it is being broken down.
Equations
- NB
- nitrogen balance (g/day)
- 4
- approximate non-urinary losses (stool, skin) plus other urinary forms, g/day
An approximate bedside formula; the 4 is an estimate.
Approximate values (adult)
| Parameter | Value | Note |
|---|---|---|
| Total body protein | ≈10–12 kg | more than half in muscle |
| Reference requirement | ≈0.8 g of protein per kilogram of body weight per day | more in pregnancy, growth and illness |
| Obligatory loss on a protein-free diet | ≈20–30 g of protein per day |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- In critical illness (sepsis, burns) cortisol and cytokines cause a strongly negative balance: muscle wasting and weakness.
- Severe liver disease reduces urea synthesis: ammonia rises.
- Kwashiorkor: inadequate protein intake with negative balance and low albumin.
Test yourself
Think of the answer, then tap the question to see it.
Why does a diet missing just one essential amino acid cause negative nitrogen balance?
Protein synthesis needs all amino acids at once; the others go unused, are broken down and their nitrogen is excreted as urea.
What nitrogen balance do you expect in a patient with extensive burns, and why?
Negative: cortisol, catecholamines and cytokines speed muscle proteolysis to feed gluconeogenesis and repair.
Why is there no protein «store» like fat or glycogen?
Every protein has a function (structure, enzymes, transport); burning it means losing working tissue, mostly muscle.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 24.4 Protein Metabolism (OpenStax (Rice University), 2022); 24.7 Nutrition and Diet (OpenStax (Rice University), 2022); Kwashiorkor: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 70). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.