[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative) › Nitrogen balance and body protein

[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# 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

**See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/) · [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- 1 g of nitrogen ≈ 6.25 g of protein (proteins are ≈16% nitrogen).
- Positive balance: growth, pregnancy, recovery, strength training (insulin, GH, IGF-1, testosterone).
- Negative balance: fasting, fever, sepsis, trauma, burns, high cortisol, immobilization, a diet missing an essential amino acid.
- Nine essential amino acids: lack one and synthesis stops even if the others are plentiful.
- ≈80–90% of urinary nitrogen is urea, made in the liver (urea cycle).

## 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

**Nitrogen balance**NB = (protein intake / 6.25) − (urine urea N + 4)

**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.

## Memory aid

«Nitrogen × 6.25 = protein»: the factor that turns urine into lost muscle.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/nitrogen-balance-and-body-protein/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Liver](https://durgical.com/en/anatomy/liver/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics

- [Prolonged fasting and starvation](https://durgical.com/en/physiology/prolonged-fasting-and-starvation/)
- [Fed and fasting states: the hormonal switch](https://durgical.com/en/physiology/fed-and-fasting-states-the-hormonal-switch/)

## Related conditions

- [Kwashiorkor](https://durgical.com/en/conditions/kwashiorkor/)

## In Biochemistry

- [Urea cycle](https://durgical.com/en/biochemistry/pathways/urea-cycle/)
- [Glucogenic and ketogenic amino acids](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-4-protein-metabolism) (OpenStax (Rice University), 2022); [24.7 Nutrition and Diet](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-7-nutrition-and-diet) (OpenStax (Rice University), 2022); [Kwashiorkor: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001604.htm) (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.

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Source: https://durgical.com/en/physiology/nitrogen-balance-and-body-protein/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/nitrogen-balance-and-body-protein/