Biochemistry · Topic

Nitrogen balance !

Nitrogen balance compares the nitrogen eaten (as protein) with the nitrogen lost, mostly as urea. It shows whether the body is building or losing protein.

Updated 2026-11-01

Key facts

Nitrogen balance = nitrogen intake − nitrogen losses (urine, faeces, skin); protein is about 16 % nitrogen, so g of nitrogen = g of protein ÷ 6.25.
Positive balance (intake > losses): growth, pregnancy, recovery from illness, muscle building, and after insulin or anabolic hormones.
Negative balance (losses > intake): starvation, a diet lacking an essential amino acid, fever, sepsis, trauma, burns, surgery, uncontrolled diabetes, cortisol excess and immobilisation.
A healthy adult with adequate intake is in nitrogen equilibrium (balance ≈ 0).
Urinary urea nitrogen is about 80–90 % of urinary nitrogen and is used to estimate losses; about 4 g/day is added for losses not measured (other nitrogen compounds, stool, skin).

In detail

1. The body has no protein store: every day about 300 g of protein are broken down and rebuilt (protein turnover), and the nitrogen not reused is excreted.
2. Amino acid nitrogen goes to glutamate by transamination, is released as ammonia, and the liver converts it to urea.
3. Clinical calculation: N balance = (protein intake in g ÷ 6.25) − (24-h urinary urea nitrogen in g + about 4 g).
4. In catabolic states, cortisol, glucagon, catecholamines and cytokines break down muscle protein to supply amino acids for gluconeogenesis, acute-phase proteins and repair.
5. If even one essential amino acid is missing, the others cannot be used for synthesis and are deaminated, so the balance turns negative even with enough total protein.
6. Minimum obligatory nitrogen loss on a protein-free diet is the baseline needed to set the safe protein intake.

Also called: Positive and negative nitrogen balance, Protein turnover

Clinical notes

In intensive care, a negative balance in sepsis or burns signals muscle wasting; nutrition aims to reduce it, although it rarely reaches zero in the acute phase.
In chronic kidney disease, urea retention limits protein intake, but too little protein leads to malnutrition.
In kwashiorkor and marasmus the balance is negative; on recovery it becomes strongly positive.

Pathways

Urea cycle

Test yourself

Think of the answer, then tap the question to see it.

How is the nitrogen content of dietary protein estimated?

Dividing grams of protein by 6.25, because protein is about 16 % nitrogen.

Which two situations give a positive nitrogen balance?

Growth in childhood and pregnancy (also recovery from illness and muscle building), when more protein is built than broken down.

Why does a diet missing one essential amino acid cause negative nitrogen balance?

Protein synthesis stops, so the other amino acids are not used, are deaminated and their nitrogen is excreted as urea.

Which urinary compound is used to estimate nitrogen losses?

Urea (24-hour urinary urea nitrogen), which makes up most of urinary nitrogen; about 4 g/day is added for other losses.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Protein in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Plasma amino acids: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Human energy requirements (Food and Agriculture Organization of the United Nations (FAO/WHO/UNU expert consultation), 2004); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.