Intake minus loss, positive in growth and negative in catabolism
Expansion
Nitrogen balance = intake - losses (urine, faeces, skin).
- Positive: growth, pregnancy, recovery, resistance training, anabolic states
- Zero: healthy adult in equilibrium
- Negative: starvation, trauma, sepsis, burns, major surgery, steroids, immobility
Protein contains about 16 per cent nitrogen, so 6.25 g of protein contains 1 g of nitrogen. Urinary urea accounts for most loss and is used to estimate it.
Requirements: about 0.8 g/kg/day in health, rising to 1.2 to 1.5 in critical illness and up to 2 in burns. Older adults need more than the standard figure because of anabolic resistance.
The important clinical point is that in the acute phase of critical illness the catabolic hormonal drive from cortisol, catecholamines and cytokines produces obligatory muscle breakdown that feeding can attenuate but not abolish. Overfeeding in an attempt to correct it causes hyperglycaemia, hepatic steatosis and excess carbon dioxide production without sparing muscle.
Loss of 40 per cent of body protein is generally incompatible with survival, which is why preserving muscle mass through early mobilisation matters as much as nutrition.