Calculation of Nitrogen Balance
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Steps to Calculate Nitrogen Balance
  1. Determine nitrogen lost in urine by a 24 hour urinary urea nitrogen test.
  2. Add 4* to the UUN to account for non-urinary losses of nitrogen.
  3. Determine nitrogen intake by dividing the daily protein intake by 6.25
  4. N-bal = value from #3 - value from #2

* 4 g is used to estimate the nitrogen losses from non-urine sources (sweat feces, etc.)

If the nitrogen balance is 0, the person is consuming the correct amount of protein for maintenance.

If the nitrogen balance is negative, increase the protein intake by a factor determined by multiplying the nitrogen balance figure by 6.25.

If the goal is repletion, increase protein intake above what would result in nitrogen equilibrium.

Example N-Balance Calculation

Protein intake: 90 g
24 hour UUN: 20 g N

20+ 4 = 24 g N (total output)
N intake = 90/6.25 = 14.4 g N

N-balance = 14 - 24 = -10 g N

This person is losing 10 x 6.25 = 62.5 g protein per day.

Practice Calculations