Nitrogen from protein

Convert between grams of protein and grams of nitrogen, and calculate the non-protein calorie to nitrogen ratio.

What is this for?

Protein averages 16% nitrogen by weight, which is where the divisor 6.25 comes from (100 ÷ 16). Nitrogen is the currency of protein adequacy assessment, because urinary nitrogen excretion can be measured directly and compared against intake.

How to use it

Choose the direction and enter the amount in grams.
To assess protein adequacy, add the non-protein calories from the parenteral or enteral worksheet.
To estimate nitrogen balance, add the 24-hour urinary urea nitrogen from a complete urine collection.

Worked example

A patient receives 80 g of protein and 1520 non-protein calories per day. Their 24-hour urinary urea nitrogen is 12 g. Assess protein provision.

Answer: nitrogen intake = 80 ÷ 6.25 = 12.8 g. NPC:N = 1520 ÷ 12.8 = 119:1, within the usual range. Balance = 12.8 − (12 + 4) = −3.2 g/day, which is negative and suggests protein provision is insufficient for the current level of catabolism.

Clinical pearls & pitfalls

The divisor 6.25 assumes protein is 16% nitrogen. This is an average across mixed proteins; individual proteins range from roughly 15% to 18%.
The constant 4 g added to urinary urea nitrogen accounts for insensible, faecal, and non-urea urinary nitrogen. Some references use 2 g in stable patients and up to 6 g where there are large wound or fistula losses.
Nitrogen balance requires a genuinely complete 24-hour urine collection. An incomplete collection makes the balance look falsely positive.
The calculation is invalid in significant renal impairment, where urea is retained rather than excreted, and unreliable during dialysis.

Assumptions & limitations

Nitrogen balance is an estimate. Insensible losses vary widely and are not measured, and large wound, burn, or fistula losses are not captured by the standard constant.
Not valid in renal failure or during renal replacement therapy.
A positive nitrogen balance does not by itself demonstrate clinical benefit; it is a surrogate measure.

References

Dickerson RN. Nitrogen balance and protein requirements for critically ill older patients. Nutrients. 2016;8(4):226.
McClave SA, et al. Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient. JPEN J Parenter Enteral Nutr. 2016;40(2):159-211.
Jones AM. The Jaffe reaction and Kjeldahl nitrogen determination — historical basis of the 6.25 factor. J Biol Chem (historical).