Nitrogen Balance Calculator

Nitrogen balance compares how much nitrogen a patient takes in against how much they lose, and is the standard bedside way to judge whether protein feeding is actually building tissue (anabolism) or the patient is still breaking down lean mass faster than it can be replaced (catabolism) -- a common concern in critical illness, major trauma, burns, and prolonged malnutrition. Nitrogen intake is estimated from protein intake using the standard conversion that dietary protein is about 16% nitrogen by weight, so grams of protein divided by 6.25 gives grams of nitrogen. Nitrogen output is measured directly from a timed urine collection (24-hour urinary urea nitrogen, or UUN) plus a conventional add-on, typically about 4 grams per day in a clinically stable adult, that covers everything UUN alone misses. That add-on is larger than "insensible losses" alone: UUN measures only the urea fraction of urinary nitrogen, so the 4 g figure has to cover both the non-urea nitrogen still excreted in urine (uric acid, creatinine, ammonia -- roughly 2 g/day) and true non-urinary losses through skin, stool, and other minor routes (roughly another 2 g/day) -- not skin and stool losses alone. This UUN-plus-conventional-add-on approach follows the estimating method described by Mackenzie, Bistrian, Blackburn, and colleagues in a widely cited 1985 review in the Journal of the American College of Nutrition.

If a lab instead reports Total Urinary Nitrogen (TUN, which already includes the non-urea urinary fraction), the add-on should be much smaller (~2 g/day, covering only the non-urinary losses) to avoid double-counting; sicker patients can lose more than the conventional estimate either way. Subtracting output from intake gives the balance in grams of nitrogen per day: a positive number means more nitrogen is coming in than going out (anabolic, building tissue), a negative number means the reverse (catabolic, losing lean mass), and the commonly cited clinical target for a patient being actively fed to rebuild tissue is a balance of roughly +2 to +4 g/day, since a much larger positive number doesn't reliably translate into proportionally more muscle synthesis. The calculator also works the formula backward to show how many grams of protein would be needed to reach exact equilibrium (balance of zero) or a positive +4 g/day target at the patient's current losses, both expressed in total grams per day and per kilogram of body weight for easy comparison against a prescribed protein order. UUN collections carry real practical error (incomplete collections, changes in renal clearance), so treat a single measurement as one data point in a trend, not a definitive verdict on nutritional status.