An adult recovering from surgery receives 100 g of dietary protein per day. His 24-hour urinary nitrogen excretion measures 12 g, and other nitrogen losses are negligible. What is his nitrogen balance?

• A Negative balance of 4 g nitrogen per day
• B Positive balance of 88 g nitrogen per day
• C Positive balance of 4 g nitrogen per day ✓
• D Neutral balance, since intake equals output

Explanation

Protein contains approximately 16 percent nitrogen, so 100 g of protein provides about 16 g of nitrogen (100 divided by 6.25). Subtracting the 12 g of urinary nitrogen loss leaves a positive balance of roughly 4 g per day, indicating net tissue protein deposition appropriate for surgical recovery. Option A reverses the arithmetic sign, and option B confuses grams of protein with grams of nitrogen. Neutral balance would require excretion equal to 16 g.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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