A student measures urinary nitrogen excretion serially in a volunteer fasted for 21 days. Compared with day 3 of the fast, urinary nitrogen excretion on day 21 shows which pattern?

• A Marked increase due to accelerated muscle proteolysis
• B No change, since proteolysis remains constant throughout fasting
• C Increased urea with decreased creatinine excretion
• D Decreased total nitrogen, with a rising proportion excreted as ammonia rather than urea ✓

Explanation

After several weeks of starvation, sustained ketosis and acidosis stimulate renal ammoniagenesis, and glutamine becomes the major vehicle for nitrogen excretion. Total urinary nitrogen falls sharply as maximal brain ketone use spares protein, protecting lean mass for survival. The urea fraction falls disproportionately while ammonia rises. This renal adaptation, alongside the brain's ketone switch, is what distinguishes late starvation from the early protein-catabolic phase.

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

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