The glucose-alanine cycle (Cahill cycle) serves which integrated purpose during fasting?
- A Recycles lactate from red blood cells back to glucose in muscle
- B Transports muscle-derived amino nitrogen to the liver for urea synthesis while carbon skeleton becomes gluconeogenic substrate ✓
- C Provides acetyl-CoA to extrahepatic tissues as an alternative fuel
- D Generates NADPH in erythrocytes through the pentose phosphate pathway
Correct answer: B. Transports muscle-derived amino nitrogen to the liver for urea synthesis while carbon skeleton becomes gluconeogenic substrate
Explanation
Muscle transaminates pyruvate to alanine using amino groups from branched chain amino acid breakdown. Alanine travels in blood to the liver, where it is re-transaminated to pyruvate, feeding gluconeogenesis, and the nitrogen enters the urea cycle. This solves two problems at once: moving toxic nitrogen safely and supplying hepatic gluconeogenic carbon. Option A describes the Cori cycle, which handles lactate and carries no nitrogen.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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