In fasting leucine metabolism, what happens to pyruvate and lactate produced by muscle?

• A. They are discarded by the body as waste
• B. They are returned to the liver to participate in gluconeogenesis
• C. They are converted to acetyl-CoA in muscle
• D. They are stored as glycogen in the liver

Answer: (B)

During fasting, skeletal muscle still generates pyruvate and lactate from glycolysis, but these products aren’t wasted. They travel through the bloodstream back to the liver, where lactate is converted to pyruvate and then funneled into gluconeogenesis to make glucose. This glucose is released into the blood to keep brain and other tissues supplied when dietary glucose is unavailable—the Cori cycle at work. So pyruvate and lactate produced by muscle are returned to the liver to participate in gluconeogenesis. They aren’t simply discarded, stored as liver glycogen, or predominantly diverted to acetyl-CoA in muscle in this fasting context.