During fasting, which statement best describes leucine metabolism in muscle?

During fasting, which statement best describes leucine metabolism in muscle?

• A. Leucine oxidation decreases, and pyruvate oxidation increases
• B. Leucine oxidation increases; acetyl-CoA is produced; pyruvate is reduced to lactate and released by the muscle for gluconeogenesis
• C. Leucine is stored as glycogen in muscle to spare energy
• D. Leucine completely inhibits gluconeogenesis in the liver

Answer: (B)

During fasting, muscle relies more on amino acid oxidation for energy, and leucine, a strictly ketogenic amino acid, is directed toward acetyl-CoA rather than glucose precursors. This means leucine oxidation increases and yields acetyl-CoA that feeds the TCA cycle for ATP production in muscle. At the same time, pyruvate from glycolysis can be reduced to lactate and exported to the liver, where lactate serves as a substrate for gluconeogenesis via the Cori cycle. So the statement reflects both the energy-producing fate of leucine in muscle and the liver’s use of muscle-derived lactate for gluconeogenesis. Leucine is not stored as glycogen in muscle, and it cannot be converted to glucose in the liver, so the other ideas don’t fit fasting physiology as well.

During fasting, muscle relies more on amino acid oxidation for energy, and leucine, a strictly ketogenic amino acid, is directed toward acetyl-CoA rather than glucose precursors. This means leucine oxidation increases and yields acetyl-CoA that feeds the TCA cycle for ATP production in muscle. At the same time, pyruvate from glycolysis can be reduced to lactate and exported to the liver, where lactate serves as a substrate for gluconeogenesis via the Cori cycle. So the statement reflects both the energy-producing fate of leucine in muscle and the liver’s use of muscle-derived lactate for gluconeogenesis. Leucine is not stored as glycogen in muscle, and it cannot be converted to glucose in the liver, so the other ideas don’t fit fasting physiology as well.