Leucine activates which pathway that strongly stimulates muscle protein synthesis?

• A. Suppresses protein synthesis
• B. Activates mTOR pathway
• C. Inhibits muscle growth
• D. Reduces anabolic signaling

Answer: (B)

Leucine acts as a nutrient signal that directly turns on the mTORC1 pathway, a central driver of muscle protein synthesis. When leucine levels are high, mTORC1 becomes activated and phosphorylates downstream targets like S6K1 and 4E-BP1, which ramps up translation initiation and ribosome activity, fueling protein production in muscle. This pathway integration explains why leucine-rich proteins or BCAAs can robustly stimulate muscle growth, especially after resistance exercise when the muscles are primed for rebuilding. The other options describe effects that oppose anabolic processes or suggest suppression of growth, which is not how leucine functions.