How leucine flips the mTORC1 switch
Leucine is sensed inside the cell by Sestrin2, which normally restrains the GATOR complex that holds mTORC1 in check. When leucine binds Sestrin2, that brake is released, GATOR activity shifts, and mTORC1 becomes active. Active mTORC1 then drives the machinery that begins muscle protein synthesis, which is why leucine is described as the dietary trigger amino acid.
This makes leucine concentration, not just total protein, an important factor in switching synthesis on after a meal.
The trigger needs full substrate to matter
Triggering mTORC1 raises the rate of synthesis only if the other essential amino acids are available to be assembled into protein. A leucine spike without complete protein produces a brief signal with little to build from, so leucine works best inside a full protein feeding rather than alone.
This is general educational information, not medical advice. Consult a qualified professional before using isolated amino acids, especially with existing health conditions.