From dietary protein to free amino acids
The cascade starts at digestion. Stomach acid and pepsin begin breaking down dietary protein, and pancreatic and intestinal enzymes finish the job, releasing free amino acids and small peptides that are absorbed across the gut wall into the bloodstream. This raises circulating amino acids, the raw material and the signal for what follows.
Among those amino acids, leucine plays a special signaling role beyond being a building block.
mTORC1 activation and ribosomal translation
Rising leucine and related cues activate mTORC1, a master regulatory complex inside the cell. Active mTORC1 promotes the assembly of the translation machinery, in part by relieving the brakes on initiation factors, so ribosomes can read messenger RNA and string amino acids into new proteins, including the contractile and structural proteins of muscle.
Resistance exercise sensitizes muscle to this signal, which is why training plus adequate protein produces more synthesis than either alone. The diagram lays out the canonical pathway; the broad outline is well established, while finer regulatory details remain under active study. General educational information, not medical or supplementation advice.