Why BCAAs alone cannot sustain muscle protein synthesis
Branched-chain amino acids are three of the nine essential amino acids: leucine, isoleucine, and valine. Leucine is a strong trigger for the mTORC1 pathway that initiates muscle protein synthesis, but a trigger is not the same as building material. Synthesizing new muscle protein requires all nine essential amino acids present together; with only the three BCAAs, the body cannot complete the new strands and the response stalls.
This is why BCAA-only products can switch the signal on without supplying what the cell needs to follow through.
Where complete EAAs and whole protein win
Essential amino acid blends contain all nine and produce a fuller synthesis response, and a normal serving of complete protein such as whey already provides them in good ratio. For most people eating enough quality protein, separate BCAA supplements add little.
This is general educational information, not medical advice. Consult a qualified professional before relying on amino acid supplements, especially if dietary protein is restricted.