....Leucine is one of three BCAAs (isoleucine and valine are the other two). When taken in isolation, BCAA ingestion results in only a relatively small increase in MPS, likely because you need the other amino acids as building blocks for muscle growth (Jackman, 2017). When BCAAs are supplemented on top of some protein, the increase in MPS is smaller compared to simply eating more protein or adding only leucine (Churchward-Venne, 2014). This may sound counter-intuitive. Why would the addition of the two other BCAAs be worse than leucine alone? Isoleucine and valine use the same transporter as leucine for uptake in the gut. Therefore, it is speculated that isoleucine and valine compete for uptake with leucine, resulting in a less rapid plasma leucine peak....
So, leucine supplementation on its own is better at producing the trigger for maximal MPS, and a complete protein is just better at providing the bricks for muscle growth. A BCAA supplement doesn’t fit anywhere in this picture; it’s worse than only leucine at doing leucine’s job, and it’s worse than a complete protein at its job.