05/07/2026
Leucine is the only amino acid your body uses as an anabolic switch. It binds to a complex called mTORC1 in muscle cells, and that binding flips the synthesis machinery on. The ribosomes start trying to build new protein. This is why "leucine triggers muscle protein synthesis" became a marketing claim. It's the basis of every BCAA powder and leucine pill on the shelf.
The mechanism is real. The conclusion drawn from it is [usually] wrong (with respect to marketing).
Building new protein requires all nine essential amino acids. Leucine alone gives you the signal but not the substrate. When you swallow BCAAs in a fasted state, your body still needs methionine, tryptophan, lysine, threonine, phenylalanine, histidine, and the other EAAs to assemble new muscle. Those amino acids have to come from somewhere. The only place available is your existing tissue. So your body breaks down skeletal muscle to supply the parts.
Robert Wolfe published a review in the Journal of the International Society of Sports Nutrition in 2017 that walked through this in detail. The theoretical maximum for muscle protein synthesis stimulation from BCAAs alone in a fasted state is the gap between breakdown and synthesis at baseline, which is roughly 30 percent. That ceiling is set by the EAAs available from breakdown. Of the two human IV-infusion studies of BCAAs alone, both showed decreased muscle protein turnover, with breakdown still exceeding synthesis. The catabolic state persisted during BCAA infusion.
Jackman et al. tested 5.6g of BCAAs after resistance exercise in 2017 and found a 22 percent increase in myofibrillar synthesis over placebo. But phenylalanine rate of appearance dropped 6 percent. Phenylalanine wasn't in the drink, so the body had to be pulling it from somewhere. That somewhere is endogenous protein breakdown.
The signaling and the substrate are two separate problems. Leucine handles the signaling. You need whole protein for the substrate. A BCAA scoop without other protein gives you the activation without the building blocks, and your body solves the math by cannibalizing what's already there.
A 2025 trial confirmed the mechanism from the other direction: when you add a full essential amino acid profile to leucine, anabolism is restored. The signal alone is not enough. Both pieces need to be present.
If you eat a meal with whole protein, you get leucine and the other eight EAAs together, and the synthesis machinery has both the signal and the substrate. That is what makes net protein accrual possible.
References:
Wolfe RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? J Int Soc Sports Nutr. 2017
Jackman SR et al. Branched-chain amino acid ingestion stimulates muscle myofibrillar protein synthesis following resistance exercise in humans. Front Physiol. 2017
Aguilera JA et al. Dileucine-supplemented essential amino acids support whole-body anabolism after resistance exercise. J Int Soc Sports Nutr. 2025
