The body may have evolved to sense leucine for muscle protein synthesis due to its metabolic pathway and passive diffusion into muscle cells.

“So he would say-- one day, I'll never forget, he said, Layne, why do you think the body evolved to just sense leucine for muscle protein synthesis? And of course, I'm like, I don't know, man. I just work here like. I'm just trying to get my PhD. And he said, well, think about it. You would want something that really wasn't extensively metabolized by the gut and liver, because you would want to show up in the blood in values that reflect what you just ate. Makes sense. And you would want it to have passive diffusion across the muscle cell because you want to be concentration-dependent, which it is.”